1890.] J. Eliot— Temperature relations of Northern India. 249 



ditions such as usually prevails in December and frequently during the 

 greater part of January. 



(2). Cloudy stormy weather accompanying rain in the plains and 

 more or less heavy and general snow in the hills. This occurs during 

 the eastward march of shallow barometric depressions from the north- 

 west frontier across Northern India. 



(3). The remarkably cool and dry weather with strong westerly 

 winds which follows the clearing up of the skies after heavy and 

 general snowfall in tbe mountain districts. The weather conditions 

 are very markedly anti-cyclonic in character. 



The chief temperature conditions of these periods are : — 



1st. High day and night temperature at the hill stations and 

 high day and low night temperature at the plains. Hence in the 

 hills the mean daily temperature is increased and the diurnal range 

 little affected, whilst in the plains the diurnal range is largely increased 

 and the mean temperature differs very slightly from the normal. It 

 is during such weather that the hill stations are occasionally warmer 

 at nights than the plain stations. 



2nd. In stormy weather both day and night temperatures are 

 largely reduced at the hill stations, whilst in the plains the day tem- 

 perature is increased and night temperature decreased. Hence in such 

 weather the mean daily temperature of the bill stations is considerably 

 below the normal, whilst the diurnal range is very slightly affected. 

 In the plains on the contrary the mean temperature is slightly affected 

 whilst the diurnal range is largely diminished. 



3rd. In the fine bright weather immediately following the clearing 

 up of the skies after heavy snowfall in the hills, both the day and night 

 temperatures are largely reduced in the plains. The amount of vapour 

 is rapidly diminished so that the air is very dry and bracing, and stands, 

 more especially in Bengal, in marked contrast to the previous weather. 

 The effect is most simply described as the passage of a cold wave 

 eastwards across Northern India at a rate usually averaging about 300 

 miles per diem. 



The last portion of the paper is devoted to a brief verbal expla- 

 nation of the causes of these remarkable and contrasted variations. In 

 the case of the prolonged fine weather of ordinary anti-cyclonic condi- 

 tions (such as prevailed throughout nearly the whole of January 1889) 

 there is little horizontal movement of the air. The adjustment of 

 pressure to the varying thermal conditions is affected in the day time 

 by expansion and by convective currents and at night by compression 

 only. It is thence shewn that in such conditions the temperature above 

 the plains of Northern India is nearly constant during such periods, 



