222 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



and conflicts between opposing winds, which occur in summer when 

 the moist warm air-mass is lifted to great heights. 



Generally, we may state the formation and amount of rain to be 

 dependent on the following conditions : 



(1) The height to which the lower air is forced upward. 



(2) The amount of vapor in the lower and upper air, respectively. 



(3) The relative coldness of the stratum into which the lower air is 

 projected. 



(4) The freedom from vapor strata and from cloud of the upper air, 

 allowing free radiation from the rain cloud. 



(5) The electrical condition of the air and cloud. 



Where mountains are high, the air warm and moist and blowing 

 toward steep slopes, very heavy rain falls either continuously at certain 

 seasons, or in thunderstorms, according to the character of the winds, 

 the heat of the sun on the earth, and to a less degree the temperature 

 of the upper air. 



The ranges of hills south of the Himalayas, the Himalayas them- 

 selves, the mountains of eastern South Africa, and the Andes give 

 examples of such effects. High mountains have the power of precipi- 

 tating as rain or snow even the rather small quantity of vapor which has 

 passed over a continent, and thus the central areas of countries remote 

 from the sea are provided with perennial fountains which flow down 

 from the high ground and pass through the land as fertilizing rivers. 



Another cause of rain is the radiation into space of the heat of vapor 

 and of water particles at a height. Recent discoveries have revealed 

 the fact that vapor does not condense into cloud globules in ordinary 

 conditions without the presence of a very fine dust which floats in the 

 atmosphere. When this dust radiates freely and moisture is deposited 

 upon it, and when a cloud is formed, the upper surface of the cloud 

 parts with more heat than the surrounding air, and the cloud globules 

 grow in size by contact with vapor. 



Now, throughout the process of increase in size, electricity is accu- 

 mulated more and more densely on their surfaces, for the electricity of 

 each molecule or particle resides on its surface, and the relative surface 

 of a globule diminishes as the size of the globule increases. If the con- 

 densation be rapid, the particles formed are very unequal in size. 

 Since surfaces only increase at half the rate of bulk, electricity is 

 much denser on the large drop. Now, it has been found by experiment 

 that large drops attract small ones when similarly electrified, and each 

 addition further increases the attractive power of the drop. The large 

 drops fall through a cloud at a much greater rate than the small par- 

 ticles and collide with many more droplets in the same time. In the 

 course of a fall of 10,000 or 15,000 feet through cloud, the drops may 

 greatly increase in size. 



The sizes of drops vary from 0.0033 inch to about 0.1 inch. An 

 ascending current of 3 miles an hour would sustain small drops; 



