1902 Encyclopedia > India > Meteorology

India
(Part 4)




INDIA - GEOGRAPHY (cont.)

Meteorology of India


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. But only within the last few years have trustworthy statistics been obtained for some of its most important registration stations.

Meteorological Geography.—After the general description of the country which has been given at the beginning of this article, it is only necessary here to sketch very briefly the meteorological geography of the peninsula. The following sentences are condensed from an interesting account in the first Report on the Meteorology of India (for 1875), by Mr H. F. Blanford. From the gorge of the Indus to that of the Dihong (Brahmaputra), a distance of 1400 miles the Himálayas form an unbroken watershed, the northern flank of which is drained by the upper valleys of these two rivers ; while the Sutlej (Satlaj), 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 Himálaya crest may be taken at not than 19,000 feet, and therefore equal to the height of the lower half of that atmosphere; and indeed few of the passes are under 16,000 or 17,000 feet. Across this mountain barrier there appear 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 Himálayas 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 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 Kahhch (Cutch), partly by the rolling sands and rocky surface of the desert of Jáisalmír and Bíkanir, and the more fertile tracts tot he eastward watered by the Lúnai. Over the greater part of this region rain is of rare occurrence ; and not infrequently more than a year passes with out a drop falling on the parched surface. On its eastern margin, however, in the neighbourhood of the Aravalli hills, and again on 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 Sírsa and Múltán (Mooltan), the average rainfall does not much exceed 7 inches.

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 peak and hillsides of the Western Himálayas. Nearly the whole surface is under cultivation, and it ranks among the most productive as well as the most densely populated regions of the world. The rainfall diminishes from 100 inches in the south-east corner of the Gangetic delta to less than 30 inches at Agra Delhi, and there is an average difference of from 15 to 25 inches between the northern and southern borders of the plain.

Eastward from the Bengal delta, two plains stretch up between the hills which connect the Himálayan 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 Himálayas, on the south by the lower plateau of the Gáro, Khásí, and Nága hills. The other, short and broad, and in great part occupied by swaps and jhils, separates the Lushái 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 Síbságar and Silchár, viz., the great range of the diurnal variation of barometric pressure during the afternoon hours,—which is the more striking, since at Rúrkí (Roorkee), Lahore, and other stations near the foot of the Western Himálayas, 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 Narbadá and the Ganges, those to the south into the Tápti, the Godávari, the Mahánadi, and some smaller streams. In a meteorological point of view it is of considerable importance. Together with the two parallel valleys of the Narbadá (Nerbudda) and Tápti (Taptee), which drain the flanks of its western half, it gives, at opposite seasons of the year, a decided easternly 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 Narbadá on the west, and that of the Son on the east, the plateau of Málwá and Baghelkhand occupies the space intervening between these valleys and the Gangetic plain. On the western edge of the plateau are Aravalli hills, which run from near Ahmadábád up to the neighbourhood of Delhi, and include one hill, Mount Abu, over 5000 feet in height. The range exerts an important influence on the direction of the wind, and also on the rainfall. At Ajmír (Ajmere), 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, and there and at Mount Abu the south-west monsoon rains are a regularly recurrent phenomenon,—which can hardly be 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 Bíkanir desert.

The peninsula south of the Sátpura range consists chiefly of the triangular plateau of the Deccan, terminating abruptly on the west in the Sahyádri range (Western Gháts), and shelving to the east (Eastern Gháts). This plateau is swept by the south-west monsoon, but not until it has surmounted the western barrier of the Gháts; 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 there or four weeks later than in tracts to the north of the Sátpuras, 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. This was formerly thought to be a north-east monsoon, and is still so spoken of by certain writers ; but the rainy wind id really a diversion of the south-west monsoon.

At the junction of the Eastern and Western Ghát rises the bold triangular plateau of the Nílgiris, and to the south of them come the Anamalais, the Palnis (Pulneys), and the hills of Travancore. These ranges are separated from the Nílgiris by a broad depression or pass known as the Pálghát Gap, some 25 miles wide, the highest point of which is only 1500 feet above the sea. This gap affords a passage to the winds which elsewhere are barred by the hills of the Ghát 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. According to Captain Newbold, this gap "affords an outlet to those furious storms from the eastward which sweep the Bay of Bengal, and, after traversing the peninsula, burst forth through it to the neighbouring sea."

In the strip of low country that fringes the peninsula below the Gháts, the rainfall is heavy and the climate warm and damp, the vegetation being dense and characteristically tropical, and the steep slopes of the Gháts, where they have not been artificially cleared, thickly clothed with forest.

In Burmah, the country around Ava, as well as the hill country to the north, has suffered from severe earthquakes, one of which destroyed Ava in 1839. The general meridianal 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, from Dr Anderson’s observations of the movement of the upper clouds, 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 Himálayas. The rainfall in the lower part of the Irawadi valley, viz., the delta and the neighbouring pat of the province of Pegu, is very heavy ; and the climate is very mild and equable at all seasons. But higher up the valley, and especially north of the Pegu frontier, the country is drier, and is characterized by a less luxuriant vegetation, and a retarted and more scanty rainfall.

Observatories.—Meteorological observatories have been established at one hundred and three stations in India (including British Burmah and the Andamans). These observatories are situated at all the elevations, from the highest, Leh (11,538 feet above mean sea-level) and Chakráta (7051 feet), to Negapatam (15 feet) and Ságar Island, the lowest, which is only 6 feet above 6 feet above mean sea-level.

Temperature of the Air.—From the average annual mean temperatures of 83 stations in this list, the average mean yearly temperature was over 82º F.:--Trichinopoli, 82·8º ; Vizagapatam, 82·7º ; Madras, 82·4º; and Madura, 82·2.º All of these stations are in the Madras Presidency. The next highest means are returned by Negapatam (also in Madras), 81·9º; Cuttack and Port Blair, each 80·5º ; False Point, 80·20º ; Goa, 79·9º ; Cochin, 79·8º ; Sagar Island, 79·5º ; Deesa, 79·4º ; and Calcutta, 79·2º. The means annual temperature of B Bombay is 78·8º, so that it is the coolest of the three presidency towns. The lowest means are obtained at the hill stations of Dárjíling, 53·9º ; Simla, 54·4º ; Murree, 55·8º ; and Chakráta, 56·1º. Between these and the next coolest stations is a great gap, Ráníkhet following with 60·4º. Pachmarhi with 68·7º, and Ráwal Pindí with a yearly mean of 69·4º. The highest mean monthly temperatures given are :—95º at Múltán, in June ; 94·3º at Delhi, in June; 94·1º at Jhánsi, in May ; 93·6º at Lucknow, in June. The lowest montly means are returned by the four coldest hill stations mentioned above, the firugres being :--Murree—January 37·7º, February 39·4º ; Simla—January 39·6º, February 41·1º ; Chakráta—January 40·8º, February 42·9º ; Dárjíling—January 40·7º, February 43·2º. The mean temperature at Leh in January is 17·6º, and in December 24·4º.





Atmospheric Pressure.—The meteorological report for 1877 contains a table showing the annual mean pressure at 72 stations, corrected (except in the case of Madras) to the Calcutta standard, which reads o·011 inch higher than that of Kew. From that table the following figures are obtained. The means yearly pressure at the highest stations is--23·274 at Chakráta, 23·371 at Dárjíling, 24·058 at Ráníkhet, 26·416 at Pachmarhi, and 26·932 at Bangalore. The greatest annual mean pressures returned are--29·862 at Negapatam, 29·856 at Madras, 29·822 at Bombay, and 29·821 at False Point.

Rainfall.—The average annual rainfall at 294 stations is recorded in the 1877 meteorological report, from which the following figures have been obtained:—

In the Punjab the highest average fall (123·21 inches) is at Dharmsála, which is situated on the face of the hills, and exposed to the full of the monsoon ; the next highest recorded is little more than half that amount, or 68·61 inches at Simla. The lowest average falls in the Punjab are--6·16 inches at Muzaffargarh, 6·93 at Múltán, 7·35 at Derá Ghází Khán, and 8·23 at Derá Ismáil Khán. All these stations are protected by the Sulámán range from the monsoon.

In Rájputána and Central India the maximum average is 20·27 at Jáípur (Jeypore), and the minimum, 60·85 at Mount Abu, the highest point in this part of India.

In the North-Western Provinces the heaviest average falls are at Náini Tál (94·17 inches) and Dehra (70·06), both of which lie high ; the minimum average fall is 24·32 at Alígarh, the next lowest figures being 26í18 Muttra (Mathura), 26·46 at Agra, and 26·74 at Etah—all stations on the plains.

In Oudh the maximum rainfall is at Sultánpur, 46·72 inches ; and the minimum at Ráí Bareli, 39· 99 inches.

The following stations of Bengal have an average rainfall of more than 100 inches:--Jalpáiguri, 122·16 ; Dárjíling, 119·25 ; and Kuch Behár, 119·05—all at the base of the hills ; Noákhalí, 107·52, and Chittagong, 105·61, both on the north-east coast of the Bay of Bengal. The lowest averages are returned by Chapra, 37·06 inches ; Patná, 38·21 and Gáya, 41·38. The average for Bengal is 67 inches.

Assam possesses in Cherra Poonjee (Chárá Punji) the station with the largest recorded rainfall in the world. The registered fall during the three years ending 1876 averaged 368·41 inches. A total fall of 805 inches was reported in 1861, of which 366 were assigned to the single month of July. In 1850 Dr Hooker registered 30 inches in twenty-four hours, and returned the fall from June to November of that year at 530 inches. In the four days 9th to 12th September 1877, 56·19 inches were registered. The following stations in Assam have also a very high rainfall:--Silchár, 121·07 ; Sylhet, 153·80 ; Dibrugarh, 116·43 ; and Tura, 115·76. The lowest recorded averages in Assam are at Samaguting (52·58 inches) and Gauháti (69·23 inches), both on the northern side of the hills separating Cachar from Assam.

In the Central Provinces the highest average falls are at Pachmarchi (82·20 inches) and Bálághát (64·11 inches) ; lowest averages, Khandwa, 32·26 inches, and Bednúr, 41·21 inches.

In Bombay, three stations on the Gháts are recorded as having an average rainfall of over 250 inches, viz. :--Matheran, 256·75 inches ; Malcolmpet (Mahábleshwar), 252·25 ; and Baura (Fort), 251·80. The lowest average rainfalls recorded in Bombay are—12·99 inches at Mandargi ; 17·25 at Dhulia ;a nd 19·93 at Gokak. The average rainfall for Bombay is 67 inches.

In Sind the average rainfall is very low, varying from 16·31 inches at Nagar, and 11·78 at Umarkot, to 5·09 at Shikárpur, and 4·28 at Jacobábad.

In Madras the highest average recorded are--135·60 inches at Cannanore ; 131·91 at Mangalore ; 125·63 at Tellicherri ; 113·62 at Calicut ; and 112·15 at Cochin—all on the west coast. The lowest falls recorded are—at Bellary, 16·06 ; Tuticorin (sheltered by the Gháts), 18·50 ; Guti (Gooty), 20·85 and Coimbatore, 20·90. All these stations lie low. The average fall at the stations on the east coast is about 41 inches. The average rainfall for Madras is 44 inches.

The rainfall along the coast of British Burmah is heavy, as might be expected, the following averages being recorded:—Sandoway, 218·58 inches ; Tavoy, 195·47 ; Maulmain, 191·34 ; Akyab, 189·23 ; Khyouk-hpyu, 170·76. The smallest rainfall is at Thayet-myo (51·04) and Prome (59·46), sheltered by the Yoma range.

The rainfall at Port Blair, in the Andamans, is also naturally heavy the average being returned as 116·25 inches.

Sun-spot Cycles.—The conclusions arrived at by the Indian meteorological department on the subject of the sun-spot cycles, which have been engaging the attention of scientific men, are thus summed up in the 1877 report:--"In conclusion, the following are the more important inferences that the meteorology of India in the year 1877 and 1878 appear to suggest, if not to established. There is a tendency at the minimum sum-spot periods to prolonged excessive pressure over India, to an unusual development of the winter rains, and to the occurrence of abnormally heavy snowfall over the Himálayan region (to a greater extent probably in the western than the eastern Himálayas). This appears also to be usually by a weak south-west monsoon. The characteristic of a weak monsoon are—great irregularity in the distribution of the rainfall over the whole of India, and the occurrence of heavy local rainfalls, which tend, by a law of rainfall and of air-motion, to recur over the same limited areas. The irregularity of rainfall distribution is often shown by the persistent and prolonged absence of rain over considerable areas. These areas of drought and famine are partly marked off by nature, depending to a certain on the geographical features and position of the district. Thus the rains are more likely to fall below the amount necessary for cultivation in the dry region of the Deccan or in Upper India, than over the Malabar coast area or the province of Bengal.


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