RELATION TO THE DISTRIBUTION OE BAROMETRIC PRESSURE. 
347 
according to the principles above discussed, the topsy-turvy movements extend, when 
the sky is clear, to an average height (in summer in England) of about 2100 feet, 
whilst in cloudy weather they can hardly take place at all. In India there is, in all 
probability, a still more rapid decrement of temperature near the ground, when the 
sky is free from cloud. From four years’ observations made at Alipore, near Calcutta,*" 
I find that at the hottest time of the day, about 2 p.m. during the months of greatest 
serenity, the temperature decreases 1*4° F. between 4 and 40 feet above the ground, 
or at the rate of 38'9° per 1000 feet. This we may take to be the most probable 
initial rate of decrease for clear weather. In the cloudy weather of July the rate is 
only 27'8° per 1000 feet, or 1*0° between 4 and 40 feet. Both these are very much 
greater than the initial rates given by Glaisher’s observations for England; which, 
by drawing tangents at the starting point of the curves on Plate 21, or differentiating 
the formulae representing them, are found to be 7‘7° and 5’2° per 1000 feet respec¬ 
tively ; and we may infer that convective action is consequently much more energetic 
in India. Whilst observations made at mountain stations thus give rates of decre¬ 
ment which are probably different from those obtaining in the free atmosphere, at all 
events for the layers of the atmosphere nearest the ground, we may still, however, 
get an idea of the relative intensities of diurnal convection currents over different 
parts of the Indian plain by a comparison like that made in Table V. 
On the Central Indian plateau, and in South Rajputana and Sindh, where there is 
usually but little cloud in January, and the range of temperature is very great, such 
convection movements doubtless take place much more frequently, and extend to a 
greater height than they do in the Punjab and North-Western Provinces. To the 
interchange thus effected may probably be attributed the anomalous wind direction 
observed at Neemuch. 
During the hot weather, vertical convection currents are extremely active over 
Northern and Central India, as testified by the constant occurrence of dust-whirls, 
familiarly known as “ devils,” as well as by the frequent piling up of cumulus clouds 
in the afternoons and the occasional occurrence of thunderstorms. Another evidence 
of such action is the gradual charging of the air with dust up to a height of 8000 feet 
or more; so that, just before the rains set in, it may be likened in appearance to muddy 
water or pea-soup. At this season, the mean temperature of the 24 hours decreases 
rapidly on ascending ; the mean rates on mountain slopes, when variations in latitude 
and longitude are eliminated, being the following t in the month of May :— 
* ‘ Indian Meteorological Memoirs,’ vol. 2, page 450 et seq. 
f See ‘ Indian Meteorological Memoirs,’ vol. 2, pages 132 to 136. 
2 Y 2 
