'SEASON OF MIST' 49 



left behind in the earth by the summer sun. For many 

 weeks the store of warmth radiating from the soil tends to 

 raise the temperature of the moist air above it ; and fog 

 is formed whenever this moist air at a high temperature is 

 chilled. In winter and spring fogs are scarcer, because the 

 temperature of the earth is lower, the moist air above it is 

 consequently cooler, and therefore cannot undergo condensa- 

 tion so readily on the inflow of chilled air from elsewhere. 

 An approach to the conditions of autumn is seen again in 

 March, when in spells of bright, hot weather there are often 

 thick morning fogs like those of September or October. 

 The sun is then sufficiently powerful to warm the earth well 

 by day, so that condensation rapidly follows when its rays 

 are withdrawn. The morning mists common in autumn 

 often occur most regularly in the finest and most settled 

 weather, which at first sight seems rather a paradoxical 

 association. But in fine settled weather the sky is free 

 from upper cloud, and is as clear by night as by day ; and 

 the earth is thus deprived of the protecting blanket which 

 hinders the escape of heat by radiation, and makes cloudy 

 nights warmer than clear ones. Before morning the 

 reduction in the temperature of the earth produces con- 

 densation in the moist air above it, and a fog is produced. 

 It vanishes as the sun once more warms the moisture-laden 

 air ; but the time comes in autumn when the sun grows too 

 weak to raise its temperature to the required height, and 

 then the fog may last all day. 



The clearness of the sky and consequent activity of 

 radiation supply one reason of the greater commonness of fogs 

 in dry calm weather. In autumn and winter fogs are often 

 thickest on the plains and along the valleys on precisely 

 those days when the hill-tops are bathed in sunshine. This 

 is especially the case when the weather is not only calm but 



