GEOLOGICAL CLIMATE. 291 



long as its temperature is higher than the average 

 radiational temperature of the atmosphere and 

 space above it. This temperature will be the 

 temperature of the under side of the cloudy 

 stratum, and will generally be above the freezing 

 point if the atmosphere is thickly clouded : it will 

 be perhaps not much above the temperature of 

 planetary space 1 when the air is at its clearest. 

 What then is there to prevent fine leaves of trees 

 or grass in any part of the world from destruction 

 by cooling down to near the temperature of space, 

 in ten minutes after the sun sets ? Answer : 

 clouds ; or if not clouds, wind; or if neither clouds 

 nor wind, dew and ground mist ; clouds by moderat- 

 ing or annulling the radiation : wind by supplying 

 heat from so considerable a mass of air that no part 

 of it is cooled to the dew point : dew by the latent 

 heat of the vapour from which it is formed when 

 clouds and wind do not suffice to prevent the 

 temperature from sinking to the dew point ; and 

 ground mist (or dew in the motionless stratum 



1 That is to say the temperature shown by a thermometer held in 

 space on the night side of the earth a few thousand miles from it. 



U 2 



