Weather 119 



influenced and determined the development of a country, and this 

 supply is controlled largely by the weather which prevails there. 



You have already studied about two forms of matter which 

 are closely related to the weather. These are air and water. Your 

 knowledge of them should help you to understand some of the 

 causes that produce changes in weather. 



Causes of Weather. The primary cause of weather condi- 

 tions is the constant change* in the amount of heat in the atmos- 

 phere. The heat energy which comes from the sun passes off 

 into space in vast amounts and greatly affects both air and 

 moisture. In acting on these it is transformed into the energy of 

 motion and causes rain, wind, tornadoes, hurricanes, waterspouts 

 and similar phenomena. Other causes that modify the weather 

 are the revolution of the earth around the sun, causing the change 

 of seasons ; the daily rotation of the earth on its axis, causing day 

 and night alternately; the fact that land and water are not uni- 

 formly distributed on the surface of the earth ; the variation of the 

 sunlight in different latitudes, and the fact that the atmosphere is 

 subject to changes of pressure as well as of temperature. 



Climate. The climate of any region is the average of its 

 weather conditions over a period of many years. We speak of 

 climate as dry climate, hot desert climate, hot rainy climate, damp 

 ocean climate or changeable climate. 



Zones. The earth's surface is divided into five zones of heat : 

 the torrid zone, which is the hottest ; the north and the south tem- 

 perate zones, which have a moderate temperature ; and the north 

 and the south frigid zones, which are the coldest. The differences 

 in the heat of these zones are caused by differences in the angle at 

 which the sun's rays strike the earth. The boundaries between 

 these zones are indicated by parallels of latitude, but their actual 

 limits are not so regular and definite as this seems to indicate ; for 

 example, some parts of the frigid zones have as high temperatures 

 as the temperate zones, as in certain sections of Alaska ; and some 

 parts of the temperate zones have summer climates that are very 

 hot, as in the very dry sections of southwestern United States. 



Causes of Variations. The causes of variations of climate in 

 the same zone may be due to winds, to the altitude of the regions, 



