Appendix F. RADIATION DURING THE NIGHT. 411 



still, being as much as 15°. During the winter months the effect 

 of radiation is often felt throughout the clear days, dew forming 

 abundantly at 4,000 to 8,000 feet in the shaded bottoms of narrow 

 valleys, into which the sun does not penetrate till 10 a.m., and 

 from which it disappears at 3 p.m. I have seen the thermometer 

 in the reflector fall 12° at 10 a.m. in a shaded valley. This often 

 produces an anomalous effect, causing the temperature in the shade 

 to fall after sunrise ; for the mists which condense in the bottom 

 of the valleys after midnight disperse after sunrise, but long before 

 reached by the sun, and powerful radiation ensues, lowering the 

 surrounding temperature : a fall of 1° to 2° after sunrise of air 

 in the shade is hence t common in valleys in November and De- 

 cember.* The excessive radiation of the winter months often gives 

 rise to a curious phenomenon ; it causes the formation of copious dew 

 on the blanket of the traveller's bed, which radiates heat to the 'tent 

 roof, and this inside either an open or a closed tent. I have experi- 

 enced this at various elevations, from 6,000 to 16,000 feet. Whether 

 the minimum temperature be as high as 50°, or but little above 

 zero, the effect is the same, except that hoar-frost or ice forms in 

 the latter case. Another remarkable effect of nocturnal radiation is 

 the curl of the alpine rhododendron leaves in November, which is pro- 

 bably due to the freezing and consequent expansion of the water in 

 the upper strata of cells exposed to the sky. The first curl is 

 generally repaired by the ensuing day's sun, but after two or three 

 nights the leaves become permanently curled, and remain so till they 

 fall in the following spring. 



I have said that the nocturnal radiation in the English spring 

 months is the great obstacle to the cultivation of many Himalayan 

 plants ; but it is not therefore to be inferred that there is no similar 

 amount of radiation in the Himala}"a ; for, on the contrary, in April 

 its amount is much greater than in England, frequently equalling 

 13° of difference ; and I have seen 16° at 7,500 feet ; but the minimum 



* Such is the explanation which I have offered of this phenomenon in the 

 Hort. Soc. Journal. On thinking over the matter since, I have speculated upon 

 the probability of this fall of temperature being due to the absorption of heat 

 that must become latent on the dispersion of the dense masses of white fog that 

 choke the valleys at sunrise. 



