34 GENERAL SCIENCE 



vapor at 32 F. weighs 2.113 grams (troy); at 70 F. it weighs 

 7.980 grams, and at 80 F. 10.933 grams. 



If the atmosphere is suddenly cooled from 80 to 70 about 

 3 grams of water will condense out of every cubic foot of saturated 

 air. This change of temperature of saturated air is the primary 

 cause of rain. 



It has been estimated that in a cubic inch of smoke there are as many 

 as 12,000,000,000 nuclei around which moisture may collect. It is possible 

 that every rain drop is built around a nucleus, or center of condensation. 

 From some observations in England and at Pittsburg it was found that from 

 500 to 1900 tons of dirt per square mile were brought down by rain in one 

 year. 



Effects of Altitude. The temperature decreases about 1 F. 

 for every 300 feet of ascent. There is less ground to radiate heat 

 into the upper layers of atmosphere, and the warm air which comes 

 up from the earth in the valleys cools very rapidly when it reaches 

 the upper strata of air. The top of a mountain 15,000 feet high, at 

 the equator, would be about 50 cooler than the land at its foot. 



Rain. Clouds increase when more vapor is turned into mist or 

 fog. Minute drops crowd together, making larger drops which again 

 join with others, making still larger drops. This can be illus- 

 trated by the dew experiment which was described on page 21. 

 First, mist is formed, then tiny drops appear, then larger drops begin 

 to run down the can. The air cannot hold up the large drops, and 

 they fall to the ground as a shower of rain. Air in circulation can 

 support a heavier weight than air which is still. Heavy winds will 

 keep rain particles up in the air until they become very large. This 

 is the cause of the difference in size of raindrops. 



Rain, briefly speaking, is caused by the chilling of air which 

 contains a certain amount of moisture. This chilling may take place 

 either through the rising of air into higher and colder levels from 

 its meeting a colder current of air, or through its contact with colder 

 surfaces, such as mountains. Rain is often caused by the rushing 

 of air from the low land up over the mountain. Some of the 

 heaviest rainfalls take place on mountains near the sea. The air 

 over the ocean gets thoroughly soaked with vapor which, while 

 warmed, it can carry. Then it suddenly comes up against a moun- 



