THE MOISTURE OF THE AIR 261 



may be true even in a desert. The explanation is as follows: 

 The air in the ground has some moisture, and during the day, when 

 the sun shines, this air is warmed. At night the air above cools 

 much more quickly than that in the ground. The cooler, heavier 

 air above then sinks into the ground, crowding up the warmer air 

 below with its water vapor. On reaching the cool pan, or other 

 object, some of the moisture is condensed. In the daytime the 

 rising moisture does not condense on the pan, because the pan 

 is warmer than the water vapor below, especially if the sun is 

 shining. The water vapor in the soil also diffuses upward, even 

 when not crowded up by the sinking of heavier air. 



Rain-making. Various attempts have been made to produce 

 rain, by means which may be called artificial. The methods tried 

 have been various, but the results have been unsuccessful always. 

 The plan most tried has been that of producing explosions of one 

 sort or another in the air well above the land. If there were cloud 

 particles in abundance in the air, such disturbances might perhaps 

 have the effect of causing them to unite, and so to become large 

 enough to fall; but the amount of rainfall which can be thus pro- 

 duced, under the most favorable conditions, is probably too small 

 to be of consequence. Other methods which have been tried or 

 suggested seem equally useless. 



Summary. The air is constantly taking up moisture from all 

 moist surfaces. This moisture, in the form of invisible vapor, is 

 diffused and blown everywhere. When it reaches a temperature 

 which is low enough (the dew-point), the moisture is condensed. 

 If it condenses in the upper air, it may fall as rain or snow, or it 

 may remain suspended in the air in the form of a cloud, and be 

 evaporated again. If it condenses on the surface of solid objects 

 at the bottom of the atmosphere, it forms dew or frost. Water 

 vapor is thus in constant circulation. Some of the water which 

 is precipitated out of the atmosphere falls on the surface from 

 which it was evaporated, but much of it falls in places far distant 

 from those whence it was evaporated. 



