THE ATMOSPHERE, WEATHER, AND CLIMATE 



25 



tains can be much more influential. Especially when 

 mountains are high, foehn winds can develop and 

 cause severe aridity in the area of rain shadow. 



The consequences of continental distribution are 

 not easily seen. Land masses display an annual 

 cycle (related to the changes in angles of solar rays) of 

 modifying air masses. Because air masses are instru- 

 mental in climate, the continental modification of air 

 masses can and does contribute to aridity. 



CLIMATE PRODUCTION 



Climates are mainly the result of solar heat reach- 

 ing the earth in variable quantities that are dis- 

 tributed according to latitude and season. The lati- 

 tudinal gradation of heat was already shown to be 

 caused by poleward increase in the angle of solar 

 rays striking the earth. The seasonal differences 

 are mostly due to the annual cycle of local change in 

 the angle of solar rays. In general, throughout the 

 year the rays striking the equator are more direct 

 than those striking the more poleward latitudes. 

 Therefore, the equator receives the greatest amount 

 of heat and there is progressive heat reduction toward 

 the poles. 



On the other hand, because air movement and 

 ocean currents mix the heat from difTerent areas, the 

 poles are not as cold as they might be, nor is the 

 equator as warm as it might be. Further curbing of 

 possible temperature extremes comes from the var- 

 iable composition of air. For example, a greater con- 

 centration of water and carbon dioxide in the at- 

 mosphere results in increased heat retention. Also, 

 any increase in clouds or dust in the air will tend to 

 shade the underlying land and, therefore, lessen the 

 amount of heat received at the surface. (In this re- 

 spect, overcast or volcanic dust can seriously restrict 

 heat reaching a particular locality.) Therefore, there 

 is a tendency for restriction of heat in potentially 

 hotter places and transportation of heat to potentially 

 colder areas. In spite of these local variations and 

 tendencies to reduce pole-equator temperature ex- 

 tremes, about two and one-half times as much heat 

 reaches the equator each year as reaches either pole. 

 These extremes and the latitudinal zonation between 

 the extremes largely account for climatic differences. 



The effects of air and water currents are influenced 

 by topography. This is best appreciated by com- 

 parison of the effects of ocean currents upon coastal 

 and interior climates. Because of the properties of 

 water, coastal areas are less subject to climatic 



extremes than are interior areas. This differential 

 effect is further influenced by the distribution of 

 elevations and depressions, that is, the relief of the 

 land. 



The local reception of heat is affected by many 

 things. Forty-two per cent of solar heat is lost to 

 space. About 43 per cent reaches the surface of the 

 earth, and 15 per cent is absorbed by the atmosphere. 

 However, none of the 58 per cent trapped by the 

 earth really remains. If it did, the earth would con- 

 stantly increase in temperature. Because tempera- 

 ture is relatively constant from year to year, the 58 

 per cent is lost to space just as fast as new solar heat 

 is absorbed. During the warm season, there is (for a 

 while at least) greater heat retention than loss; but 

 with the coming of winter, loss is greater than reten- 

 tion. The general exception to this seasonal heat 

 variation is found at the equator, where temperature 

 is fairly constant. However, as one progresses toward 

 the poles, seasonal differences in temperature in- 

 crease. 



The loss of the 58 per cent of solar heat that 

 briefly remains at the surface of the earth and in the 

 atmosphere does not necessarily involve a direct loss 

 to space. The 43 per cent usually reaching the sur- 

 face is never completely absorbed. In most cases, 15 

 per cent to 20 per cent of the solar heat is immediately 

 lost to the air, but a greater or lesser percentage can 

 be lost at once. Large heat loss results from such 

 things as ice reflection; small loss results from the 

 absorption of heat by water, black soil, and dry vege- 

 tation. However, absorption need not be related to 

 retention. For example, the ocean does not lose its 

 heat readily, but black soil does. Eventually, how- 

 ever, surface heat is lost to the atmosphere and at- 

 mospheric heat to outer space. Again, this loss need 

 not be direct. Original air heat can be transported 

 to the surface and surface heat to the air, to be cycled 

 back again to the surface. Still, as stated before, on a 

 worldwide basis, there is (except for seasonal varia- 

 tion) a balance between solar heat input to atmos- 

 phere and surface, and heat output to space by at- 

 mosphere and surface. Because of air and ocean cur- 

 rents, input in one area often causes heat to be moved 

 to another site prior to its output. 



CLIMATIC DISTRIBUTION 



The fact that climate is graded geographically and 

 tends to display regular changes with latitude and 



