116 FIRST YEAR COURSE IN GENERAL SCIENCE 



piece of paper; viz., the vibration of air particles that have 

 been disturbed. 



After a shower, the atmosphere is usually free from 

 clouds, and hence the earth is cooled by rapid radiation of 

 heat. It is cooled also by the evaporation of rain water, 

 for the heat required in evaporation is taken from the sur- 

 face of the ground. From one or both of these causes, it is 

 often true that "a shower cools the air." 



126. Climate. Climate may be said to be the average 

 of weather for a long series of years. It is not correct to 

 say that the climate is much more severe in a given place this 

 year than last year, for it requires many years of observa- 

 tion to determine the climate of a place. It could properly 

 be said that a winter was severe or a season rainy. A 

 drouth may prevail one year, but the next year conditions 

 may be normal or there may be an excess of rain. 



A single word is not sufficient to describe climate. The 

 climate may be hot and dry with certain prevailing winds, 

 or cold and dry with some other winds. It is customary 

 to speak of an island climate, or a coast or inland cli- 

 mate, according to the situation of a place, because various 

 physical conditions have an important effect on climate. 



Variations in temperature, rainfall, and wind cause all the 

 differences in climate. 



127. Conditions Affecting Temperature. There are 

 three conditions which determine the temperature of a place. 

 The first condition we are accustomed to call " distance from 

 the equator," as if the equator were the source of heat. 

 The source of most of the earth's heat is, of course, the sun, 

 and because of the nearly spherical shape of the earth dif- 

 ferent latitudes receive different quantities of heat. Where 

 the sun is directly overhead, as it is somewhere in the torrid 

 zone all the time, every square mile of the earth receives more 

 heat than an equal area where the rays are slanting. At 

 the tropic of Cancer on June 21, the heat is received ver- 



