1890.] Eelations between the Hills and Plains of Northern India. 49 



effect is to diminish the daily range of temperature by decreasing the 

 day and increasing the night temperature. 



Finally, when the stormy weather passes away unusually dry clear 

 weather sets in. In the hills the snow probably extends down to a level 

 of 4,000 or 5,000 feet. The temperature of the air at and above that 

 level is mainly determined by that of the snow surface with which it is 

 in contact, and hence, even in the middle of the day, differs little from 

 32°. Hence a period of low and nearly constant temperature conditions 

 sets in until the snow is melted and the snow line retreats. The snow 

 melts very rapidly, at a rate of six to nine inches per diem in clear 

 weather in exposed positions, and a snow fall of 3 or 4 feet will melt 

 away and disappear in five or six days in favourable weather except in 

 sheltered positions. Consequently, temperature in the hills at such periods 

 is at first low, but rapidly rises with the melting of the snow, and after 

 a few days of fine clear weather the conditions merge into those of 

 normal ordinary anticyclonic weather, which have been already stated. 



In the plains the conditions and actions are different. Solar radia- 

 tion during such periods is more active than usual in consequence of the 

 great clearness of the atmosphere, the absence of dust, &c. Hence not 

 only is the upward convective motion over the plains dui'ing the day 

 greater than usual, but in consequence of the low temperature over the 

 snow-covered surface of the hills there is a rapid flow of air from the 

 hills towai'ds the plains, which in consequence of the first action is pro- 

 bably greater by day than by night. This mass of air starting from, say, 

 a level of 4000 feet above the plains at a temperature of 32° will by rapid 

 descent be heated about 20° and hence will arrive at the level of the 

 plains at a temperature of about 32° + 20° =52°, or 20° lower than the 

 maximum temperature prevailing in the plains in ordinary anticyclonic 

 weather. Hence there will be a steady flow of cool air towards the 

 plains from the hills, the temperature of which, when it arrives at the 

 level of the plains, will be very low when compared with the ordinary 

 day temperature at the period. As the snow melts and the snow line 

 ascends, the temperature of the descending current at the level of the 

 plains will increase. Hence in the plains immediately after a severe 

 storm in the hills there will be, 



1st. A strong and steady current from the hills towards the plain s 

 and hence a strong easterly current from the north north- 

 west and west down the Gangetic Plain. 

 2nd. This current will be fed from a source of nearly constant 

 temperature above the elevation of the snowline, and hence 

 the temperature of the descending current at the base of 

 the hills will be least immediately after the clearing up 



