RAIN AND SNOW. 637 



All animals and plants depend upon moisture in the atmosphere as 

 much as upon the varying degrees of warmth. A dry east wind effects us 

 all prejudicially ; warm, soft airs influence us again in other ways. Air will 

 be found drier as a rule in continents than in islands or maritime districts, 

 and this will account for the clearness of the sky in continental regions. 

 Fogs and mists arise when the air is what is termed saturated with moisture, 

 and colder than the earth or waters upon it. So the celebrated and dangerous 

 fogbanks of Newfoundland arise from the warm water of the Gulf Stream, 

 which is higher in temperature than the air already saturated. And the 

 same effect is produced when a warm wind blows against a cold mountain ; 

 the air is cooled, and condenses in cloud. 



The cooling of the breath by the exterior air is exemplified in winter 

 when we can perceive the vapour issuing from our mouths as we speak. 



Fig- 719- Storm clouds. 



. RAIN, SNOW, AND DEW. 



Rain is produced by the condensation of vapour. " Vesicular vapours, 

 or minute globules of water filled with air," compose the clouds, and at 

 last these vesicles form drops, and get heavy enough to come to the ground 

 Perhaps they are not sufficiently heavy to do so, and then they are absorbed 

 or resolved into vapour again before they can get so far, because the lower 

 strata of air are not yet saturated, and can therefore contain more moisture. 



On the other hand, we may experience rain from a cloudless sky. This 

 is no very uncommon case, and occurs in consequence of the disturbance of 

 the upper strata when warm and cold currents come into collision and 

 condense the vapours. 



Rain is very unequally distributed. We shall find that the region of 

 calms, which we mentioned in a former page, is also the zone of the greatest 

 amount of rain. The heated air rises and falls back again, there being little 

 or no wind to carry it away. The rainy season, therefore, sets in when a 

 place enters the zone of calms. Equatorial districts have two rainy seasons, 

 as they enter twice a year into the region of calms, but most places have 



