54 Meteorological Observations. 



to' 



occur, is from the end of November to the beginning of 

 April. The mists of November and December depend on 

 different causes from those that give rise to the fogs of 

 February and March. The first arise from the peculiar in- 

 fluence of radiation, and the second from diurnal variations 

 in the winds. 



During the day the surface absorbs heat from the clear 

 rays of the sun in proportion to the degree of moisture con- 

 tained on it. If the surface be arid, the absorption of heat 

 will be great under the action of the sun's rays, but when 

 these are withdrawn, the radiation and cooling will be 

 equally rapid and great ; hence the diurnal variations of dry 

 sandy tracts, as the desert of Scind, will necessarily be 

 great, but here the atmosphere and the surface are both dry 

 together. In Bengal, on the other hand, where a larger pro- 

 portion of the surface is covered with water and vegetation, 

 the moisture of the air is greater and the dew-point conse- 

 quently lower, and wherever this last happens to come with- 

 in the sphere of diurnal variation, precipitation must take 

 place.* Since the extremes between the lowest and highest 

 diurnal temperature is greater on clear open spaces than in 

 forests, it is in the former we commonly perceive a thin 

 horizontal stratum of vapour suspended of a morning at sun- 

 rise over the surface. 



Fogs, on the other hand, are more general, and arise 

 from diurnal changes of the wind. The south-west monsoon 

 generally sets in as a southerly wind about the beginning of 

 March or the middle of February ; it is naturally moist, and 

 blows steadily during the day, when the temperature of the 

 air, and consequently its capacity for moisture is greatest, sub- 

 siding gradually after night-fall; from this hour till sim-rise 

 temperature gradually diminishes, and we find every thing en- 

 veloped in fog. Night fogs which are not uncommon about 

 the change of the monsoon, when the winds are variable, 



* The. diurnal variation in Calcutta during the N.E. monsoon amounts to as 

 much as 30°. 



