1881.] and rainfall to temperature in India. 75 



It must not be overlooked that, both in the dry season and in such an 

 autumnal season as that of 1877, cloudless weather is accompanied by hot 

 ■westerly winds, while cloudy weather is usually characterised by compara- 

 tively cool easterly winds ; and it may therefore be objected, that a large 

 part of the temperature difference shown in the above table, is dependent 

 on the wind and not on the local effect of cloud and rain. And this objec- 

 tion may be admitted, in so far, that the temperature effect is not strictly 

 of local origin. But the heat of the westerly wind, itself, is simply owing 

 to the dryness of the adjacent tract For the heaviest rain that falls in 

 the North- West Provinces in July and August is brought by westerly 

 winds, which come from the Arabian Sea. These blow across Rajputana 

 and Central India, the surface of which has then been cooled by the rain 

 already fallen ; and under these circumstances westerly winds are cool 

 winds. The sujDposed objection, therefore, has no real validity. 



The above data show that, both in the dry season and the rainy 

 season, the anomalous temperature of the air depends principally on the 

 cloud and rainfall ; the effect of both these being to lower the tempera- 

 ture ; in the case of the former, by obscuring the sun, in that of the latter, 

 by the evaporation which ensues, and which reduces the temperature of both 

 the ground and the air in contact with it, not only on the days of rainfall, 

 but generally for one or two days afterwards. In November and Decem- 

 ber, however, when the temperature is falling rapidly, the influence of these 

 agents is relatively less powerful, and the final result is of a different 

 character. In these months, the total loss of heat by radiation from the 

 ground, under a clear sky, exceeds the total gain from solar radiation under 

 similar conditions ; and, accordingly, the presence of cloud, which tends to 

 arrest both, results in maintaining the temperature above the average. 

 The winds, which bring the vapour to form the cloud, also contribute to 

 maintain a high temperature ; since they come from the seas around India, 

 the temperature of which, at this season, is higher than that of the land. 

 It is true that, in the cold season, as in the hot dry season, a warm period 

 due to southerly winds and cloud, if rain falls, is almost always followed 

 by a few days of excessive cold, as in the dry season ; but, on the whole, 

 the former effect is preponderant ; and in November and December accord- 

 ingly, the rule which holds good for the greater part of the year is revers- 

 ed, cloudy and rainy months having a positive, and clear dry months a 

 negative, temperature variation. This is well shown by the following 

 table, which gives the mean temperature anomaly, cloud proportion, rain- 

 fall and number of rainy days of the six Punjab stations, Dera Ismail 

 Khan, Rawalpindi, Sialkot, Lahore, Ludhiana, and Delhi, for November 

 and December, in each of the five years 1875-79 : — 



