2 U. V. Blanford— r//e Theory of the Winfer [No. 1, 



the phenomena which precede and accompany the cold- weather rainfall 

 of Northern India. In each of the annual reports on the Meteorology 

 of India, in recent years, two or three instances of this cold weather 

 rainfall have been described and illustrated at some length ; and at the 

 present time, although many important points still require further eluci- 

 dation, it is at least possible to set forth some generalizations on the 

 conditions which usher in the precipitation of the cold- weather rains, 

 and on the probable source of the vapour which feeds them. 



The four charts on Plate I exhibit the average distribution of at- 

 mospheric pressure in the months of November, December, January, and 

 February. These charts, being based on the registers of duly verified 

 baronieters during the last seven years, corrected to a common standard 

 and reduced to sea-level values from elevations, determined in all but 

 a few exceptional instances, by actual spirit-levelling to the mean sea- 

 surface, may be accepted as representing, witli a near approximation to 

 truths the relative differences of pressure which characterize the winter 

 months in India.* Certain characters common to all, may be regarded 

 as distinctive of the season. The seat of highest pressure is in the 

 neighbourhood of Peshawar. Whether this may be taken as indicating 

 that the pressure on the highlands of Cabul is also greater than at similar 

 elevations over the plains of India is, however, very doubtful. The 

 situation of Peshawar on a plain of moderate extent, girt around with 

 mountains, is such that the high pressure may be and very probably 

 is a local effect of the cooled air, draining on all sides from the sur- 

 rounding slopes and filling the basin from which its escape is much 

 obstructed. A similar high pressure is shewn by some other stations 

 near the foot of the N. W. Himalaya, of which Dehra is a notable 

 example. The conditions of pressure at higher elevations over the 

 Himalaya, will be noticed presently. 



The next feature to be noticed is that, throughout the winter 

 months, the axis of average high pressure on the plains and plateaux 

 of India, occupies nearly the same situation as that of low pressure at 



* Rigorously speaking any such representation must of course involve an 

 element of unreality, which is the greater, the greater the diifference of land levels 

 in the area embraced in the chart ; and, where, as in the case of India, largo portions 

 of the area differ by 2,000 feet and upwards, this element attains to some im- 

 portance. Although it may not seriously impair the value of the chart as an illus- 

 tration of the pressure-differences or potentials which maintain the system of wind- 

 currents, the fact that the lower strata of air, resting on low alluvial plains, have 

 no horizontal extension to the higher plateaux and cannot therefore be directly and 

 immediately influenced by the atmospheric pressure there existing, is one that must 

 be kept in view in discussing the relation of the winds to the pressui'e-distribation. 



