220 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



sun's rays would strike down upon us oppressed with an air cloudless 

 and saturated, and all objects would be perpetually streaming with 

 moisture. An approach to such a state of things sometimes actually 

 occurs on high mountains when the air is saturated and at the same 

 time remarkably free from dust. 



Clouds are often caused and maintained by mixture of winds or cur- 

 rents at different temperatures, the colder current reducing the tem- 

 perature of the other below the dew point. Such clouds may be very 

 wide in extent, but are not often dense, except in sudden and violent 

 disturbances. 



Radiation from a stratum of highly vaporous air may produce a 

 cloud, and, when once formed, every cloud which has a clear sky above 

 it radiates strongly and tends to maintain its existence by the conse- 

 quent deposition of vapor upon its particles which it induces. The 

 intensity of radiation into space depends largely on the dryness of the 

 air above; and since dryness increases rapidly with height, the radia- 

 tion from a high cloud is much more rapid than from a low one. Other- 

 wise high clouds would dissolve much faster than they do in the rather 

 dry air about them. If the heat of the sun's rays falling upon a cloud 

 exceeds the loss by radiation, the cloud diminishes in bulk and density. 

 Thus a fog frequently dissipates toward the middle of the day. But 

 the farther the fog or cloud lies from the surface of the earth, the less 

 is the heating effect of the sun, for loss by radiation proceeds faster 

 and is not compensated by terrestrial warmth. 



Sometimes, but rarely, cumulus clouds may be seen to precipitate 

 fine rain suddenly, about sunset, owing to the sudden, uncompensated 

 loss of heat by radiation. The appearance may be compared to a veil 

 suddenly let fall which does not reach the ground. An example of this 

 phenomenon occurred in the south of England on April 13, 1894. 



The edges of clouds are always changing, and, in fact, a cloud is in 

 constant process of formation and solution. Sometimes, especially 

 in fine weather, or with a strong wind, the edges are hard, rounded, 

 and well marked. This may be owing to a property which has recently 

 been discovered to belong to aggregations of very small drops when 

 moderately or slightly electrified — they attract one another. The higher 

 regions of the air are strongly electric, especially in stormy weather, 

 and the particles are held in proximity by mutual attraction and by the 

 attraction of the mass of cloud. 



Fog and clouds of a stratiform character, and cumulus clouds, and 

 cirrus may commonly exist without rain, and in most countries there are 

 many days in the year wholly overcast but rainless. This happens most 

 often in quiet and uniform conditions of weather. There is no strong 

 disturbance in the upper air ; horizontal currents of somewhat differing 

 temperature give rise to a stratum of cloud about their borders, and 

 this soon evaporates when carried into the drier air above or falling 

 into the warmer air below. Cumulus may often be seen to sink and 



