492 



NA TURE 



[March 24, 1892 



show that for a day or two previously to the appearance of 

 the storm-centre in North-Western India hght southerly 

 or easterly winds prevail over Northern India, accom- 

 panied with increasing but not dense cloud, and a steady 

 rise of vapour tension, which, as Mr. Eliot points out, is 

 only in part explained by the increase of temperature. 

 The distribution of the cloud is well marked and 

 characteristic. "An examination of a large number of 

 these storms shows that they invariably give light cloud 

 to the south-east of their tracks, moderate cloud to the 

 east, and overcast skies to the north. In fact the largest 

 development of cloud usually occurs in the northern and 

 north-eastern portion of the depression, and at a con- 

 siderable distance to the north of the depression, as 

 indicated by the isobars, . . . and usually in the hill 

 districts, and adjacent plain districts of Northern India. 

 Hence the cloud distribution appears to indicate that the 

 ascensional motion accompanying these storms com- 

 mences to the south, and proceeds slowly in the eastern 

 quadrant, and is completed in the northern quadrant. 

 The cloud thins off rapidly in the north-west quadrant, 

 and in the west and south-west portions of the depression 

 the skies are clear." These observations are very 

 important ; they show that the damp, warm air of 

 southerly origin is absolutely restricted to the eastern 

 side of the depression, for it is, so to speak, hall-marked 

 by its freight of vapour, and this manifests its presence, 

 and also indicates the region of its ascent and withdrawal 

 from the earth's surface, by forming a thick cloud canopy, 

 on the north and north-east chiefly. This too, of course, 

 is the region of heaviest rain and snow fall. 



The wave of warmth that precedes the depression is 

 thus clearly explained by the unseasonable replacement 

 of northerly by southerly winds ; and since the warmth 

 depends not only on the derived temperature of the air, 

 but also, and in a very great measure, on the check given 

 to nocturnal radiation by cloud and vapour, while it is 

 not dense enough greatly to obstruct the solar rays, it is 

 sufficiently obvious why the night temperatures should 

 show a greater excess than those of the day-time. 



For the reduction of the temperature in the cold wave 

 that follows the depression, three distinct causes may be 

 assigned : viz. the contact of a snow surface down to low 

 levels on the Himalaya and Afghan mountains, from 

 which are drawn the north-west winds of the western 

 half of the storm ; the evaporation of the rain that has 

 fallen on the plains, and is rapidly taken up by the very 

 dry air that has descended from the mountains ; and 

 lastly, the increased nocturnal radiation in this dry 

 atmosphere under a cloudless sky. Of these it would 

 seem that the first and second are the most influential. 

 This seems to be indicated by the facts already quoted — 

 that the intensity of the cold largely depends on the 

 rain and snow fall, and the level to which the latter 

 descends,^ also that when the storm centre lies so far 

 south of the mountains that little or no snow is precipi- 

 tated on them, the cold wave does not always follow. It 

 is further supported by the fact noticed by Mr Eliot, that 

 the relative depression of the day temperature in the 

 cold wave is frequently greater than that of the night 

 hours. This effect of the snow is the more remarkable, 

 when we consider that, in descending to the level of the 

 Indo-Gangetic plain, the air must, of course, be dyna- 

 mically heated about 5^' for each thousand feet of 

 descent. Nearly all this heat must have been- expended 

 in melting the snow, since the air arrives at the level of 

 the plains dry and cold ; so much so, indeed, that the 

 further cooling it undergoes, in consequence of the 

 evaporation of the rainfall and nocturnal radiation, brings 



I Some of the lowest temperatures hitherto recorded in Norihern India 

 were thoss observed in the first week of February 1883, four or five days 

 after a storm which covered with snow the plain around Rawulpindi, only 

 1700 feet above the sea; down to a level therefore unprecedented in the 

 meteorological annah of India. Ses Report on the Meteorology of India in 

 1883 ; also " Climates and Weather of India," p. 204. 



NO. I 169, VOL. 45] 



down the night temperature of the plains sometimes as 

 much as ly below that of mountain ridges at 7000 feet. 



It remains to notice the barometric features of these 

 storms, for, if less striking than the vicissitudes of tem- 

 perature that accompany them, they afford matter of much 

 interest, and suggest questions of a wider bearing than 

 such as have reference merely to the local circumstances 

 of the sub-tropical zone. The barometric depression of 

 these winter storms is as a rule very small, and the gradients 

 are very low. On the Indian weather charts the isobars 

 are laid down for increments of one-twentieth of an inch ; 

 but it is seldom that the storm vortex, when launched on 

 the plains of India, is encircled by more than one or two 

 closed isobars, and sometimes its position can only be 

 gathered from the figures in the accompanying table of 

 the reduced barometric readings, or recognized by the 

 oblique inflection of the wind arrows. Such was the case 

 with both the January storms, noticed above, which 

 travelled across India to Burma, and also the Punjab 

 •secondary depression of January 22, 1889. But that 

 which formed in the Punjab on January 28, and which is 

 described as a stationary storm, was of a much more pro- 

 nounced character, and for three days determined the 

 leading features of the distribution of pressure all over 

 North-Western India, the isobars being only slightly 

 modified by the travelling depression further south. Of 

 a similar type was the storm rit the end of January 1883, 

 of which illustrations are given in the report on the 

 meteorology of India for that year. This too appears to 

 have lingered in the angle of the Punjab inclosed by the 

 Himalaya and the Afghan mountains, and the question 

 forces itself upon us, whether there is not something in 

 the physical features of this part of the country that 

 favours the development of such vortices, and detains 

 them. while the feebler and shallower depressions that 

 form part of the same general disturbance, pursue their 

 course eastward across the plains. Mr. Eliot remarks 

 on the effect of the Himalaya in causing a forced ascent 

 of the southerly vapour-bearing winds, thus localizing the 

 maximum precipitation on the north and north-east of the 

 depression ; and it is possible also that the angle at which 

 the mountain chains meet on the north of the Punjab, 

 inclosing the plain of that province, may not be without 

 its influence in favouring the development and detention 

 of the vortex. 



It is still very obscure what are the general causes that 

 determine the appearance of these storms in India. At 

 one time it seemed to me probable that their origin was 

 to be sought for in the local conditions of India itself, and 

 even now I see no reason to question that, as distinct 

 vortices, very many, perhaps most, of them originate on 

 the plains of India ; especially in such cases as the 

 succession of storms in the earlier part of January 1883, of 

 which a brief description was given in the " Climates and 

 Weather of India." Mr. Eliot, too, seems to hold a similar 

 view, since he speaks sometimes of storms "forming" in 

 Rajputana, &c., sometimes merely as " first appearing" in 

 Sind, Rajputana, or the Punjab. But in some of these 

 instances there is evidence that the appearance of the 

 storm on the plains of the north-western frontier was 

 preceded by stormy weather in Afghanistan, indicating 

 that the disturbance had reached India from that country 

 or Baluchistan ; and Mr. Eliot distinctly identifies a storm 

 that appeared in the Punjab on January 9, 1889, with one 

 that had been experienced at Bushire at the head of the 

 Persian Gulf, on the 7th and 8th. 



In this matter, we must distinguish between the 

 barometric depression, which appears like the trough of 

 a great atmospheric wave of very great extent both in 

 longitude and latitude, sweeping across the country from 

 the westward, and the vortex, or in some cases vortices, 

 which are, as a rule, merely local and subordinate features 

 of the former. No doubt, it is the passage of one of these 

 troughs that in all cases determines the formation of the 



