1890.] Relations between the Hills and Plains of Northern India. 45 



and diminish in temperature at a rate not much less than that of a 

 rapidly ascending current of dry air, which is very approximately 1° 

 for every 193 feet. Assuming the rate of decrease of temperature in 

 these ascending currents to be 1° for every 200 feet, the motion of the 

 atmosphere would tend to give a temperature of (72° — 30° or) 42° at the 

 elevation of 6000 feet above the plains of the Punjab. Little or no 

 change would occur at night, when there are practically no convection 

 currents, and hence at that elevation above the plains of Northern India 

 the temperature in such periods would remain permanently at about 42° 

 and hence be about 6° higher than the average night or minimum tem- 

 perature at the level of the plains below. 



The day temperature at the hill stations would be considerably 

 higher than 42° in consequence of the heating of the air by contact 

 with the land surface, and average about 60° in such weather. About 

 sunset temperature would fall quickly and a short period of rapid de- 

 crease of temperature would occur until the temperature reached that 

 of the same level above the plains, viz., 42°. The continuous decrease 

 of temperature in the hills and plains for some time after sunset would 

 evidently give rise to a compressive movement over the hills and plains 

 aud also to a very slow downward movement of air from the hills to- 

 wards the plains and to a nearly horizontal upper movement from 

 above the plains towards the hills. Hence the air which cools by contact 

 with the mountain sides and moves down towards the plains is replaced 

 from a large source (that of the whole mass above the plains at the 

 higher levels), and hence arrives at a nearly constant temperature 

 corresponding to that level. Thus air brought in from the level of 

 7000 feet would arrive during the night at that level in the hills at a 

 nearly constant temperature at 42°, and hence when the temperature at 

 the hill stations has fallen to a little below 42° it would remain fairly 

 steady during the night at about that temperature.* As the tempera- 



* In order to verify this statement I had two series of temperature observations 

 taken in a suitable open position on the top of a ridge at Simla on the nights of the 

 9th and 11th of December last, when ordinary antioyolonic weather prevailed in 

 Northern India. They are given in the following table and it will be seen fully 

 to confirm the conclusion given in the text. 





Temperature of the air. 





Date. 



16 



hrs 



1630 

 hrs. 



17 

 hrs. 



17-30 

 hrs. 



18 

 hrs. 



1830 

 hrs. 



19 

 hrs. 



1930 

 hrs. 



20 



hrs. 



2030 

 hrs. 



21 



hrs. 



21-30 

 hrs. 



22 

 hrs. 



cd "a 



F3 r£ 



December 9th 

 „ 11th 



549 

 507 



52-4 

 48-5 



492 

 439 



47-7 

 438 



45-4 

 42-0 



466 



42-2 



469 



42-7 



466 

 431 



467 



42- f 



44-9 

 433 



432 

 43 6 



45-6 

 449 



45-9 



45-2 



410 

 412 



