Meteorology: India (basic data compiled during the British Raj)

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This article was written around 1909 when conditions were different.It has been archived for its historical value as well as for the insights it gives into British colonial writing about India. Indpaedia neither agrees nor disagrees with its contents. Readers who wish to add fresh information can create a Part II of this article. Readers will be able to edit existing articles and post new articles directly on their online archival encyclopædia after its formal launch. See examples and a tutorial.

Extracted from:

THE IMPERIAL GAZETTEER OF INDIA

THE INDIAN EMPIRE

HENRY FROWDE, M.A.

PUBLISHED UNDER THE AUTHORITY OF HIS MAJESTY'S SECRETARY OF STATE FOR INDIA IN COUNCIL

OXFORD

AT THE CLARENDON PRESS

1909

PUBLISHER TO THE UNIVERSITY OF OXFORD

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Meteorology of India: record from 1909

singular interest mFirst, on account of its variety and con- trasts.

General Meteorology of India

THE area dealt with in the present chapter includes the whole of India proper, with Burma and Baluchistan. It is not a self- contained area, as its meteorology depends very largely upon the oceanic area to the south and also, to a slighter extent, upon the regions to the north and northwest, more especially Central Asia and the Persian area or plateau. The object of this chapter is not only to give a statement of the larger seasonal weather changes, but also to indicate, so far as at present possible, their relation to each other, and their co- ordination to meteorological conditions and actions in the neighbouring areas of land and sea.

India probably presents a greater variety of meteorological conditions, actions, and features than any area of similar size in the world. The normal annual rainfall varies from 460 inches at Cherrapunji in the Assam hills, and from between 300 and 400 inches (probably) at suitably exposed positions on the crests of the Western Ghats and the Arakan and Tenasserim hills, to less than three inches in Upper Sind.

The largest rainfall actually measured in India in one year was 905 inches, at Cherrapunji in 1861, while at stations in Upper Sind it has been nil. A rainfall exceeding 25 inches within twenty-four hours is of occasional occurrence, and falls exceeding 15 inches are comparatively frequent. At one period of the year parts of India are deluged with rain ; at another persistent dry weather with clear skies prevails for weeks or months. During the rains the air is almost supersaturated with moisture in some of the coast districts and in the hills, while in the hottest weather it is occasionally so dry in the interior that the methods employed for calculating humidity in Europe have given negative and hence impossible results.

The coasts are occasionally visited by cyclones fiercer and more concentrated than have probably ever occurred in Europe. These bring up storm-waves that sweep over the low coast lands of Lower

Bengal or the deltas of the Mahanadi, Godavari and Kistna, destroying the crops, and drowning the inhabitants by tens of thousands.

In one season of the year India is the scene of the most wonderful and rapid growth of vegetation ; in another period the same tracfc becomes a dreary brown sunburnt waste, with dust-laden skies and a heated atmosphere that is almost unendurable even by the natives of the country.

The transition from the latter to the former phase over the greater part of the interior often occurs in a few days. In one year the rains may be so distributed as to cause a severe and extensive famine over several Provinces, necessitating for months afterwards the continuous relief of millions of the population ; in another the meteorological conditions may be so favourable that the crops far more than suffice for the normal food demand.

India again presents a noteworthy combination of tropical Secondly, and temperate region conditions. Tropical heat, heavy and ^| n ^" frequent rain, and fierce cyclones are prevalent at one period tropical

of the year : while moderate temperature and rain, with shallow ancl tem " . , . . , , . - L perate re-

extensive storms conditions resembling those of South-eastern gi on con- Europe obtain at another. ditions. In the third place India is par excellence the area in which Thirdly the contrast of what are termed monsoon phases or conditions a * a is exhibited most strongly over a large area. These conditions nounced

are the prevalence of dry land winds, with little cloud and monso p n conditions, ram, during one half of the year, and of winds of oceanic origin, with high humidity, much cloud and frequent rain, during the other half.

The work of meteorological observation was begun by the Initiation East India Company at the end of the eighteenth century. ai y e " e t Observatories of the first rank were established at Madras in of meteoro- 1796, at Simla in 1840, at Bombay in 1841, and on the 1( ?& ical Dodabetta Peak (Nilgiris) in 1847.

The observatories at tion and Simla and Dodabetta were closed after a few years' work, but inquiry in those of Bombay and Madras have been continued until the present time and have furnished most valuable series of observations. Observations (chiefly of temperature and rain- fall) were also taken at many hospitals, but in many cases these were carelessly recorde^l and are of little scientific value. The progress of meteorological inquiry in England, and its utilization for the purposes of storm warning after the Crimean War, suggested the commencement of systematic meteorological observation in India.

Isolated and independent Meteorological Departments were started by four of the Provincial Governments -in Bengal in 1865, in the United Provinces in

1864, in the Punjab in 1865, and in the Central Provinces in

1868. The Bengal Meteorological Department was initiated chiefly for the purpose of conveying warnings to the port of Calcutta ; the other three for supplying information to the local medical authorities, in the hope thafcit might enable them to trace out the relations between weather and disease. These local departments were of some service in collecting meteoro- logical data, but they were found to be of little use for the investigation of the larger phases and changes of weather in India. The Government of India accordingly decided to im- penalize the system, and sanctioned the necessary arrangements for the extension of the work of observation to the whole of India,for the adoption of uniform met hods of observation, and for Present the systematic discussion of the observations as a whole. The tion^oHhe I" 1 ? 6 *" Department thus formed was placed under the control

Meteoro- of a scientific officer with full powers to carry out the sanctioned logical reforms. This change was effected in 1874-5, and the present ment. system has thus been in operation for thirty years. At the end of 1902 the following observatories were in existence :

A magnetic and meteorological observatory at Bombay (of the first rank). A solar physics, magnetic, and meteorological observatory at Kodaikanal, on the Palni hills in Southern India (of the first rank).

An astronomical and meteorological observatory at Madras (of the first rank). A central meteorological observatory (of the first rank) at Calcutta (Alipore), where all instruments are tested before

issue and their corrections to the India standards deter- mined.

231 meteorological observatories (of the second and third rank) maintained by the Government of India or by Native States working in co-operation with the Govern- ment. Twenty-three of these are mountain observatories situated at elevations exceeding 5,000 feet, the highest being those at Leh in Kashmir (elevation 11,503 feet), at Kyelang (10,087), and the (Shaur Peak (11,200) in the Punjab.

2,390 rain-gauge stations, recording and reporting rainfall only.

The gazetted staff of the Department includes in addition to its head, who is Director-General of Indian observatories the Directors of the Kodaikinal and Colaha observatories and the First Assistant Meteorological Reporter to the Government of India (all of whom are whole-time officers) ; the Provincial Meteorological Reporters at Allahabad, Bombay, Calcutta, and Madras, of whom the three latter issue Provincial Daily Weather Reports ; and an Assistant Meteorological Reporter in charge of the Alipore central observatory. The five officers last mentioned are half-time officials, who hold other appoint- ments in the Educational or Telegraph departments ! .

For many years the Indian region, including India proper, India not Burma, the Arabian Sea, and the Bay of Bengal, was considered JJJ^J** as an independent meteorological area, in which the weather logical was supposed to be determined chiefly, if not solely, by the area - conditions within that area. It was assumed that India was protected on the north by the lofty barrier of the Himalayas, and on the west by the moderately high range of the Sulaimans, from the cold winds coming from northern regions, and that it was only exposed to the influence of equatorial sea currents.

The presence of this northern mountain barrier does un- doubtedly exercise a very considerable influence on the meteorology of India and more especially of the Indo-Gangetic plain ; for a comparison of the temperature data of Northern India with those of the south and centre of the United States in the same latitudes indicates that the intervention of the Hima- layas increases the temperature of the Indo-Gangetic plain from 3 to 5 above what it would have been if a low-level plain had extended northwards to the Arctic regions. Nevertheless, as will be seen later on, meteorological conditions in India generally are very largely determined by outside influences.

The physiographical and geographical features of India are Physio- of great importance, in so far as they modify more or less futures considerably the lower air movement, and hence the distribution of India, of temperature, pressure, humidity, and rainfall ; and it is necessary to bear them in mind in any scientific discussion of the meteorological conditions and actions of that country.

India is the middlemost of three great Asiatic peninsulas which project southwards into the Indian Ocean, and which are more or less dependent on that ocean for their broader meteorological features. It t consists of a peninsula proper (to the south of latitude 22 N. or the Tropic of Cancer), and of a broad low alluvial plain the axis of which runs east and west.

1 KoU by Editor. The constitution and present efficiency of the Indian Meteorological Department are due mainly to two men who have successively been at its head Mr. H. F. Blanford and Sir John Eliot, K.C.I.E., the Utter of whom has contributed the material for the present chapter.

The Peninsula is of comparatively low elevation and has a backbone of hills, near the west coast, from which the land slopes slowly eastwards 1 . To the north of the Peninsula is the low plateau of Central India, gradually falling to the ex- tensive Indo Gangetic plain, which nowhere rises, except in the immediate neighbourhood of the hills, above 800 feet.

To the north of this extensive plain is the lofty continuous barrier of the Himalayan mass, the central axial range of which averages over 20,000 feet in elevation. Farther north is the elevated Tibetan plateau, an extensive area 2,000 miles in length from east to west and 200 to 500 miles in breadth from north to south, averaging over 10,000 feet in altitude.

The continent thence falls northwards by a succession of slopes to the Arctic Ocean, to which it presents a vast low unbroken plain similar in general character to that of the corresponding plain in North America. Any general air movement on the Tibetan plateau would, if it extended into the plains of Northern India, have to rise 10,000 feet and then be preci- pitated in cascade form over the Indian hills.

There is no evidence of any such general movement in the lower atmo- spheric strata. Actual observations indicate that the air motion over the Western Himalayas is mainly an interchange between the hills and plains due to local actions and conditions, and is not the continuation of general air movements over Central Asia, or the Indian Ocean and its two arms, the Arabian Sea and the Bay of Bengal. Any such general air movement is confined to the middle and higher atmospheric strata.

Again, the north-west frontier of India proper consists of a series of hill ranges running north and south, forming the escarpment of a plateau stretching westwards for at least 1,000 miles into Western Persia. The average elevation of these ranges north of Jacobabad is at least 6,000 feet, and of the greater part of the plateau to the west 4,000 to 5,000 feet.

The lower air movements upon that plateau are not at any period of the year continued over the frontier ranges into Northern India. There is a drift down the passes, but it is insignificant Hence the important conclusion that India is, in its lower air movements, cut off towards the rjprth, north-east, and north- west.

Northern India is at one season the goal of the sea winds that blow in the lower strata, and at another season the source of the land winds that blow from it to the neighbouring seas; but the origin of these movements is not to be found

1 It is this tract which is hereafter referred to as 'the Peninsula. 1 directly in the heating and cooling of the large central area of Central Asia, as has been stated by some meteorologists.

It will, however, be shown later on that the cold weather Outside rainfall in India is largely dependent upon storms which infl f u e nce origmate in the higher atmosphere to the north-west of India, f ec t Indian and that the distribution of the monsoon rains is in part meteoro- dependent upon conditions in the distant Indian Ocean, and in conditions. part upon conditions in East Africa, the Persian area (probably), and perhaps to a slight extent Central Asia.

The primary fact in the meteorology of India is the alterna- The altertion of seasons known as the south-west and north-east natlon of seasons monsoons.

If the Indian Ocean extended northwards over the known as whole or greater part of Asia, the air movement, and meteoro- tlie mon Al _ i A o o j soons the logical conditions generally, over the area between 35 S. and primary 35 N. lat. would be similar to those obtaining in the corre- feet of spending belts of the Atlantic and Pacific Oceans.

North- ^^ east winds would blow steadily and permanently over the logy. northern, and south-east winds over the southern half. Between these two systems of winds there would be a narrow belt near the equator, which would move slightly northwards or south- wards during the year, and in which variable winds, with frequent squalls and showery weather, would prevail. These wind systems in the Pacific and Atlantic Oceans are due to the large and permanent differences of temperature between the tropical and polar regions.

Over the belt of high temperature in these oceans near the equator there is a more or less con- tinuous ascensional movement, and an outflow in the upper atmosphere northwards and southwards with an indraught from north and south in the lower strata. Pressure is lowest in the area of squally weather and variable winds near the equator, and increases northwards and southwards to about 35 or 40 N. and S. lat., near which latitudes high-pressure belts or narrow elongated areas lie over these oceans. These pressure features are as permanent as the trade winds.

The presence of the Asiatic continent, extending from the neighbourhood of the equator northwards, modifies the air movement and pressure conditions very largely in the Indian Ocean and seas, and concerts, in the northern portion, a permanent into a periodic, or monsoon, air movement. During one period of the year Central and Southern Asia is much cooler (from 15 to 25 in the winter months), and during the remainder of the year its temperature is considerably higher (probably from 10 to 15 in the summer months), than the Pacific and Atlantic Oceans in the same latitudes. These are toons.

Subdivi- ion of Cftcb rnon-

soon into two the fundamental facts of temperature that underlie the explana- tion of the monsoons in India. North-cast The north-east monsoon is a season of winds of continental

and south- origin, and thus of great dryness except where they have passed monsoons over some extent of sea, and hence in Imfca of clear or lightly- clouded skies and of light occasional rain, due either to feeble depressions or to local disturbances.

The south-west monsoon, on the other hand, is a season of winds of oceanic origin, of high humidity, and of frequent and heavy rain over nearly the whole area. The average annual rainfall of India, as determined from the returns of about 2,000 stations, is 45 inches, of which nearly 90 per cent, falls during the south-west monsoon.

From this point of view the terms dry and wet monsoon seem to be more appropriate, as suggesting the most prominent and impor- tant feature of the seasons in the land area of India.

The dry monsoon, which lasts from about the middle of December to the end of May, may be divided into two periods, one of corn- paratively low, and the other of increasing and high, tempera- ture. The former, the cold weather of Northern India, includes the months of January and February, and the latter extends from March to May. The wet monsoon may also be sub- divided into two periods. The first, from June to September, is the monsoon proper, when general and heavy rain falls occasionally or frequently in nearly all parts of India.

The second is the retreating monsoon period, when the area of rainfall contracts southwards, and the intensity and volume of the fall diminish, until the humid currents pass out of the Indian land and sea areas and withdraw to the equatorial belt : this period extends from October to December.

The north-east monsoon air movement is fully established ver ^ e I n< ^' an ' an d and sea areas by the beginning of January, when temperature is lowest in the Asiatic continent. A belt of j^gjj p ressure anc j anti-cyclonic conditions then stretches from the West Mediterranean to Central Asia and North-east China. This corresponds to the belts of high pressure in about latitude 30 N. in the Pacific and Atlantic Oceans, but lies farther north. Pressure decreases southwards to a belt a little to the south of the equator, and thence f increases again to a broad belt stretching from South-west Australia across the South Indian Ocean to the Cape.

The northern belt of high pressure appears to separate completely the region traversed by the storms of Northern Europe (most of which enter from the North Atlantic) from the areas of the local storms of the Mediterranean Sea, Persia, and India, and limits to the north Indian Ocean rhir- weather period. what may be termed the Indian monsoon region. The pressure conditions evidently accompany and determine northerly winds (deflected to north-east in consequence of the earth's rotation) in the Indian seas, and south-easterly winds in the centre of the Indian Ocean.

The winds in the land area are modified by the trend and elevation of the mountain ranges, and by the direction of the mountain valleys. Temperature begins to rise in general in the month of February over the Asiatic continent, but the changes are usually small until the end of that month. Hence the normal air movement and pressure conditions are practically the same during these two months over the whole area between lat. 40 S. and 35 N., and the air movement in that large area virtually consists of two independent circulations. The rough sketch below shows the general character of the

VERTICAL SECTION ALONG MERIDIAN OF 70 E. IN JANUARY

pressure conditions and air movements in these areas. The meteorology of India during this period is only concerned with the actions and changes in the northern circulation. The Central Asia high pressure or anticyclone, although intense, is ineffective so far as the lower air movement in India is con- cerned, as it not only lies over very elevated ground but is shut off from India by the Himalayan barrier range.

Clear skies, fine weather, low humidity, large diurnal range Sketch of of temperature, and light land winds are the characteristic ^ n ^ e " features of the weather in India during this period. These India dur- conditions are as a rule initialed in the Punjab in the beginning in K co]( * of October, extend slowly eastwards and southwards, and finally prevail over the whole Indian land and sea area to the north of lat 8 N. before the end of December. The months of November and December are, on the whole, the pleasantest of the year in Northern India.

The air movement in Northern and Central India is from the west down the Gangetic plain, recurving across Bengal through north-west and north, and in the Bay of Bengal, where it is from north-east to east This latter movement is continued across the Peninsula from the east, and passes out into the Arabian Sea. The west coast districts from Gujarat southwards and tfie adjacent sea area are, however, protected from this general movement by the Western Ghats, and hence light to moderate land and sea breezes obtain in a well-marked form in these districts. The most noteworthy feature of the air movement in India at this time is that it is from practically opposite directions in Northern India and in the Peninsula, viz. from the west in the former and from the east in the latter area.

The intermediate belt, which includes the greater part of Khandesh, BerSr, and the southern half of the Central Provinces, is, as might be expected under these conditions, a region of light unsteady variable winds. It is also the seat of the largest deviations from the normal weather conditions (more especially of temperature), to which attention will be directed later.

The most interesting feature of the northern wind system at this time is the return of the upper air current over India from the south (deflected to the south-west and west-south- west by the earth's rotation). It is the continuation of the ascensional movement of the humid currents in the equatorial belt, which have previously passed over a large extent of sea surface, but which, by the action of ascensional movement, have been deprived of a considerable portion of their aqueous vapour in that belt. This current is not fully established until the end of December, by which time the circulation of the south-west monsoon has been fully replaced by that of the north-east monsoon.

These facts explain to some extent the character of the storms and disturbances of the period in Southern Asia. A succession of shallow storms passes eastwards across the phenomena Persian area and Northern India during this period. These upper storms are not the continuation of European disturbances, current and are land-formed. The Precipitation accompanying them is small in amount over the greater part of the area they cover, but is very large on the higher elevations of the mountain ranges to the north, including the Hindu Kush and Himalayas. A unique feature of these storms is the formation of brief subsidiary or secondary depressions in the Punjab, accompanied by moderate to heavy rain in the plains, and by violent gales and heavy snowstorms in the middle and higher Western Himalayas

Storms of the cold- weather period

These storms are attended with remarkable temperature effects. Temperature usually rises very considerably in front of them, owing in part to the presence of cloud which diminishes largely the fall of temperature by night, and in part to the shift of winds to easterly and southerly directions. In the rear of the storms unusually dry clear weather obtains as a rule, with stronger and cooler westerly winds than usual.

This change is, of course, emphasized by the previous rise of temperature over the same area when it was in the advancing quadrant of the storm. Temperature occasionally falls as much as 20 or even 30 in forty-eight hours in Baluchistan and Northern India owing to the passage of these warm and cold waves. It should, however, be noted that these tempera- ture changes are more directly related to the extent of snowfall in the neighbouring mountain and plateau areas than to the intensity of the storms.

The few cloud measurements that have been taken at Allahabad and Simla during these storms establish that the cloud, and hence the field of condensation, in these storms is at an elevation of 15,000 to 20,000 feet. The distribution of the Precipitation, and the occurrence of stormy winds and weather, not in the plains, but in the mountain districts of Northern India, indicate clearly that the storms are disturb- ances in the upper humid, and not in the lower dry land, current. The great majority of them originate either over the Syrian desert area or over the Persian plateau, and usually begin to affect Baluchistan and North-western India in December.

They are, in their full development, a phenomenon of north-east monsoon or cold-weather conditions only, and are, compared with the storms which visit Northern Europe at this time, very feeble disturbances. The barometric de- pression in the primaries rarely exceeds two-tenths, and in the secondaries five-tenths, of an inch. The number and character of these storms vary very largely from year to year. The conditions determining these variations have not yet been fully investigated ; they appear to have no relation to the local conditions in the lower atmosphere over India.

In Northern and Central Jndia periods of bright sunny Weather in weather, with clear or lightly clouded skies, alternate with ? ndia (hu ' periods of cloudy showery weather accompanying the passage ^father of the shallow extensive cyclonic or cold-weather storms of period. the period ; and light to moderate showers fall in the plains of Northern India during the advance or passage of the storms. In RSjputSna, Central India, and the Central Provinces the rain is much less frequent and more local in occurrence, and usually accompanies thunderstorms.

Occasionally severe hailstorms occur during the passage of these cold-weather storms across Rajput5na and Central India, and do much damage to the poppy ad other crops. The influence of these storms rarely extends to the south of the hill ranges lying across the head of the Peninsula. On the other hand, the north-easterly to easterly winds which blow across the Coromandel coast districts are more or less largely charged with vapour, due to their passage over a considerable extent of sea area in the Bay, with the result that some cloud prevails in Southern India during this period, especially over and near the hills.

The intermediate belt, including the Konkan and the Deccan, has less cloud than any other part of India at this time. The cool north-west winds in the rear of the storms advancing from Persia and passing over Northern India occasionally extend across the north-east of the Arabian Sea, and reduce temperature considerably as far south as Goa and Karwar.

Precipitation

During this season the rainfall is very small in amount

ofthe over Lower Burma, and practically nil over the greater part

weather f l ^ e Peninsula. The rainfall in Northern India is greatest

period. in the submontane districts and decreases in amount with

increase of distance from the hills. The chief feature of

the Precipitation is the heavy snowfall in the Western Himalayas

and the higher mountain ranges of Afghanistan and Baluchistan.

It increases rapidly with elevation and probably exceeds on

the average 30 feet at 15,000 feet of altitude, and varies

between that amount and at least 60 to 100 feet over the

higher ranges.

Fine weather prevails generally in Southern India at this period ; but occasional showers, frequently accompanying thunderstorms, are received in the coast districts, and on the Palni, Nilgiri, and Anaimalai hills, usually immediately after the breaking up or disappearance of the more vigorous cyclonic storms of the period in Northern India, when strong north-east winds flow down the Bay and are continued as easterly winds across the Coropiandel coast. This is, it may be pointed out, one of the many examples of a marked contrast of conditions between northern and tropical India. Higher temperature or cloudy weather in one area often accompanies lower temperature or weather finer than usual in the other.

It has been pointed out that there is a debatable tract between the areas of westerly winds in Northern and Central India and of easterly winds in the Deccan. During the period from November to February the rainfall in this tract, which includes Khandesh, Berar, and the southern half of the Central Provinces, is usually very scanty. In normal years heavy fogs or misty occur nightly, contributing largely to the progress of the great wheat crops of these districts ; but in years when the cold-weather rains are late and scanty, temperature rises very largely above the normal in this area and fogs are then comparatively rare.

The second half of the dry season includes the months Metcoro- of March, April, and May. During this period increase of !^f y s f n< | temperature in the northern region, more especially in the half of the Indian land area, accompanies a fairly continuous decrease dr ? , or

north-cast of pressure ; and decrease of temperature in the southern m0 nsoon. region, including the Indian Ocean and adjacent land areas of Africa and Australia, causes a slight but continuous intensification of the southern anticyclone referred to on page no. These pressure changes accompany corresponding changes in the two air movements. The south-east trades movement increases in intensity and volume, and the north- east trades movement not only decreases, but is to some extent broken up, by the establishment of local circulations in India, and perhaps in Arabia and East Africa.

An important feature in connexion with these changes is a slow transfer northwards of the equatorial low-pressure belt. This north- ward advance is probably due chiefly to the strengthening of the southern, and in slight part to the decrease of the northern, circulation. It is noteworthy that during this period the area of general rainfall due to the south-east trades is transferred northwards from South-east Africa to the Equatorial Lakes region of that continent

The conditions existing during the first half of the dry monsoon season begin to be modified over the whole land and sea area of India in the latter half of March, owing to rapid increase of temperature. North-east winds continue in the Arabian Sea and Bay of Bengal, but fall off in strength, and in April and May light winds obtain in the centre and south of the two seas, more* especially in the Arabian Sea. Very large and important changes in the pressure, temperature, and humidity conditions are initiated in the interior of India during these two months. Temperature increases rapidly and steadily 1 , and pressure diminishes pari fassu, over the heated

1 It is moft satisfactory, on the whole, in discussing the meteorological conditions and changes accompanying the prevalence of increasing and

Changes of pressure conditions and air movement accom- panying local in- crease of tempera- ture in Indian land area.

land area, owing to the transfer of air to neighbouring cooler regions, more especially the sea areas to the south. India thus becomes a low-pressure area relative to the adjacent seas. The depression is very slight at first, but increases in intensity with the advance of the season. The most important local features in this general depression are the formation and development of a deepish low-pressure focus or sink in the hottest area, including Upper Sind and the adjacent districts of the Punjab and Rajputana, and the formation of a shallow depression in Chota Nagpur.

There is thus increasing local indraught from the adjacent seas across the Bengal and Bombay coasts, which is, however, only clearly marked over comparatively narrow belts round the coasts landwards and seawards. These belts slowly increase in width with the season ; but in the Arabian Sea the belt, even in May, probably does not at the utmost extend more than 300 miles seawards, thus leaving a large area of light northerly or variable winds in the centre and south of that area. In the Bay of Bengal, on the contrary, the local sea winds extend in May over the greater part or the whole of its area, in consequence chiefly of its comparatively small extent. These changes convert the air high temperature to consider the maximum day temperature as the best indication of the intensity of the thermal conditions and actions. The fol- lowing gives a brief statement lor the months from February to June of the maximum day temperature conditions in the plains of India.

In February the mean maximum temperature ranges fiom a mean of 66 3 in the Northern Punjab to one of 90 to 95 in the Deccan and interior of Southern India. It increases in March to about 100 in the latter area, and ranges between that amount and 76 in the Northern Punjab. In April it ranges between 100 and 105 over by far the greater pait of the interior of India except the north-west and submontane districts. The area of greatest day temperature is transferred in this month to the Central Pro- vinces and Western Rajputana. In May the mean maximum day temperature varies between 105 and 110 over the greater part of the interior, and the thermal focus, as the area of greatest temperature was termed by Mr. Blanford, now includes Upper Sind, the South-western Punjab, and Rajpu- tana, in parts of which it slightly exceeds 1 10. In June the highest mean maximum temperatures are, as in May, between 105 and 1 10, but occur in a comparatively small area including the Western and South-western Punjab and Upper Sind, f

The hottest area in India just before the invasion of the south-west mon- soon currents is Upper Sind and the South-western Punjab, as represented by the observatories at Jacobabad and Montgomery* The hottest period is usually the last week of May or the first week of June, just before the first advance of the Bombay monsoon current up or across the western coast. The highest maximum day temperature hitherto recorded in India it 126, at Jacobabad, on the nth of June, circulation in India and the adjacent seas from a general into a local movement, marked by strong hot winds down the river valleys in Northern India as well as by the strong sea winds above mentioned. It is no longer part of the previously established north-easfrtrades, nor is it, as some have maintained, the commencement of the south-west monsoon. The period is undoubtedly a transition from the general air movement of the north-east to that of the south-west monsoon, but the final transformation occurs under conditions, and in the manner, explained later.

In normal years, India is not disturbed during the hot Storms of weather months (March to May) by storms appearing in ^ e j^ Central or Western Asia *, for this is the only period of the period. year, it should be remembered, in which India is, to a large extent, an independent meteorological area. It is in conse- quence of this isolation that the storms of this period are local in character. They are due to various large actions and contrasts of conditions accompanying high temperature, large diurnal range of temperature, and great contrasts of humidity. These storms all belong to a class which may be termed hot- weather storms.

They include the dust-storms of the dry heated interior 1 (the Punjab, Sind, Rajputana, and the Gangetic plain), and thunderstorms in the regions in which there is more or less interaction between the damp sea winds and the dry winds from the interior, viz. Assam, Bengal, Burma, the west coast districts, and the Deccan. Hailstorms are more common in Assam than in any other part of India. They also occur frequently in Central India, and occasionally in Bombay, Rajputana, the United Provinces, Bengal, the Central Provinces, and Sind.

They are of very occasional occurrence in Upper Burma, Hyderabad (Deccan), and Kathiawar, and rarely, if ever, occur in Mysore, South Madras, Malabar, the Konkan, and Lower Burma. The interaction of the dry and damp winds is, in Bengal and Assam, supplemented by the action of the hills in giving rise to vigorous forced ascent, and these thunderstorms, or ' nor'-westers ' as they are locally called, are of frequent occurrence, and occasionally of great violence and intensity. Sometimes they develop into tornadoes, the most

1 Sometimes under very special conditions, as in the year 1903, a series of cold-weather storms continues to cross Baluchistan and Northern India in March*

1 These are sometimes, but rarely, accompanied by destructive hail. In the Moradabad hailstorm of April 30, 1888, at least 230 persons were killed. intense form of small revolving whirls. These are rarely more than a few hundred yards in diameter, but advance rapidly, overturning and destroying houses and trees, and causing great destruction of life and property. Their path of destruc- tive violence is seldom more than 5 t6 10 miles in length. They are comparable in violence and intensity with the tornadoes of certain districts of the United States. The rain- fall due to these hot-weather storms is very important in North-eastern India, especially in Assam, where it is of great service for the tea crop.

These storms, which are peculiar to the hot weather, are evidently due to the vigorous convective air movements of the period, and usually occur in the afternoon hours when these movements are strongest. Their frequency and strength increase, as might be expected, with the advance of the season, and with the increasing intensity of the thermal conditions and actions. This is very clearly shown by the following data for the rainfall of the period in Bengal and Assam :

The wet season or fouth-west monsoon. Division.

RAINFALL (inches).

March.

April.

May.

Assam Valley .... Assam Surma .... North Bengal .... East Bengal .... Deltaic Bengal ....

The amount of aqueous vapour in the air increases slowly, but fairly steadily, during this period, owing to the increasing influence and extension into the interior of the sea winds. This increase is most marked in the coast districts. In the interior the humidity percentage decreases considerably in consequence of the rapid increase of temperature, and the air is as a rule exceedingly dry in the afternoon hours during the months of April and May.

We have next to deal with the south-west monsoon. In the first or cold-weather period the weather in India is mainly determined by continental, and * in the second or hot-weather period by local, conditions. During the south-west monsoon the weather is determined chiefly by oceanic conditions, which we have now to consider.

The first portion of the wet season, from June to September, witnesses the extension of the south-west monsoon currents over the whole of India and their persistence during these months as rain-giving currents. This is the season of the south-west monsoon proper. It is the most important season of the year, as it is upon the amount and distribution of the rainfall of this period that the prosperity of at least five-sixths of India mainly depends.

Accompanying the rapid increase of temperature in the Asiatic land area in May, pressure generally decreases, so that at the end of May the Asian winter anticyclone has been replaced by a deepish low-pressure area, the axis of which stretches from the Sudan to Sind and Western Rajputana and thence to Bengal, and is hence farther south than the median line of the winter anticyclone. A graphical representation of the chief pressure conditions in India and the Indian Ocean in May is given below. As compared with that given on p. 1 1 1

VERTICAL SECTION ALONG MERIDIAN OF 70 E. IN MAY

SOUTHtMN CIRCULATION IN

FIG. a.

it indicates the replacement of a stable by an unstable system. Finally by a spasmodic and sudden burst, due chiefly to the momentum of the current and the diminution of resistance, and hence to vis a tergo and not to indraught towards the low- pressure area in India, the equatorial belt closes up, and the south-east circulation extends northwards across the equator into the two areas of the Arabian Sea and the Bay of Bengal, whence it advances more or less quickly into the neighbouring land areas of Burma, India, and Abyssinia.

This advance in many respects resembles, on a large scale, Ext ^ nsion what the ' bore ' in a river like the Hooghly is on a small south-cast one. The front of the advancing current is in each of the tra <* es sea areas invariably an area of very disturbed weather, strong equator C winds, and frequent heavy rain squalls. Often a cyclonic to the storm forms in this area and is when it occurs the most ^ii* characteristic feature of the advance. The storms in the areas.

Cyclonic Arabian Sea usually advance parallel to the coast until they the^ra*^ reac ^ ^ ie latitude of Bombay, when they generally recurve bian Sea. to the west and march to the Arabian coast, where they break up. Occasionally they form farther west and advance past Sokotra into the Gulf of Aden.

Thesfc storms are on the whole the most dangerous that are encountered by mariners in the Arabian Sea. The most disastrous in recent years was the Aden cyclone of May 30 to June 3, 1885, in which the English SS. Spckc Hall, the French man-of-war Renard, the German corvette Augusta^ and numerous native craft were lost with all hands. The storms in the Bay of Bengal, unlike those of the Arabian Sea, advance landward and usually strike the Bengal or Orissa coast and thence march in direc- tions between north and west, giving more or less general rain '. If the humid currents are strong they quickly spread over the whole of India, and the transformation from the hot weather to damp cloudy rainy weather is effected in a very short period.

During the next three or four months the horizontal air movement over the Indian Ocean and seas into the adjacent land areas is converted over India into an irregular ascensional movement accompanying rainfall, the actions connected with which not merely maintain the low pressure established by the high and increasing temperature in May, but intensify it to some extent in June and July. The figure on the opposite page represents the lower gradients and air circulation in June, July, and August over the area in question.

When this current is established over the Indian seas and adjacent land areas, a continuous air movement extends over the Indian Ocean, the Indian seas, and adjacent land areas between latitudes 30 S. and 30 N., the southern half 01

1 It may also be noted that storms occasionally form in the Bay of Bengal and the Arabian Sea before the establishment of the monsoon currents in those areas. They are initiated under different conditions, but are on the whole similar in character, to the October cyclones. These storms are usually generated in the centre of the Bay after a period of light unsteady winds and hot sultry weather, and hence probably of the accumulation ot much aqueous vapour in the air by evaporation. They at first advance very slowly, but increase considerably in velocity as they approach the coast.

They usually move westwards to the Madras coast or north -north-cast wards to the Pegu or Arakan coasts. It is very remarkable that these storms sometimes form in several years in succession about the same time and advance along the same route. For example, one such storm has formed during four out of the five years ending with 1903 in the last week of April or the first fortnight of May and has marched along the same general track to the Burma coast.

which is the south-east trades and the northern half the south-west monsoon current. The strength of this movement differs very considerably in different parts of its area of extension, owing to various causes. The most important fact is that it is a continuous horizontal air movement in the lower atmosphere that occurs mainly over an extensive tropical oceanic area, in the greater part of which it is passing from cooler to warmer areas and in which weather is generally fine with clear or lightly clouded skies. Hence when it enters the Indian seas and approaches the coasts of Southern Asia the air is highly charged with aqueous vapour.

The south-east trades, previous to their extension across Complete the equator, are largely directed to the African coast, and field of ex give more or less general rain, during the dry monsoon period t h e sou th- of India, in South and East Africa. During April and May west moa- the rainfall occurs most largely in the Equatorial Lakes area and soon cur " rents.

VERTICAL SECTION ALONG MERIDIAN OF 70 E.

adjacent districts. This rainfall produces the first seasonal rise of the Nile in May and June. The extension of the south-east trades as south-west winds over the Arabian Sea is followed by a diversion of a part of the monsoon current to Abyssinia and the Sudan. The rainfall in the mountain districts of Abyssinia is similar in period and character to that of Northern India. *Sir W. Willcocks estimates that an average of 60 to 70 inches falls over that area from June to September. This gives rise to a much higher and greater flood in the Nile than does the rainfall in Central Africa. The Nile floods hence reach their culmination in September.

Arabia, so far as is known, shares only slightly in the Arabian Sea monsoon current, which is chiefly deflected east- wards over the centre and hence parallel to the Arabian coast. Some of the higher regions in Southern Arabia, more especially Yemen, receive moderate rain during the period. Persia, Afghanistan, and Baluchistan are practically outside the sphere of the monsoon currents.

The Arabian Sea monsoon current is mainly determined to India, across the Bombay coast The Bombay current prevails exclusively over the Peninsula, Central India, Raj- putana, and Northern Bombay. Another portion of the mon- soon current passes up the Bay of Bengal, and part of it enters India proper, giving rain to Assam, Bengal, and the Gangetic plain. The volume of the Bombay current is pro- bably three times as great as that of the Bengal current. The remaining portion of the current in the Bay passes from the Andaman Sea across the Lower Burma coast and up the river valley of the Irrawaddy, giving frequent rain to the whole of Burma. Siam and South China receive most of their rainfall during the same period ; but this, it would seem, is due chiefly to an influx from the China Sea and the Gulf of Siam, and not from the Indian seas.

It will thus be seen that the south-east trades, after they have crossed the equator, pass in part northwards up the Arabian Sea and in part up the Bay of Bengal, dividing into two semi-independent movements. Each of these again gives rise to two separate movements, one to the west and the other to the east. Abyssinia and Burma are each served by one of these four subdivisions, and India proper by two. Bay of The following paragraphs describe the action of the Indian

Bengal monsoon currents in greater detail. The current in the south of the Bay of Bengal is from west-south-west to south-west, and is therefore at this stage largely directed towards Burma and Tenasserim ; and probably at least one-third of its volume proceeds to that area, and is either forced across the Tenas- serim hills or passes up the Irrawaddy valley. The remaining portion advances up the Bay and is deflected westwards by the action of the Arakan hills, witfc the result that at the head of the Bay the mean direction of the air movement is from south-east and south. The advancing current chiefly crosses the coast between Chittagong and Purl, and comes almost immediately under the influence of the Assam hills and the Eastern Himalayas. That portion of the current which passes into the cut de sac formed by the Assam and Chittagong hills is forced upwards, and gives excessive rain (the heaviest probably in the world) to the southern face of the Assam hills.

The remainder of the current is deflected westwards, owing largely to the action of the lofty barrier of the Himalayas, to the lower ranges *>f which it gives almost daily rain along its whole length from Sikkim to Kashmir. The field of this portion of the monsoon current is therefore Assam, Bengal, and the greater part of the Indo-Gangetic plain. It is more effective as a rain-distributing current than the Bombay current, as it is not directly impelled against and across a line of hills, and forced by ascent to part with the greater part of its moisture before reaching the interior, but advances up a broad river plain, the elevation of which increases very slowly with distance from the sea.

The greater portion of the Arabian Sea current on reaching Arabian the Bombay coast is directed from west-south-west to west Sea C UF- against the west coast districts. Over a large portion of that area it meets with an almost continuous hill range, from 3,000 to 6,000 or 7,000 feet in elevation and rising abruptly from the coast, and is forced across in part by the momentum of the whole current.

This forced ascent cools down the air considerably, and gives rise to frequent and very heavy rain upon these hills. Heavy rain, too, falls in the low coast districts, the total averaging roughly 100 inches, nearly the whole within about four months. The current, after depositing the greater portion of its aqueous vapour in the coast districts and the western face of the Ghats, advances across tlie Peninsula, giving strong winds but only occasional rain to the Deccan, and passes out into the Bay of Bengal, where it mixes with the Bay current, giving rise to occasional squalls.

The northern portion of the west coast current, which crosses the Kathiawar, Cutch, and Sind coasts, passes over the sandy plains of Western Rajputana, and gives little rain, except in the coast districts, until it reaches the Aravalli hill range. It thence passes on north-eastwards and northwards, being deflected from the hottest and driest area (the Sind permanent low-pressure area of the period) by the action of the earth's rotation, and reaches the Eastern Punjab, where it intermingles with the current from the Bay and is in part deflected west- wards, and in part forced upwards by the Himalayan range, where the two currents combine to give moderate to heavy rain in the Eastern Punjab, Eastern Rajputana, and the Western Himalayas.

An important feature of the period, not yet noticed, is the Monsoon trough of low pressure. Date of establish- ment of wet mon- soon in different parts of India. debatable area in the south of the Indo-Gangetic plain be- tween the independent fields of the two currents. It usually stretches from Sirsa, through Agra, Allahabad, and HazSribflgh, into OrisMi, and may be termed the monsoon trough of low pressure. It is characterized by lower pressure, and by the prevalence of much more variable and unsteady but stronger winds, than the areas immediately to the north or south. A very marked tendency exists for cyclonic storms forming in the north of the Bay during the period to advance along this trough. It is mainly due to this that the rainfall is locally heavy in the rice-growing districts of the Central Provinces.

The normal dates on which the monsoon currents advance into the larger Provinces of India are given below :

Normal I ate of the Arc

a. Commencement of the South- weft Monaco n Rainfail. 3MaliLne 4.Bum bay Coast,Deccau ,Central Provinces ,Central India Rajputana,Bengal Bihar,United Provinces, East United Provinces, West Eastern Punjab .The dates do not, as a rule, vary much in the coast districts, but range over a considerable period in North-western India. The monsoon is usually established over the whole Indian area by the end of June. July and August are the months of greatest extension of the currents to the northern limits of the Punjab, and of heaviest and most general rain.

The strength of the currents, and the accompanying rainfall, begin to diminish in the first or second week of September and decrease rapidly during the latter half of the month. Pulsatory The period of about three months during which the monsoon character usua iiy obtains its full strength is, however, not one of continuous rainfall of rain in any part of India.

More orjess general ram is received the period, during periods varying very considerably in length, with inter- vals of fine weather and passing clouds in the interior and showery weather in the coast districts. In other words, bursts of general rain alternate with breaks, partial or general as the case may be.

This is due to what may be described as the self-feeding actions accompanying rainfall. Some ascensional movement is requisite to cool the air in order to give rise to condensation and rain. The process of condensation releases energy, which strengthens the ascensional movement and hence up to a certain point increases the rainfall. It however drains and partially exhausts the atmosphere of aqueous vapour. The rainfall then diminishes in amount, and part passu the ascen- sional movement decreases, until the rainfall ceases or has become local in character and distribution l . A short period of lighter winds of indraught follows, with a slow accumulation of energy, in the humid currents over the south and centre of the Indian seas.

This is followed by a repetition on a small scale of the actions accompanying the first great advance of the monsoon. In other words, the strong humid winds in the Bay tend to press forward by their momentum, and squally rainy weather sets in over the front of the current. A cyclonic storm forms, and the storm, with the strong humid winds in its rear, advances landwards and gives another burst of heavy and more or less general rain. The pulsatory character of these actions and of the rain Precipitation is one of the important features of the monsoon period.

The majority of the storms which form in the Bay of Bengal Cyclonic during this period rarely develop so far as to have an inner s ^ orins . of calm central area, and only about one storm in four is of season in sufficient intensity to give winds of force 10 (gale) or upwards 2 . ^ e Ka y OI " They are chiefly remarkable for the rapidity with which they enga ' form and begin to move landwards, for the heavy rain they distribute to the belt they traverse, and for the general moderate rain that follows during a few days over the greater part of Northern India.

They are thus of great importance in the economy of the season, as they not only modify considerably the rainfall distribution, but are apparently an essential element in the dynamics of the current for the production of general rainfall. In some parts of the interior, rainfall during the wet monsoon occurs only in connexion with these cyclonic storms.

The great majority of these storms advance far into the interior, into RajpuUlna or the Punjab. In fact their vitality is one of their most characteristic features. Their tracks depend largely upon the distribution of pressure at the time of their advance. They almost invariably march in a west-north-westerly direction, except at the commencement and near the end of the period,

1 There is a tendency to increased rain in the Deccan and the southern districts during these periods, an example of the opposition of conditions and actions that frequently subsists between Not them and Southern India.

9 Beaufort notation.

Normal rainfall May to October, Humidity and tem- perature conditions during the south-west monsoon.

when they occasionally advance northwards into Bengal. The great majority form in the north of the Bay, and occasionally, under special conditions, over South Bengal, where their origin is associated with concentrated heavy rain.

The following table shows the general distribution of rainfall month by month from May to October. These are the averages of a large number of stations not equally distributed, values which are only approximate but sufficiently exact for the purpose intended.

Inches.

May a6o

June 7.10

July 1105

August 9-53

September ......... 6-78

October 3.15

Total 40.40

An important feature of the rains is the marked tendency to uniformity of temperature, cloud, and humidity conditions over by far the greater part of India. This will be seen from the following data for twelve representative stations in India for the month of July :

Mean (8 a.m.)

Station.

Mean

temperature.

Mean (8 a.m.) humidity.

cloud amount. (o = clear, and 10 = overcast,

Per cent.

Lahore

93-0

70-5

4-3

Karachi

85-6

8l. 5

7-3

Jaipur

85-9

75-9

68

Allahabad

85-5

83-7

8.2

Calcutta

83-3

88.5

8.4

Sibsagar

840

9 2.8

8.9

Nappur

81-5

83-4

8-4

Horn bay

81-0

86.a

9-0

Rangoon

806

93.8

9.0

Sholapur

80-6

72-4

8.1

Trichmopoly

87-7

62.3

7- 1

Madras

87.3

687

7-4

Variations of the strength of the south-west monsoon currents related to correspond* ing varia- tions in the

As the south-west monsoon currents are the direct con- tinuation of the south-east trades* it is a priori probable that they will more or less reproduce any large variation in the strength and character of the latter. The data for estimating the intensity, volume, or humidity of these two great currents are, however, too few and uncertain to be of any value for detailed comparison except as regards the accompanying rain- fall. The experience of the past ten years has shown that there is from this standpoint a close connexion. The period 1895- south-cast 1902 was one of almost continuously deficient rainfall in India, trades of and of severe droughts in 1896 and 1899.

The same period Ocean. was notoriously one of scanty rain and droughts in Australia, and of severe droughts in different parts of South and East Africa. Mauritius suffered from drought in the years 1896-97 and 1899-1900, or immediately subsequent to the scanty rainfall in India that brought on the famines of 1897 and 1900. It is not possible to make a more exact comparison at present; but the evidence is sufficiently strong to justify the conclusion that deficient rainfall in the south-east trades region in the period November to April is preceded or followed by a weaker south-west monsoon than usual, more especially in the Arabian Sea branch of the monsoon currents.

The rains are in such years late in their establishment over Western and Northern India, and the Bombay current retreats earlier than usual and gives considerably less rain than the normal over the whole area dependent on it. In such years Abyssinia Rainfall in usually suffers equally with Western India, as it is now fully ^ * stcrn India and established that years of drought in Western or North-western Abyssinia India are almost invariably years of low Nile flood.

The relation is further confirmed by the fact that years of heavier rain than usual in W'estern India are also years of high Nile rainfall in flood. It is also found, as might be anticipated, that the ^^indi^T Precipitation in these areas due to the humid south-west and Burma monsoon currents is not invariably distributed in the same varie in " if versely proportion. Local conditions in one area may be more favourable than usual and its share is hence increased. There are no data available for Abyssinia, and the variations in the height of the Nile floods are not sufficient evidence, as they are probably due in part only to the Abyssinian rainfall There is, however, marked opposition between the rainfall in Burma (and to a less extent in Bengal and Assam) and the rainfall in North-western and Western India.

The table on the next page, giving the annual variation of the rainfall in different areas from 1878 to 1902 as compared with the strength of the Nile flood, illustrates the preceding remarks.

India shows, moreover, large local variations from the normal Variations distribution of the rainfall. When such is the case, it is found m loc ? 1 that the current and rainfall are determined largely by the ab- ofmonsoon normal pressure conditions set up during the hot weather and hence antecedent to the arrival of the rains. The currents are directed more largely than usual towards any area over which anomalies.

There has been for some time previously a local deficiency, and less largely to areas in which there is from any cause a local excess, of pressure. These conditions cause a variation of rain- fall from the normal in the earlier stages of the monsoon, viz, relative excess in one area and relative dafect in another. This special rainfall distribution tends to maintain the pressure and other conditions which determined the initial set of the cur- rents and consequent rainfall, and hence the areas of increased rainfall continue to be areas also of deficient pressure. Such

RAINFALL VARIATION. Year. NILB FLOODS. Whole of India (excluding Burma). Burma. North- western India. Inches Inebes Inches 1878 Very high . + 6-34

-27.32

4-6-67

1879

Above normal .

1-69

+ ^54

-1-04

1880

Below

-1.56

+ 3-78

-3-49

1881

Normal

4 O-IO

+ 5*45

4-2-87

1882

Normal

4- 2-64

4- I I -00

4/2.5;

1883

Normal

0-12

+ M3

-3.18

1884

Below

+ I-73

4-6.01

1885

Normal

+ * -05

4-4.72

I- 22

1 886

Normal

-3.02

- 2-22

4- 0.33

1887

High. . .

-2.42

+ 5-55

4-1-48

1888

Below

1-54

4- I I SI

3.84

1889

A bove

+ 2-41

-5.84

4 0-37

1890

Above

4- 0-68

6-23

-1.24

1891

Below

~.V54

4-2-48

-2.32

1892

Above

4-5-09

-7.28

4 6-8S

1893

High. . .

49-07

+ 7-04

+ 7-53

1894

High. . .

46.47

4-11.47

4- 8.84

Normal

2*19

-u 63

-5-20

1896

Below

4-83

+ 379

-2.87

1897

1898

> Normal .

( -0-15

1 +0.43

4 0-40

-2.03

-2-S8

1899

Much below

-11.14

+ 6' 33

-15,56

1900

Above

0.57

0-91

-1. 08

1901

Above

~4' I 3

4-0-07

-8.18

1902

Below

-2-05

-7.21

-5-7 persistency of abnormal pressure conditions is one of the most striking features of the meteorotygy of India, and it is mainly for this reason that the study of pressure anomalies in India must be the basis for any successful system of long period or seasonal forecasts from meteorological data. The continuity of pressure anomalies is sometimes persistent throughout the whole monsoon period, but occasionally interfering actions occur, such as snowfall in June or July in the Western Himalayas.

One of the more potent influences in modifying the pressure Influence

conditions antecedent to the advent of the monsoon is late and ?

in ri i maprolonged snowfall in the Himalayan regions. Mr. Blanford i a y a on first fully investigated the question of the strength of the mon- ttie soon and distribution* of the monsoon rainfall so far as it was modified by Himalayan snowfall, and upon an incorrect idea of his conclusions has been based what newspapers term the snowfall theory of the variations of the monsoon rainfall.

Ac- cording to this theory, snowfall is either the predominant or the only important factor, scanty snowfall preceding a strong, and excessive and prolonged snowfall a more or less deficient, monsoon. This snowfall theory is, however, not a theory of the Meteorological Department. All that Mr. Blanford established was that, in certain cases which he investigated, heavy and late snowfall in the Western Himalayas was followed by a late and scanty monsoon, more especially on the Bombay side, and that the relation held so closely in these cases that it might be used as an empirical guide for forecasting droughts. The relation was employed successfully in the last period of excessive and prolonged snow in the Himalayas to fore- cast drought in Central Rajputana and adjacent districts in 1891.

The preceding remarks show that the distribution of the monsoon rainfall in India is not a simple but a very complex problem. It depends upon a large number of factors, every one of which must be taken into consideration if a rational explanation of the variations from year to year is to be given ; and in attempting to forecast future conditions not only must each large factor be taken into account, but proper weight or value must be given to each factor.

It should also be remem- Peculiar bered that rainfall is, as a meteorological factor or element, c ^ aract r . cr , of rainfall quite different from pressure, temperature, and air movement. as a dis- It is a discoatinuous element, the only discontinuous element continuous of importance at the present stage of meteorological discussion. It is, however, found that the relative or local distribution of any one south-west monsoon rainfall is similar in character throughout the whole period in about four years out of five. The application of statistical jnethods based on this general fact or principle (which may be termed the persistence of weather or meteorological conditions) will, it is hoped, continue to give more and more exact and numerous relations, all of which will be of use for seasonal forecasting in India.

Important The following are the most important variations of the

rariations p er j oc j anc j distribution of the monsoon rains from the normal of monsoon rains from which may occur :

coimal.

(1) The commencement of the rains, may be considerably delayed over the whole or a large part of India. This is, on the whole, most frequent in Northern Bombay and North-western India. In 1877 the commencement of the rains on the Konkan coast was a fortnight later than the normal, and in 1896 a week later. The delay was most marked in the year 1900, when it amounted to three weeks in the Konkan, Kathiawar, and the Central Pro- vinces, and to nearly a month in Central India and Rajputana.

(2) There may be a prolonged break lasting over the greater part, or the whole, of either July or August (most usually August). Thus, in 1880 there was a break from August 14 to September 3 over the whole of North-western and Western India. In 1883 there was a break lasting nearly six weeks, from July 19 to the fourth week of August, in North-western and Central India.

(3) The rains may terminate considerably earlier than usual. The economic effect of this is occasionally very serious. The absence of rain may cause the chief crops, e.g. rice, to dry up, with the result that the harvest may fail either partially or completely. In 1874 the rains ceased in Bihar early in September. In 1883 they terminated about a month before their normal date over the greater part of Bengal. Similarly in 1896 they ceased four weeks earlier than usual in North-eastern India and six weeks before their normal date in the Central Provinces. In 1899 they terminated four weeks, and in 1902 more than a fortnight, earlier than usual in the Central Provinces.

(4) The rains may, as above observed, be directed more largely than usual throughout the whole or great part of the monsoon period to a given area and less than usual to another. This is the most common variation, examples occurring every year. For example, Assam, Eastern Bengal, and Burma almost invariably have heavier rain than usual when the fall is very deficient in the east of the Punjab and the Gangetic plain.

The following chart shows clearly the normal distribution of Distribu- te rainfall of the period : tic ? rainfalL

CHART SHOWING THE NORMAL RAINFALL IN INCHES IN THE PLAINS OF INDIA,' i.e. AT STATIONS BELOW 3,000 FEET IN

ELEVATION, DURING THE PERIOD JUNE TO SEPTEMBER FIG. 4. The second half of the wet season forms a transition period The re- leading up to the establishment of the conditions of the dry trta t l m y t season. During this period the area of rainfall contracts and monsoon extends southwards and diminishes in intensity, but is deter- period, mined by the same winds or air currents, and by the same general conditions, as in the preceding period, June to September. The transition from the wet to the dry season commences in the latter part of September and is usually not completed until the third week of December. It is first

Gradual withdrawal of the monsoon mittent actions. currents from the Indian area. exhibited in Northern India, and frequently follows upon a final burst of rain and thunderstorms accompanying the march of a cyclonic storm from the head of the Bay. The skies clear with the dispersion of the storm, and light and dry westerly airs replace the damp winds which previously prevailed. The night temperature decreases considerably with the increased dryness of the air, and the day temperature increases slightly on account of the absence of cloud.

Hence the mean daily temperature is at first almost unchanged, but the diurnal range is largely increased.The Northern India humid current retreats down the Gangetic plain and across Central India by a series of inter- As a rule the lower air current of the wet monsoon withdraws from the Punjab in the third week of September, from the western half of the United Provinces in the last week of September, from the eastern half of the United Provinces and Bihar in the first or second week of October, and from Bengal and Upper and Central Burma in the third or fourth week of that month.

The dry land westerly winds during this period extend eastwards down the Gangetic plain, and at the same time increase slightly in intensity, and are usually established over the north of the Bay, and the whole of Northern and Central India, before the end of October. While these changes are in progress in Northern India and the Bay area, similar alterations occur in Western India and the Arabian Sea.

The south-west humid current usually obtains at the beginning of September over the whole of that area, but decreases in strength and probably also in elevation and volume. It begins to retreat from the head of the Arabian Sea, Northern Bombay, and Rajputana in the second or third week of September. This process, as in the corresponding retreat of the Northern India or Bay current, occurs by a series of intermittent actions. The rains usually cease in Central India at the end of September, and in Berar and the Central Pro- vinces in the second or third week of October. The further retreat of the currents in both sea areas is marked by novel features, more especially in the case of the Bay of Bengal.

These features are most easily explained by reference to the pressure changes. During the south-west monsoon proper pressure is lowest changes in in a belt across the Persian Gulf, the north of the Arabian Sea, Baluchistan, Sind, Eastern Rajputana, and the southern districts of the United Provinces. As the air temperature diminishes, at first slowly and then rapidly, until the beginning or middle Pressure area. of December, the air contracts over the cooling area and there is also a flux in the higher regions from the areas to the south, i.e. the Indian Ocean.

This double action causes a continuous increase of pressure over the Indian land area, largest in amount where temperature flecreases most rapidly and pressure was previously lowest, i.e. in North-western India. The low-pres- sure conditions previously prevailing in India are thus almost obliterated by October, when pressure is remarkably uniform.

The only important feature during the remainder of the period is a residual shallow depression stretching eastwards and west- wards across the Bay and Arabian Sea, with slightly higher pressure in the Deccan and South Indian areas. This depres- sion is slowly transferred southwards during the period, and lies over the centre of the Bay at the beginning of November and over the south of the Bay at the beginning of December.

The humid current of the retreating south-west monsoon continues over the area to the south of the belt and varies very largely in strength. When it is vigorous, it recurves round the eastern portion of the low-pressure belt and extends west- wards in the area immediately to the north of it.

At the same time, and as part of the general actions, a cyclonic storm forms and passes westwards, carrying the humid currents of the retreating monsoon with it and giving heavy rain. In the intervals between these actions the westerly winds in the south of the Bay are light in force. Light variable winds with fine weather prevail in the centre of the Bay, and light to moderate north-easterly winds in the north and north-west of the Bay. Periods of fine dry weather thus alternate with periods of showery and disturbed weather in the east coast districts of the Peninsula.

The monsoon current recurves at the head of the Bay and Recnrva- in Bengal, in the same general manner (i.e. through north and t? re f the north-west to west) as during the south-west monsoon proper, rent in The recurvature is now, however, due not to the obstructive O c *b er and Noaction of mountain ranges, but to special pressure conditions vember in the Bay.

The retreating south-west monsoon gives occasional towards or frequent general rain during this period to the Madras coast. RairuTin In October and the first half of November the Precipitation Madras, occurs chiefly in the North Coromandel and Circars districts, and in the second half of November and December chiefly or solely in the South Coromandel districts. In the second or third week of December the belt of low pressure usually passes out of the Bay limits into the equatorial belt, where it forms a permanent feature of the meteorology of the Indian Ocean during the next five months.

The heaviest and most general rainfall during this period accompanies the passage of the cyclonic storms of the period, which almost invariably advance westwards or north-westwards to the Madras coast. The Bay islands and the Malayan peninsula receive occasional rain during this period, but to a less extent than the Coromandel coast.

Probable Similar conditions probably obtain in the Arabian Sea.

of retreat 5 The humid current retreats slowly over the southern half ofmonsoon of that area in October and November, and is in slight part current in directed to the Malabar coast, to which it gives occasional j^^ rain. It is probable that a belt of slightly lower pressure than elsewhere demarcates the northern extension of the current, which recurves to west and determines more or lessgeneral rain from lat. 16 N. southwards to the eastern coast of Africa, and also in the Equatorial I^akes region, the area of rainfall passing slowly southwards with the advance of the season. Cyclonic storms occur much less frequently in the Arabian Sea than in the Bay of Bengal during this period.They rarely form in that sea, but advance into it from the Peninsula and Bay area.

Storms of The humid current in the Bay continues to exhibit the the period. same intermittent or pulsatory character as in the wet season. Periods of rain and strong winds alternate with periods of dry weather and light winds. Each period of general and heavy rain is as a rule initiated by the advance from the Bay of a cyclonic storm, which gives concentrated rainfall over long narrowish belts of country. These bursts of rain occur at longer intervals than in July and August, owing to the decreasing intensity of the humid monsoon current.

It is, however, interesting, as an example of the opposition which frequently characterizes meteorological actions, that the ante- cedent conditions of uniform pressure and temperature and light variable winds during the intervals of fine weather between the rain periods favour the slow incubation of storms in the centre of the Bay, with the result that they are occa- sionally very intense and violent in character. The conditions are, on the whole, favourably for the formation of the most severe cyclones in October, when the humid currents that provide the motive power are still of moderate strength. These October cyclones are examples of the most intense tropical storms. They differ in several respects from the cyclonic storms of temperate regions. They are usually of small extent, occasionally not exceeding 200 miles in diameter.

Hence the shifts of wind accompanying them are very rapid and dangerous to vessels, and October is the most critical period of navigation in the Bay of Bengal. The pressure gradients are very steep and the winds of hurricane intensity in the inner |torm area. The Precipitation is excessive, the rain being commonly described as 'falling in torrents/ The most characteristic feature in the worst storms is an inner central area of calms or light variable winds, occasionally 10 to 20 or even 30 miles in diameter, which is termed by sailors 'the eye of the storm.' The transition from the calm area to the belt of hurricane winds is usually exceedingly rapid.

Another characteristic feature of these cyclones is the piling up of a mass of water in the inner storm area and area of lowest pressure. This advances with the storm and strikes the coast as a 'storm-wave/ The effect of this in flooding the coast districts depends largely upon the phase of the ordinary tidal wave at the time when the storm-wave strikes the coast.

If the storm-wave strikes the coast about high water or shortly after it may produce the most disastrous results, flooding low coast districts in a few minutes to the depth of ten, twenty, or even thirty feet above tidal high-water level. In such an inundation, caused by a storm-wave which spread up the Hooghly in 1737, 300,000 people are said to have perished, but the number is probably exaggerated. The storm-wave accompanying the Calcutta cyclone of October, 1864, drowned 50,000 people and caused immense destruction of shipping.

That accompanying the Backergunge cyclone of 1876 was one of the most disastrous on record: probably over 100,000 people were drowned in less than half an hour in the islands and low districts at the mouth of the river Meghna, while an equal number died from epidemics of fever, cholera, and other diseases which almost invariably follow a storm-wave.

The total height of the wave was in some districts as much as 30 to 40 feet. Cyclones of the most dangerous type are fortunately rare, not more than one, on the average, occurring in five years.

A moderate to rapid increase of pressure in October follow- Further ing the last burst of rain initiates cold-weather conditions, j^tem with fine dry weather, almost continuous cloudless skies, and perature light variable winds in Upper India, the monsoon current ? h *}" g *? m being, during the first fortnight or three weeks of that month, ig period. determined chiefly to North-eastern India and Burma, which areas usually receive moderate to heavy rain. The cessation of the rainfall in that area is followed by a rapid local rise of pressure in Upper and Central Burma, Assam, and Bengal, and a general decrease of temperature and increase of pressure continue during the next two months.

These changes are greatest in Upper India, and are small in amount in the extreme south of the Bay and in Southern Ipdia. Gradients for northerly winds increase more or less steadily throughout the period in the Indian area and the air movement increases in intensity. North-westerly and westerly winds obtain in the Gangetic plain, are continued as north-easterly winds in the north of the Bay, and extend southwards over the centre and south of the Bay as the season advances.

A narrow belt intervenes in which winds are irregular and unsteady, ranging between south and east, while further south-south-west humid winds continue but decrease in general strength and volume. Thi$ sea- The most important feature of the air movement of the atransi ly w hole P er ^d is thus the slow and gradual withdrawal of the tional south-west humid winds from the Indian area, and the gradual period of extension of the winds of the dry season from Upper India able dura- eastwards and southwards over the whole land and sea area. This change is usually completed before the end of December, when true north-east monsoon winds are established over the whole Indian region. The season above discussed is thus a transitional period of considerable length, and is in this respect unlike the transitional period at the commencement of the south-west monsoon. That monsoon is usually estab- lished rapidly over the Indian area, and extends over the greater part of the whole of the interior in the course of a week or a fortnight. The advancing current is a vigorous move- ment, while the retreating current, and the current which gradually displaces it, are both feeble.

Precipitation

The rainfall due to the decaying and retreating current is i>eriod not on ty muc ^ smaller in amount than during the previous three months, but also much more irregular. The whole area of North-western and Central India and Northern Bombay receives practically no rain during this period. The Punjab and the adjacent districts receive a small amount, chiefly due to one or more cold-weather storms in the last fortnight of December. North-eastern India and Upper Burma obtain light to moderate rain, chiefly iri October, and Lower Burma moderate to heavy rain in October and the first half of November. On the other hand the Madras coast districts receive their chief rain supply during this period. The amount increases southwards from Ganjam to Negapatam, but decreases very rapidly in amount from the coast to the interior.

The important variations to which the weather is subject in this period are as follows :

A. The number and character of the storms which form More im- in the Bay, and hence also the distribution of the rainfall ^^ifea accompanying the rcireat of the south-west monsoon humid turcs of current, vary very largely from year to year. In some years, we * thc . r as for example in 1896, no storm of importance occurs, while f a n d ls .

in others two to four severe storms or cyclones may be ex- tribntion of perienced. The years 1876, 1886, and 1891 were remarkable for the number of fierce and extensive cyclones which occurred during this period.

B. The character of the large general and local pressure changes which accompany the gradual withdrawal of the monsoon current in India, and which appear in part to deter- mine and in part to be determined by rainfall, varies largely from year to year. Thus an abnormally large increase of pressure in October over North-western and Central India is almost invariably accompanied by absence of rain in the Deccan. Similarly, a delay in the establishment of the high- pressure conditions in North-eastern India and Burma accom- panies, and in part at least determines, a prolongation of rainfall in that area to the exclusion of the Peninsula. The late rains are then not only delayed in that area, but are scanty and more irregular than usual.

The most important variations from the normal of the rainfall distribution of this period may be summarized as follows :

(1) Prolonged rainfall in Burma, Tenasserim, and the Bay islands, and consequent deficient rainfall in Madras and the Deccan.

(2) Early termination of the rains in Burma, accompanying increased rainfall in the Peninsula.

(3) Deficient and scanty rainfall in the Deccan and Northern Madras, accompanying normal or increased rain in Southern India.

(4) Early termination of the rains in both the Burma and Madras areas, due to more general actions and conditions than the local conditions in the Indian area ; these have not yet been fully investigated.

(5) Early commencement of the cold-weather rains in Northern India, almost invariably preceding prolonged stormy weather with much snow in the northern mountain ranges.

77. Special Remarks on Rainfall in India, and Droughts

Aqueous The air is always more or less charged with aqueous vapour.

rapour. rp^ e arnount j s measured (in English Treasures) absolutely by the weight in grains per cubic foot, and varies greatly with the season and with the elevation and position. In the Bombay and Bengal coast districts, during the height of the wet mon- soon, it is about 10 to 12 grains per cubic foot ; whereas in the interior of India during the driest periods it is not more than four grains per cubic foot, and occasionally even less. ' Rela- tive humidity' is humidity in relation to temperature, and indicates how far the air departs from saturation with respect to aqueous vapour.

Unsaturated air can always be cooled down until the space is saturated and further cooling gives rise to condensation. There are many complex problems connected with the pro- cesses of condensation and rainfall, but the main actions are Rate of simple. Ascending air always cools down at a rate depending cooling of U p On the amount of aqueous vapour it contains and whether ascendir condensation is m progress.

If the air be not saturated, it cools down at a rate of about i F. per 200 feet ; but as soon as it reaches the saturation stage, followed by rain, condensation commences and continues so long as rapid ascensional move- ment continues. The heat given out in this process acts me- chanically on the atmosphere and tends to increase the ascen- sional movement. The rising air cools down, but at a much slower rate than previous to condensation. In this stage the rate of cooling may be as low as i F. per 500 feet. Uptake or ascensional movement is thus a very powerful means of pro- ducing the cooling necessary to condensation and rainfall.

Ascensional There are other processes, as for example the mixture of cool and sional or warm masses of air, but they are probably of comparatively rare corrective . air move- occurrence and seldom give rise to heavy general rain. Hence ment the j t j^y fo e assumed that rainfall in a humid current is chiefly, if chief cause of rainfall. not almost entirely, due to upward movement, which may be initiated by various actions. Whatever the initial cause, the continuance of the movement is in large part strengthened and maintained by the energy set free by the condensation of the aqueous vapour, and converted by physical processes into the energy of motion of the atmosphere affected.

Ascensional

The initial ascensional movement may, in part at least, be or( ]j nar y diurnal changes in progress caused by largely the heating of the land surface during the day. When the ascensional movement is very large, as in the hot weather, due to it is possible to get this action even although the column of ^** b ascending air may be very dry near the earth's surface. This solar is shown by the occasional occurrence of showers of rain or action, hail in the hottest ind driest districts. There is hence a marked tendency, especially in the hot-weather period, to the occurrence of rain at the same time of the day.

The uptake or ascensional movement may also be due to Ascen- the forced ascent of a humid current against and across a range ^vement of hills, initiated by the momentum of the moving mass in the O f humid rear and further accentuated by the rainfall to which it gives current forced up rise. Rainfall is therefore usually heavy on and near any range an a across of hills facing a humid current, but diminishes very rapidly on a line of passing beyond the crest of the hill range, when the current not only ceases to rise but may descend slightly, and has, during the process of rising over the mountain ridge, been deprived of a considerable portion of its moisture. There is in such cases a very marked contrast between the rainfall on the two sides of hill ranges across which humid currents pass and the adjacent plain districts.

Ascensional motion also occurs on a large scale in cyclonic Ascen-

storms, and hence heavy rainfall is the most prominent feature Slonal

morement of these storms in India, i he rainfall is usually greatest in the i n cyclonic advancing quadrant of these storms. The heaviest downpours storms, frequently occur in the interior of India and not in the coast districts, and more especially in the neighbourhood of the hills athwart the track of a storm. Falls ranging between 20 and 35 inches in twenty-four hours occasionally occur under these con- ditions. Such was the heaviest fall on record in the plains of India : namely, 35-38 inches at Purnea, in Bihar, on Septem- ber 13, 1879.

The rainfall of the ascensional motion may, lastly, be due to Aaccn- the increased resistance to the horizontal movement experienced Slonal movement by a humid current crossing a coast and advancing landwards, due to re- as, for example, across Bengal and up the Gangetic plain, "stance tohorizontal

1 his is undoubtedly one action determining a considerable movement portion of the monsoon rainfall. at the

The above are the chief actions producing rainfall in the humid south-west current Other minor actions might be enumerated ; but it is sufficient to refer to Mr. Blanford's mono- graph on The Rainfall of India for a full statement and a fairly complete explanation of the causes co-operating to deter- mine the rainfall in each district of India, and also to some extent the variations from year to year.

Distribution

A brief account of the character and distribution of the rain* ie seasons has been given under the meteoro- fall by sea- son in logy of that season. The following gives a statement of the India, mean or normal rainfall in India, derived from the average data of about 2,000 stations distributed somewhat unequally over the whole of the country : Cold weather (January-February) ..... 0.99 inches. Hot weather (March-May's 4.58 South-west monsoon (June-September) .... 34-65 Retreating south-west monsoon (October-December) . . 4.95 Of the mean annual rainfall 12 per cent, occurs during the dry, and as much as 88 per cent, during the wet, season. The ratios differ very considerably in different parts of India, vary- ing for the dry season from 3 per cent, in Bombay to 19 per cent, in Bengal, 21 per cent, in the Punjab, and 30 per cent, in Assam. These figures show the importance of the hot-weather or spring rainfall in Assam, and of the cold-weather rainfall in the Punjab.

Appendix A (p. 153) gives data for the meteoro- logical divisions lately adopted in the India Daily Weather Report. Cold- The cold-weather rainfall is due almost entirely to disturb-

weather ance and condensation in an upper current, probably over rainfall. r . , . .. s . % , ,

15,000 feet in elevation. Hence its distribution is very slightly

modified by the configuration of the country over which the current passes, except over and near the only mountain range sufficiently elevated to give rise to forced ascent on the large scale, viz. the Himalayas. The Precipitation is large over the interior elevated ranges, and moderate over the outer ranges, of the Western Himalayas. It diminishes slightly from the sub- montane districts southwards, and from the Punjab eastwards to Bihar, but thence increases again to Assam. It is very irregular in its occurrence, but the normal amount differs very slightly over the plains of Northern and Central India, varying from half an inch to one inch, except in the Punjab and Assam, where it averages two inches.

Bnc to As already pointed out, the Precipitation of the cold-weather storms. period occurs during the passage of shallow extensive depres- sions across Northern India from west to east. These storms are almost the only source of rain in Persia, Afghanistan, and Baluchistan, and to a less extent on the higher and interior ranges of the Western Himalayas. The cold-weather rainfall is small in absolute amount in Northern and Central India, but is nevertheless of great economic importance over the larger part of that area, as it is upon this rainfall that the wheat and other cold-weather crops of the non-irrigated districts in Northern India depend. In the districts to which irrigation extends the crops are practically independent of the rainfall ; but over the unirrigated districts, including the greater part of Rajputilna, Sind, Central India, and parts of the Punjab and the United Provinces, such cultivation as there is largely depends upon the amount and time distribution of this limited rainfall.

The distribution of the hot-weather rainfall presents much Hot- greater contrasts than that of the cold weather. It is due, as W( r a * h 5i r previously stated, to large convective movements over the in- terior plains raising air masses to a sufficient height to produce condensation, and to forced ascent of local sea winds across hill ranges in Assam, Bengal, Orissa, and Malabar, supplemented more or less largely by convective air movements due to the intense thermal actions of the period. The rainfall often accompanies dust-storms in the interior, and is small in amount, averaging only about an inch, over Bombay, Berar, the Central Provinces, Central India, Rajputana, and the Gangetic plain. It accompanies thunderstorms (occasionally of extra- ordinary violence and intensity) in Assam, the greater part of Bengal, Arakan, and Lower and Upper Burma, and is largest in amount in Assam, where it averages 31 inches. Economically it is of great importance for the tea crop of Assam, and in Bengal, where it favours an early spring crop of rice. Over the greater part of the interior it is of little agricultural value, and the violent winds and heavy rain accompanying the storms occasionally do great damage.

It is upon the south-west monsoon rainfall that the prosperity Rainfall of of India chiefly depends. As already stated, nearly 90 per the et or cent, of the whole rainfall of the year falls on the average m0 nsoon. during this part of the year, being due to the invasion of India by oceanic currents of great volume and elevation from the Arabian Sea and the Bay of Bengal. The goal of these currents from June to September in India is a persistent low-pressure area in Upper Sind and the neighbouring dis- tricts of Rajputana and the Punjab. The transformation effected by the invasion of these currents is comparatively slight in the regions of frequent thunderstorms in April and May : that is, in Lower Burma, Bengal, Assam, Orissa, and the Madras coast. Over the remainder of India, and more especially in the interior of the Peninsula and in Northern and Central India, it is very pronounced. Previous to its advance

Action of forced ascent of aqueous vapour. strong land winds, almost unbearably high temperature, exces- sive dryness of the air, and a dust-laden atmosphere are the chief features. Afterwards strong to moderate sea winds, moderate temperature, great humidity, much cloud, frequent rain, and a comparatively clear atmosphere are prevalent. From June to September the two currents give rain chiefly to Burma, Northern and Central India, and the northern half of the Peninsula, while over the southern half of the Peninsula rain occurs very irregularly (chiefly during breaks of the rains in Northern India) and is scanty except on the Malabar coast. The distribution of the rainfall during this period (June to September) is determined chiefly by ascensional movement due to (i) forced ascent across hills, (2) convective ascent in cyclonic storms, and (3) irregular ascensional movement caused by resistance to horizontal air movement, or due to actions accompanying the heating of the atmosphere during the day hours over the drier parts of the interior.

The action of forced ascent across or against hill ranges gives heavy rain in (a) the west coast districts of Malabar, the Konkan, and Gujarat, (b) Tenasserim and Arakan, (c) the Assam hills and Sylhet, and (d) the Lower Himalayas and submontane districts, more especially the eastern half. Actual data show that in the Western Ghats the rainfall is on the average at least three times as great on the crest of the hills as at sea-level. The average rainfall of the period is roughly 100 inches in the coast districts of the Konkan, and thence increases up to an average of 300 inches at stations situated on the crest of the hills facing the sea at an elevation of 3,000 to 4,000 feet.

A similar law of distribution almost certainly holds in the case of the Tenasserim, Arakan, and Assam hills. Sylhet at the foot of the Assam hills has an average total of 157 inches, while Cherrapunji in the Assam hills, at an elevation of about 4,000 feet, has an average of 458 inches. The inverse action to that of forced ascent is found in the districts on the landward side of ranges of hills which face the monsoon currents.

The current, which has been deprived during ascent of a large portion of its aqueous vapour, after it passes over such a range, either moves horizontally or descends slightly, and is then, unless some other special actions come in, virtually a non-rain- giving current. Hence the Bombay and Madras Deccan Districts of Sholapur, BijSpur, Ahmadnagar, Poona, and Bellary are very dry areas and specially liable to drought. Similarly, a portion of Central and Upper Burma, including the Districts of Ma^we, Minbu, Myingyan, Kyaukse, and Sagaing, is a very dry and precarious area, with a scanty rainfall averaging not more than 30 inches for the year. The Himalayas are not exposed to the full burst of the annual monsoon currents. The Bay current advances towards them, but is deflected, and though there is much ascensional movement, it is not forced ascent accompanying movement across the line of hills.

Hence the distribution of the rainfall does not follow exactly the same law as in the Western Ghats or the Assam hills. The rainfall is, however, considerable over the Lower Himalayan ranges up to levels of 8,000 to 10,000 feet, but is slight on the higher elevations or in the interior valleys. The rainfall again decreases considerably southwards in the submontane districts, and over the Gangetic plain it is determined partly by ordinary ascensional movement and partly by cyclonic storms.

A considerable proportion of the wet monsoon rainfall over Of cyclonic the greater part of India is due to the ascensional movement storms - accompanying cyclonic storms. On the average eight storms of moderate to considerable intensity pass from the Bay of Bengal into India between June and September. They travel comparatively slowly and frequently traverse the whole breadth of Northern India. Their characteristics, and the accompany- ing rainfall, have already been noticed on page 125.

After the Bay current has withdrawn from Northern India and Upper Burma, and is recurving over the Bay, it is, as already stated, directed to the Peninsula. It is now, however, comparatively feeble, and probably its elevation as well as its volume diminishes and it becomes less effective as a rain-giving current. Cyclonic storms occur at longer intervals, but con- tinue to give large amounts of rain to the areas they pass over. Their path is, usually in the latter part of October and almost invariably in November and December, westward to the Circars and Coromandel coasts. The rainfall in the Deccan and Southern Madras coast districts during this period occurs almost entirely in connexion with these storms, and is hence extremely irregular in its distribution. The rainfall is heaviest in a narrow belt of the Coromandel coast, where it ranges between 20 and 30 inches, and decreases rapidly on proceeding into the interior. It is occasionally very heavy in the Nllgiris, when the storms advance into the interior of Southern India and break up against the high hill ranges.

A characteristic feature of Indian meteorology is the excessive Examples downpours which occasionally occur, chiefly during the passage < ^ excessiv of the cyclonic storms of the south-west monsoon, and also in twenty-

four hours in the Western Ghats and Lower Himalayas during a strong influx of monsoon winds. The fail at Purnea in Bihar of 35-38 inches in one day, in September, 1879, has already been mentioned (p. 139). Other phenomenal downpours, exceed- ing in each case 24 inches within the twenty-four hours, have been:

wet mon soon. AmountYear.

Date and month.

District Station.

of rainfall 24 hours.

1876

June 14

Khasi and Jaintia Hills

Cherrapunji

40.80

I88o

Sept. 1 8

Bijnor .

Nagina .

33-40

1 880

Sept. 1 8

Hijnor

Dhampur

30-40

1888

May 27

Khasi and Jaintia Hills

Jowai

30-20

1890

June 13

Khasi and Jaintia Hills

Cherrapunji

30-04

1885

June 15

KhfUi and Jaintia Hills

Jowai

29-20

1880

Sept. 18

liijnor .

Najibabad

38.50

'893

May 30

Cachar .

Nemotha

7-.V>

1890

June 29

Kangpur

Bhawaniganj

27-00

1891

July 28

Surat

Jalalpur .

25-85

1888

Tune 27

Kha^i and Jaintia Hills

Mahadeo

25 So

1886

June 1 8

Colaba .

Roha .

24.80

Another noticeable feature is the very heavy downpours which occur in very short periods, chiefly accompanying thunderstorms. The most striking examples have occurred at Calcutta and Nagpur, at each of which places slightly over an inch has fallen within ten minutes, equivalent to a rate of nearly seven inches per hour.

Variability A most important feature in determining the economical of rainfall. va [ ue o f ra infall in any given district is its variability. This can be estimated in various ways. Mr. Blanford, in his mono- graph on The Rainfall of India, has investigated the variability in the twenty-two rainfall divisions which he adopted for the seasonal returns submitted regularly to the Secretary of State for India by the Meteorological Department. According to this method (based on the law of errors and method of least squares) the variation from the normal rainfall for which the probability is \ (i.e. the chances are i to i) is determined, and then expressed as a percentage of the normal rainfall. Some of the results of Mr. Blanford's calculations will be given on the opposite page. This is undoubtedly the most scientific method of determining estimates showing the probable range of variation of rainfall. It is, however, too elaborate for general use over a very large area like India. The simplest method is to adopt as the measure of variability the

IHJ

percentage ratio of the difference between the highest and lowest actuals for the period of observation to the normal or average rainfall of the period. The largest percentages of variability as thus calculated are those of the areas named below :

Percentage

vai lability

Annual

Meteorological Division.

of

annual

rainfall in inches.

rainfall

Gujarat ....

161

27.64

North-western Dry Area .

150

10-24

Baluchistan

133

8-66

Indo-Gangetic Pin in, West

ICO

30-89

l)cccan ....

<J3

29.68

Eabt Coast, south

90

51-59

The most noteworthy feature is the very great variability of the rainfall in Gujarat. This is due to the fact that it is liable not only to very scanty rain in dry years, but to excessive downpours from cyclonic storms in years of ordinary or strong monsoon conditions.

The following table gives Blanford's estimates of variability for thirteen areas :

Area.

Mean annual percentage deviation from the aver- age rainfall. (Blautord's method.)

Sincl and Cutch

37

United Provinces

^

Khandesh and Ucrlr

20

Bihar

is

Hyderabad .

17

Central India .

15

Gujarat . . .

15

North Dcccan

'5

Myx>re .

• 5

Carnatio .

• 3

Rajputana

13

Punjab

Orissa and Northern rin.ars

I*

When the rainfall in any area is too scanty for the staple Deficient crops of that area, and a partial or complete failure of the rainta ll harvest ensues, the season is one of drought which may, if droughts. severe and long continued, produce famine 1 . A large deficiency of rain in an area of heavy rainfall, as for example Arakan,

1 Famine is the subject of a separate chapter (Vol. Ill, chap. x). VOL. I. L

East Bengal, &c., rarely, if ever, affects the staple crops to an extent sufficient to justify the use of the term 'drought.' Droughts chiefly occur in the interior districts where the normal rainfall, when properly distributed, is sufficient for the crops, though not ample or abundant. *The table on the pre- ceding page gives a list of the larger areas liable to drought in the order of the variability of the rainfall as determined by Mr. Blanford (vide The Rainfall of India, p. 129).

Mr, Blanford, from examination of the data, inferred that droughts leading to famine were especially likely to occur in all districts in which the variability, as measured by the process described on p. 125 of his memoir, is twelve or upwards.

Scanty and ineffective rain in the wet monsoon in any area is due to one or more of the causes referred to on p. 130. The rainfall of the first half of that monsoon is much less valuable and effective, on the whole, for agricultural purposes than the rainfall of the second half. It is essential that there should be occasional to frequent rain in August and September, and occasional rain in October, in order to bring the chief food crops to perfection. An abrupt termination of the rains in August may cause the whole of the rice crop in a Province to fail to come to maturity.

In North-western and Central India not only may the rains of the south-west monsoon fail more or less completely, but also the preceding or following cold-weather rains, and failure of the first is, in about two cases out of three, accompanied by failure of the second. This double drought entails loss of the harvest of both seasons and is most disastrous. The most severe droughts in the Punjab and the Gangetic plain have been of this kind. Finally, a severe drought leading to famine may occur after a series of bad years, owing to excessive or deficient rain, either of which may diminish the crop returns for several years by large percentage amounts. This was the case in the Central Provinces in 1896.

///. Special Remarks on Indian Temperatures

Measure- Temperature is perhaps next to rainfall the most important meiit of air feature of meteorological observation in India from the luxe!* 1 ** economic standpoint. Temperature in relation to the amount of aqueous vapour present in the air and the rainfall deter- mines the character and abundance of the staple crops. The temperature of the air in India is carefully measured under, so far as is possible, approximately similar con- ditions. The thermometers are exposed in open sheds, through which the air freely circulates, and are protected from the direct influence of the sun by a thick roof or thatch of straw. Tfoe bulbs of the instruments are placed as nearly as possible at the same standard height above the ground, viz. four feet.

The sun is practically the only hot body which materially modifies and determines the temperature of the earth's atmosphere. The solar radiation is absorbed in part during its passage through the earth's atmosphere and in part by the earth's surface, which again gives up a portion of its heat to the adjacent air.

The average temperature of the ground surface in India is, at Variation the hottest time of the day in the cold weather, from io c to 20 ^ ro ) nd above the temperature of the air four feet high. The difference tempera- increases until the months of April and May, when the excess tl L re and

11 i o j c or effect on

is usually as much as 40 and sometimes 45 or 50. ihe a j rmovc . difference falls off rapidly during the rains, and in August is as small as in the height of the cold weather. At Jaipur the ground has been found to be cooler than the adjacent air four feet above for sixteen hours out of the twenty-four in December and for twelve hours in May.

The air in contact with the earth's surface is largely and rapidly heated during the day time, and is then carried upwards by virtue of the convective movements thus initiated. These movements tend to distribute the heat taken in by the air at the earth's surface throughout the superjacent mass, and give rise to changes of temperature of great importance.

The temperature of the air at any fixed position or place Variation

is in a constant state of flux or change due to a variety of ofairtem perature.

actions, more especially (a) the absorption of heat or solar energy either directly or indirectly, () the radiation of heat to the earth, clouds, or space, (c) the vertical or horizontal movement of the air in the lower strata. The most effective cause of rapid temperature changes is the displacement of air of one character by that of another, such as almost invariably occurs in the alternation of land and sea breezes, or accom- panies hailstorms, thunderstorms, cold-weather cyclonic storms, &c. These changes are usually very irregular in character. The temperature of the air varies (i) regularly in consequence of the regular variations in the causes or actions modifying it, the most important of which is that of the sun's thermal action

Diurnal variation of the air temperature.

Dinrnal rorge.

Annual

tempera- ture.

as modified by elevation, and (2) irregularly owing to the actions enumerated above.

The temperature of the air is increased by the absorption of solar radiation or radiant energy directly or indirectly during the day hours, while it is affected during the whole diurnal period by radiation to clouds, space, &c. The result of these two actions is that in the lower atmosphere the temperature of the air is lowest very shortly before sunrise, and increases until two or three hours after noon, after which it decreases until next morning before sunrise. This fairly regular variation, with a maximum and minimum, which divides the day into unequal periods of about eight hours and sixteen hours on the average of the year, is the diurnal variation.

The difference between the maximum and minimum tem- peratures of any twenty-four hours period, as for example the ordinary civil day, is the ' diurnal range/ The diurnal range depends chiefly upon the amount and distribution of cloud and also upon the humidity. The following summarizes the chief features of the diurnal range :

(1) It is much smaller in the wet than in the dry season, the difference being most marked at stations in the interior.

(2) It is smallest in amount at the coast stations, more especially at those on the Konkan and Malabar coasts, where it averages io y , and increases on proceeding into the interior to a maximum of 30 on the mean of the year in the Punjab and Upper Sind.

(3) It has, over the greater part of Northern and Central India, two maximum and minimum values. It is usually absolutely greatest in November, the most serene month of the year ; falls slightly up to January or February ; increases again up to April and May ; and falls to the absolute minimum in July or August, the height of the rains.

Again, during one part of the year, from January to May or June, the increase of temperature by the solar action is greater than the decrease or loss by radiation and other actions, and hence temperature rises more or less steadily during this period of increasing elevation of the sun. During the remainder of the year the balance is the other way and temperature steadily decreases from June or July to December. This regular increase during one half of the year, followed by decreasduring the other half, is known as the annual variation. The annual variation is estimated in various ways. The most usual methods are (a) as the difference between the highest and lowest monthly means, (6) as the difference between the highest and lowest d^ily means, (c) as the difference between the highest normal daily maximum and lowest normal daily minimum temperatures during the year, and (d} as the mean of the annual absolute range of temperature : that is, of the differences between the highest maximum and lowest minimum observed in each year. The data establish that the annual variation (calculated by any of these methods) is small in amount at the Bay islands and in Malabar.

It increases rather rapidly northwards in proceeding along the east and west coasts of India. It is twice as great at Bombay and Rangoon as in Malabar, three times as great at Madras and Chittagong, and over five times as great at Karachi. It is from eight to ten times as great at the stations in the North Deccan and Northern and Central India, and is absolutely greatest in the most inland and the driest districts, including Upper Sind and the Punjab.

The highest day temperatures are usually observed in Maximum Northern India in the month of May. The hottest area is J^pera- undoubtedly that part of Upper Sind known as the Pat desert, turcs. of which Jncobabad is imperfectly representative, and after that, the adjacent tracts, including the South-western Punjab and North-western Rnjputana. In seventeen out of the last twenty years the highest day temperatures, ranging from 117 to 126, were recorded at Jacobabad. As there is much irrigation in and near this town, which is more or less sheltered by trees, it is almost certain that at Sibi and other places in the driest parts of the Pat desert the maximum temperature probably reaches 130 in the hottest periods. Maximum temperatures exceeding 120 have been recorded at the stations named below:

JacobabSd. Jodhpur.

Pachbhadra. Sirsa.

Hyderabad. Multan.

Montgomery. Khushab.

Dera Ismail Khan. Lahore.

Agra.

The conditions which determine the night temperature are Minimum very different from those on which the day temperature depends. Radiation into space and conduction to the ground surface are turcs.the chief actions. The effect of these actions in reducing tem- perature is greatest in clear dry still nights, such as usually obtain in the cold and hot weather in North-western and Cen- tral India and the North Deccan. The air cools almost con- tinuously during the night at rates depending upon the amount of cloud, the humidity of the air and other conditions ; and the lowest temperature is, on the average of the year, observed at 5.30 a.m. or about half an hour before mean sunrise. The lowest minimum night temperatures are almost invariably recorded in Northern and Central India during the cool periods after the passage of cold-weather storms. The follow- ing gives the lowest temperatures (26 or below) that have been recorded at the chief meteorological stations in the plains of North-western India :

Mean diurnal tempera- tures.

Hilt

tempera?

tures.

Lowest

Station.

minimum tempera-

Date.

ture.

-

Punjab and Frontier Province

/Peshawar 1 Rawalpindi 1 Khu>hab L>era Ismai Khan

'5-7

33-9 35.0 26-O

December 19, 1878. December 28, 1878. January 15, 1899. January 9, 1876.

Rajputana

LmUitana Pachbhadra

24.0 35-4

December 6, 1879. February 18, 1882.

Mean daily temperature is usually defined by meteorologists as the mean of twenty-four readings of a standard thermometer, properly exposed, taken at hourly intervals. Hourly observa- tions, however, are recorded at only three or four stations in India. Hence in the India Daily Weather Reports the mean temperature of any day is taken as the semi-sum of the maximum and minimum temperatures recorded during the day. This differs from the true mean as defined above by small amounts which have been approximately determined from short series of hourly observations in different parts of India.

Appendix B (p. 154) gives comparative data of this element for twenty-three representative stations in India and illustrates sufficiently the mean or normal temperature conditions over India.

The temperature at hill stations is determined only very slightly by the direct action of the sun, and chiefly by the large convective air movements over the plains, and by the inter-

m] METEOROLOGY 151

change or diurnal air movement between the plains and hills which is a permanent feature in Himalayan meteorology. Appendix C ip. 155) gives the average mean temperature of twelve representative hill stations for each month.

At stations in mountain valleys, e.g. Srinagar and Leh, the maximum day temperature is determined chiefly by solar action and local conditions of cloud, &c. At stations on the crests of the outer mountain ranges it is, in dry clear weather, conditioned almost solely by the convective movement over the plains in which dry air, rising rapidly, cools down at the rate of i per 186 feet During the rains, when there is much forced ascent of the humid current against and over the hills, it is conditioned by that movement, in which humid air at the mean temperature conditions cools down at rates which vary from about 200 to 500 feet per degree.

The night temperatures at hill stations are, more especi- ally in fine dry clear weather, determined by actions and conditions very different from those which give rise to the day maximum. The air cools rapidly by radiation and con- duction to the ground on the higher elevations, and sinks down into the valleys, where it tends to accumulate and give decreasing temperature as the night advances. On the moun- tain crests it is modified by the upper air currents which set in from the plains towards the higher ranges. These actions tend to give much lower night temperatures at the same eleva- tion, and a larger diurnal range in valleys than on the mountain crests.

In fine dry weather, when there is little air movement, temperature falls rapidly at stations such as Murree, Simla, and Chakrata on open mountain ridges from 2 p.m. to 6 p.m., and is then practically constant during the whole night from 7 p.m. to 5 a.m. Another important feature is that when the conditions obtaining in fine dry weather in Northern India are most marked that is, when the temperature at night falls rapidly in the plains owing to radiation in a clear calm atmo- sphere until shortly before sunrise, and when it remains nearly constant on the crests throughout the whole night the minimum temperature at hill stations may be considerably higher than at neighbouring stations in the plains 5,000 and 6,000 feet below. This frequently occurs in continuous anticyclonic periods during the cold-weather period when the cold-weather rains are late and scanty. One example will be sufficient to illustrate these remarkable temperature conditions. The minimum tempera- ture at Murree on the night of January 2,* 1889, was 12-4

i 5 2 THE INDIAN EMPIRE [CHAP.

higher than at Rawalpindi, and 8-2 higher than at Sialkot ; at Simla it was 12-1 higher than at Ludhiana, and 11-8 higher than at Lahore; and at Ranlkhet it was 13-4 higher than at Roorkee, 18-2 higher than at Bareilly, 6-4 higher than at Agra, and 7-3 higher than at Allahabad. TJ^is was on the whole the most remarkable example of the inversion of the tempera- ture with elevation that has occurred during the past twenty years in the cool season in Northern India.

III]

METEOROLOGY

'S3

APPENDIX A

RAINFALL DATA OF 21 METEOROLOGICAL DIVISIONS OK INDIA (IN INCHES)

NORMAL RAINFALL.Hnrma, Coast .

0.71

18.08

130.63

3-52

18.79

'34- ] 5

152.94

Burma, Wet .

0.49

7-5i

55-29

1-69

8-co

56-98

64.98

liuima, Dry .

0-15

6.36

24.49

1.65

6-51

26.14

32.65

Delta of Bengal

1.64

'5-5*

60.78

1.38

17-22

62.16

79-38

Brahmaputra Valley

2-37 ' 23.99

64.99

1. 01

26.36

66.00

92-36

Indo-Gangetic Plain,

1.31

3-72

41.85

O..S*

5-03

42-43

47.46

I-ast

Himalayas and Sub-

1.27

6-35

56.51

-33

7-62

56.^4

64.46

Himalayas, Kast

Himalayas and Sub-

^So

2.92

34.85

1.18

681

36.03

4284

Himalayas, West

Further Kashmir .

3-3 r >

5-57

2-34

'-45

8-93

3-7<>

12.72

Iiv'o-Gangetic Plain,

1*^2

J.20

2758

O* (sQ

2-72

28-! 7

30-89

West

North-West Dry Aren

1.02

1.30

7-49

Cii ^l

2-32

7.92

10-24

Baluchistan .

.V78

2-52

0-94

I. 4 2

6-30

2.36

8-66

West Coast .

0-89

9-4

85.08

4-83

10-29

89-91

100.20

Gujarat .

0-12

0-22

26-96

o-34

o-34

27-30

27.64

South India .

0.58

6.21

18-55

5-83

6-79

24-38

3M7

Deccan .

0*16

2.36

25-35

i.Si

2.52

27-16

29-68

WestSatpuias. .

0.56

i-oo

31-74

L02

i- 5 6

32-76

34-32

Central India Plateau

0-75

0-87

30.02

0-6 1

1.62

3063

32-25

East Satpnras

1.14

2.91

5 -76

0-86

4-05

51-62

55-67

Ea&t Coast, North

0-78

5.18

41.30

4-24

5-96

45-54

51-50

East Coast, South

3.29

3.06

22.91

33-33

5-35

46.24

51-59

w

x Chakrata

Mussoorie

Ranikhet

Darjeeling

Pachmarhl

Wellington

i 5 6 THE INDIAN EMPIRE

BIBLIOGRAPHY

The following publications of the Indian Meteorological Department, which are procurable at Calcutta, deal with the matters treated in this chapter :

1. Handbook- of Cyclonic Storms in the Bay of Bengal for the use of Sailors, vol. i, Text; vol. ii, Plates.

2. Rainfall Map of India in two sheets, scale 64 miles to I inJi, showing the annual distribution of rainfall in In<Jia (in colours).

3. Instructions to Observers of the Indhn Meteorologi - al Department, in which are described in detail the various instruments in use at the observatories.

4. Barometer, Wind, and Current Charts of the Bay of Bengal and adjacent sea north of the equator.

5. Charts of the Bay of Bengal and adjacent sea north of the equator, showing the specific gravity, tem| erature, and currents of the sea surface.

6. Barometer, \\ ind, and Current Charts of the Arabian Sea and the adjacent portion of the North Indian Ocean.

7. Monthly We.ithcr Review of India, giving a summary of the chief feature^ of the weather in India and containing discussions on atmospheric pressure, barometric depressions, temperature of the air, winds, rainfall, &c. This also contains all available information about snowfall in the mountain districts to the north and north-west of India, and an abstract of observa- tions taken at about 200 observatories in India.

8. Annual Summary of the Monthly Weather Review, giving a discussion of the meteorological data of India for the year.

9. India Daily Weather Report and Chart, containing observations rCv.orded at 8 a. m. at 168 observatories, with a chart showing the distribu- tion of atmospheric pressure and rainfall all over India, and a brief summary of the chief features of the weather of the day.

10. Indian Meteorological Memoirs, giving discussions and researches of meteorological data for India and neighbouring countries, illustrated by charts, plates, and curves, vols. i-xi. Blanford's memoir on The Rainfall of India, published in 1886, forms one of the volumes of this series.

11. Indian Special Cyclone Memoirs, giving full accounts of the most important cyclones and cyclonic storms which have occurred in India during the past thirty years.

12. A Handbook of the Meteorology of India, by Sir John Eliot, is now (1906) under preparation.

13. A Climatological Atlas of India, by Sir John Eliot, is also now (1906) under preparation.

This article was written around 1911 when conditions were different. It has been archived for its historical value as well as for the insights it gives into British colonial writing about India. Indpaedia neither agrees nor disagrees with its contents. Readers who wish to add fresh information can create a Part II of this article. Readers will be able to edit existing articles and post new articles directly on their online archival encyclopædia after its formal launch. See examples and a tutorial.

Extracted from:

Encyclopaedia of India

1911.

No further details are available about this book, except that it was sponsored in some way by the (British-run) Government of India.

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Meteorology: details from 1911

The great peninsula of India, with its lofty mountain ranges behind and its extensive seaboard exposed to the first violence of the winds of two oceans, forms an exceptionally valuable and interesting field for the study of meteorological phenomena.

Indus to Brahmaputra

From the gorge of the Indus to that of the Brahmaputra, a distance of 1400 m., the Himalayas form an unbroken watershed, the northern flank of which is drained by the upper valleys of these two rivers; while the Sutlej, starting from the southern foot of the Kailas Peak, breaks through the watershed, dividing it into two very unequal portions, that to the north-west being the smaller. The average elevation of the Himalaya crest may be taken at not less than 19,000 ft., and therefore equal to the height of the lower half of the atmosphere; and indeed few of the passes are under 16,000 or 17,000 ft. Across this mountain barrier there appears to be a constant flow of air, more active in the day-time than at night, northwards to the arid plateau of Tibet. There is no reason to believe that any transfer of air takes place across the Himalayas in a southerly direction, unless indeed in those most elevated regions of the atmosphere which lie beyond the range of observation; but a nocturnal flow of cooled air, from the southern slopes, is felt as a strong wind where the rivers debouch on the plains, more especially in the early morning hours; and this probably contributes in some degree to lower the mean temperature of that belt of the plains which fringes the mountain zone.

At the foot of the great mountain barrier, and separating it from the more ancient land which now forms the highlands of the peninsula, a broad plain, for the most part alluvial, stretches from sea to sea. On the west, in the dry region, this is occupied partly by the alluvial deposits of the Indus and its tributaries and the saline swamps of Cutch, partly by the rolling sands and rocky surface of the desert of Jaisalmer and Bikaner, and the more fertile tracts to the eastward watered by the Luni. Over the greater part of this region rain is of rare occurrence; and not infrequently more than a year passes without a drop falling on the parched surface. On its eastern margin, however, in the neighbourhood of the Aravalli hills, and again in the northern Punjab, rain is more frequent, occurring both in the south-west monsoon and also at the opposite season in the cold weather. As far south as Sirsa and Multan the average rainfall does not much exceed 7 in.

The alluvial plain of the Punjab passes into that of the Gangetic valley without visible interruption. Up or down this plain, at opposite seasons, sweep the monsoon winds, in a direction at right angles to that of their nominal course; and thus vapour which has been brought by winds from the Bay of Bengal is discharged as snow and rain on the peaks and hillsides of the Western Himalayas. Nearly the whole surface is under cultiva tion, and it ranks among the most productive as well as the most densely populated regions of the world. The rainfall diminishes from ioo in. in the south-east corner of the Gangetic delta to less than 30 in. at Agra and Delhi, and there is an average difference of from 15 to 25 in. between the northern and southern borders of the plain.

The Bengal delta

Eastward from the Bengal delta, two alluvial plains stretch up between the hills which connect the Himalayan system with that of the Burmese peninsula. The first, or the valley of Assam and the Brahmaputra, is long and narrow, bordered on the north by the Himalayas, on the south by the lower plateau of the Garo, Khasi and Naga hills. The other, short and broad, and in great part occupied by swamps and jhils, separates the Garo, Khasi and Naga hills from those of Tippera and the Lushai country. The climate of these plains is damp and equable, and the rainfall is prolonged and generally heavy, especially on the southern slopes of the hills.

A meteorological peculiarity of some interest has been noticed, more especially at the stations of Sibsagar and Silchar, viz. the great range of the diurnal variation of barometric pressure during the afternoon hours, - which is the more striking, since at Rurki, Lahore, and other stations near the foot of the Western Himalayas this range is less than in the open plains. The highlands of the peninsula, which are cut off from the encircling ranges by the broad Indo-Gangetic plain, are divided into two unequal parts by an almost continuous chain of hills running across the country from west by south to east by north, just south of the Tropic of Cancer. This chain may be regarded as a single geographical feature, forming one of the principal watersheds of the peninsula, the waters to the north draining chiefly into the Nerbudda and the Ganges, those to the south into the Tapti, the Mahanadi, the Godavari and some smaller streams. In a meteorolgical point of view it is of considerable importance. Together with the two parallel valleys of the Nerbudda and Tapti, which drain the flanks of its western half, it gives, at opposite seasons of the year, a decided easterly and westerly direction to the winds of this part of India, and condenses a tolerably copious rainfall during the south-west monsoon.

Separated from this chain by the valley of the Nerbudda on the west, and that of the Sone on the east, the plateau of Malwa and Baghelkhand occupies the space intervening between these valleys and the Gangetic plain. On the western edge of the plateau are the Aravalli hills, which run from near Ahmedabad up to the neighbourhood of Delhi, and include one hill, Mount Abu, over 5000 ft. in height. This range exerts an important influence on the direction of the wind, and also on the rainfall. At Ajmer, an old meteorological station at the eastern foot of the range, the wind is predominantly south-west, and there and at Mount Abu the south-west monsoon rains are a regularly recurrent phenomenon, - which can hardly be said of the region of scanty and uncertain rainfall that extends from the western foot of the range and merges in the Bikaner desert.

The peninsula south of the Satpura range consists chiefly of the triangular plateau of the Deccan, terminating abruptly on the west in the Sahyadri range (Western Ghats), and shelving to the east (Eastern Ghats). This plateau is swept by the south-west monsoon, but not until it has surmounted the western barrier of the Ghats; and hence the rainfall is, as a rule, light at Poona and places similarly situated under the lee of the range, and but moderate over the more easterly parts of the plateau. The rains, however, are prolonged some three or four weeks later than in tracts to the north of the Satpuras, since they are also brought by the easterly winds which blow from the Bay of Bengal in October and the early part of November, when the recurved southerly wind ceases to blow up the Gangetic valley, and sets towards the south-east coast.

The Eastern and Western Ghats

At the junction of the Eastern and Western Ghats rises the bold triangular plateau of the Nilgiris, and to the south of them come the Anamalais, the Palnis, and the hills of Travancore. These ranges are separated from the Nilgiris by a broad depression or pass known as the Palghat Gap, some 25 m. wide, the highest point of which is only 1500 ft. above the sea. This gap affords a passage to the winds which elsewhere are barred by the hills of the Ghat chain. The country to the east of the gap receives the rainfall of the south-west monsoon; and during the north-east monsoon ships passing Beypur meet with a stronger wind from the land than is felt elsewhere on the Malabar coast. In the strip of low country that fringes the peninsula below the Ghats the rainfall is heavy and the climate warm and damp, the vegetation being dense and characteristically tropical, and the steep slopes of the Ghats, where they have not been artificially cleared, thickly clothed with forest.

In Lower Burma the western face of the Arakan Yoma hills, like that of the Western Ghats in India, is exposed to the full force of the south-western monsoon, and receives a very heavy rainfall. At Sandoway this amounts to an annual mean of 212 in. It diminishes to the northwards, but even at Chittagong it is over 104 in. annually.

The country around Mandalay, as well as the hill country to the north, has suffered from severe earthquakes, one of which destroyed Ava in 1839. The general meridional direction of the ranges and valleys determines the direction of the prevailing surface winds, this being, however, subject to many local modifications. But it would appear that throughout the year there is, with but slight interruption, a steady upper current from the south-west, such as has been already noticed over the Himalayas. The rainfall in the lower part of the Irrawaddy valley, viz. the delta and the neighbouring part of the province of Pegu, is very heavy; and the climate is mild and equable at all seasons. But higher up the valley, and especially north of Pegu, the country is drier, and is characterized by a less luxuriant vegetation and a retarded and more scanty rainfall.

Within the boundaries of India almost any extreme of climate that is known to the tropics or the temperate zone can be found. It is influenced from outside by two adjoining areas. On the north, the Himalaya range and the plateau of Afghanistan shut it off from the climate of central Asia, and give it a continental climate, the characteristics of which are the prevalence of land winds, great dryness of the air, large diurnal range of temperature, and little or no precipitation. On the south the ocean gives it an oceanic climate, the chief features of which are great uniformity of temperature, small diurnal range of temperature, great dampness of the air, and more or less frequent rain. The continental type of weather prevails over almost the whole of India from December to May, and the oceanic type from June to November, thus giving rise to the two great divisions of the year, the dry season or north-east monsoon, and the rainy season or south-west monsoon. India thus becomes the type of a tropical monsoon climate. For the origin of the monsoon currents and their distribution see Monsoon.

The two monsoon periods are divided by the change of temperature, due to solar action' upon the earth's surface, into two separate seasons; and thus the Indian year may be divided into four seasons: the cold season, including the months of January and February; the hot season, comprising the months of March, April and May; the south-west monsoon period, including the months of June, July, August, September and October; and the retreating monsoon period, including the months of November and December. The temperature is nearly constant in southern India the whole year round, but in northern India, where the extremes of both heat and cold are greatest, the variation is very large.

In the cold season the mean temperature averages about 30° lower in the Punjab than in southern India. In the Punjab, the United Provinces, and northern India generally the climate resembles that of the Riviera, with a brilliant cloudless sky and cool dry weather. This is the time for the tourist to visit India. In south India it is warmer on the west coast than on the east, and the maximum temperature is found round the headwaters of the Kistna. Calcutta, Bombay and Madras all possess the equable climate that is induced by proximity to the sea, but Calcutta enjoys a cold season which is not to be found in the other presidency towns, while the hot season is more unendurable there.

The hot season begins officially in the Punjab on the 15th of March, and from that date there is a steady rise in the temperature, induced by the fiery rays of the sun upon the baking earth, until the break of the rains in June. During this season the interior of the peninsula and northern India is greatly heated; and the contrast of temperature is not between northern and southern India, but between the interior of India and the coast districts and adjacent seas. The greater part of the Deccan and the Central Provinces are included within the hottest area, though in May the highest temperatures are found in Upper Sind, north-west Rajputana, and south-west Punjab. At Jacobabad the thermometer sometimes rises to 125° in the shade.

The south-west monsoon currents usually set in during the first fortnight of June on the Bombay and Bengal coasts, and give more The or less general rain in every part of India during the next three months. But the distribution of the rainfall is very uneven. On the face of the Western Ghats, and on the Khasi hills, overlooking the Bay of Bengal, where the mountains catch the masses of vapour as it rises off the sea, the rainfall is enormous. At Cherrapunji in the Khasi hills it averages upwards of 500 in. a year. The Bombay monsoon, after surmounting the Ghats, blows across the peninsula as a west and sometimes in places a north-west wind; but it leaves with very little rain a strip 100 to 200 m.

in width in the western Deccan parallel with the Ghats, and it is this part of the Deccan, together with the Mysore table-land and the Carnatic, that is most subject to drought. Similarly the Bengal monsoon passes by the Coromandel coast and the Carnatic with an occasional shower, taking a larger volume to Masulipatam and Orissa, and abundant rain to Bengal, Assam and Cachar. The same current also supplies with rain the broad band across India, which includes the Satpura range, Chota Nagpur, the greater part of the Central Provinces and Central India, Orissa and Bengal. Rainfall rapidly diminishes to the north-west from that belt.

A branch of the Bombay current blows pretty steadily through Rajputana to the Punjab, carrying some rain to the latter province. But the greater part of north-west India is served as a rule by cyclonic storms between the two currents. In September the force of the monsoon begins rapidly to decline, and after about the middle of the month it ceases to carry rain to the greater part of north-western India. In its rear springs up a gentle steady north-east wind, which gradually extends over the Bay of Bengal, and is known as the north-east monsoon. A wind similar in character, but rather more easterly in direction, simultaneously takes possession of the Arabian Sea. The months of November and December form a transition period between the monsoon and the cold season. The most unhealthy period of the year follows immediately after the rains, when malaria is prevalent, especially in northern India.