50 AUTUMN AND WINTER 



frosty. Fog is favoured by calm anticyclonic weather in 

 other ways. The best conditions for its formation are when 

 a cold current drifts slowly over a moist and warmer layer 

 next to the earth ; and this is just what happens when the 

 light, cold airs of a winter's anticyclone wander over the 

 damp surface of the soil. Strong wind almost always dis- 

 sipates fog, rain carries the condensed particles of moisture 

 to the earth, and moderately warm air does not cause con- 

 densation. Strong wind and rain and moderately warm air 

 are all forthcoming in spells of typical cyclonic weather ; and 

 thus we find by experience that a wet autumn is generally 

 not a foggy one, or at any rate, that the wet and foggy periods 

 do not coincide. By another apparent paradox, the stormiest 

 winter weather is generally also the sunniest. Stormy 

 weather has spells of clear limpid sunshine between the 

 storms ; and in spite of the brilliance of bright frosty days, 

 the total amount of sunshine in a wet and warm winter is 

 generally much greater than in a dry and frosty one. In our 

 climate, frost and sunshine seldom persist together for very 

 long ; if it is calm, on creeps the fog, and if there is a strong 

 wind, it brings either snow or rain. 



It has been established by experiment that the particles of 

 fog can only form round a nucleus provided by some form 

 of floating dust, or of some of the gaseous products of the 

 combustion of coal or wood. This supplies one very good 

 reason for the density of fogs in large towns, with their 

 myriad chimneys, and the ceaseless grinding and shaking of 

 machinery and heavy traffic. But it is equally true of the 

 clearest country air, and gives a remarkable indication of 

 the abundance of minute floating particles in the earth's 

 atmosphere. Many of these particles are mineral, consist- 

 ing of dust carried up by storms or ejected by volcanoes ; 

 others are vegetable, including grains of pollen shed from 



