154 A YEAR IN SCIENCE 



On December 21 we have our shortest day. The days 

 then gradually become longer until on March 21 the 

 nights and days are again equal. 



The hottest days in our latitude do not come in June 

 at the time when the sun's rays are most nearly vertical. 

 During the long days at that time more heat is received 

 by the earth than is given off during the short nights. 

 There is thus a gradual accumulation of heat, and our 

 hottest days come later in the summer. During the fall 

 the earth slowly loses more heat than it receives, but it 

 is some time after our shortest day, December 21, that we 

 have our coldest weather. 



The change of seasons and the difference in the length 

 of day and night vary greatly with the latitude. In the 

 temperate regions there are four seasons, summer, fall, 

 winter, and spring. In the polar regions the difference 

 is chiefly a matter of daylight and of darkness. Seasons 

 differ but little in the tropics because the days and the 

 nights are always nearly equal, and the sun's rays are 

 nearly vertical all the time. 



All the variations which are due to day and night, 

 seasons, rainfall, winds, etc., are of great importance 

 to the life of man and to human industries. The dis- 

 tribution of life is largely the result of climate. 



Weather maps. Weather maps are prepared every 

 day by the United States Weather Bureau, which is a 

 part of the Department of Agriculture. Telegrams 

 are received daily at the Weather Bureau from sta- 

 tions in different parts of the country. Each telegram 



