AEROGRAPHER'S MATE 3 & 2 



northern continents. For these reasons, the 

 northern continents serve as manufacturing plants 

 for dry polar air. The polar air cap is no longer 

 symmetrical, but is displaced far to the south, 

 particularly over the interior of Asia. 



The large temperature difference between 

 the land and water surface, which reverses 

 between the two seasons, determines to a great 

 extent the seasonal weather patterns. 



You will note in figure 13-1 that in the 

 Northern Hemisphere, the temperature gradient 

 is greater in winter than in summer, and is 

 more regular in winter. Note also how the iso- 

 therms are more closely spaced and parallel 

 in winter. In the Southern Hemisphere, the 

 temperature gradient does not have as great a 

 seasonal change as it does in the Northern 

 Hemisphere. This is due to the unequal distri- 

 bution of land and water on the two hemispheres. 

 Since the Southern Hemisphere has less land 

 and more water surface than the Northern 

 Hemisphere, the change due to the greater water 

 surface is less with consequently more nearly 

 uniform isotherms. Too, the continents of the 

 Southern Hemisphere taper toward the poles 

 and do not extend to as high a latitude as in 

 the Northern Hemisphere. 



The nature of the surface affects the local 

 heat distribution. Color, texture, and vegetation 

 influence the rate of heating and cooling. 

 Generally, dry surfaces heat and cool faster 

 than moist surfaces. Plowed fields, sandy 

 beaches, and paved roads become hotter than 

 surrounding meadows and wooded areas. During 

 the day, air is warmer over a plowed field 

 than over a forest or swamp; during the night 

 the situation is reversed. 



The distribution of water vapor and cloudi- 

 ness is another important factor influencing air 

 temperature. Although areas with a high per- 

 centage of cloudiness have a high degree of 

 reflectivity, the energy which is not reflected 

 is easily trapped in the lower layers due to the 

 greenhouse effect. Thus, it must be expected 

 that areas of high annual moisture content will 

 have relatively high annual temperature. 



Influence on Air Circulation. — The higher 

 mean temperature of the Northern Hemisphere 

 is not only an effect of its greater land cover, 

 but the oceans are also warmer than in the 

 Southern Hemisphere. This is partly due to the 

 movement of warm equatorial waters from the 



Southern Hemisphere into the Northern Hemi- 

 sphere, caused by the southeast trades which 

 cross the Equator. Another factor conducive 

 to higher mean temperatures in the Northern 

 Hemisphere is the partial protection of its 

 oceans from cold polar waters and Arctic ice 

 by land barriers. There is no such barrier 

 between the Antarctic region and the southern 

 oceans. 



TOPOGRAPHY. — Over land climates may 

 vary radically within very short distances 

 because of land forms and the variations with 

 altitudes. In this section we will discuss these 

 two general effects. 



Altitude. — The height of an area above sea 

 level exerts a considerable influence on its 

 climate. For instance, a place located on the 

 Equator in the high Andes of South America 

 would have a climate quite different from a 

 place located a few feet above sea level at 

 the same latitude. 



A change of all climatic values is observed 

 as a function of elevation. 



Mountain Barriers. — A powerful influence 

 on climates is mountainous terrain, especially 

 the long high chains of mountains that act as 

 climatic divides. These obstacles deflect the 

 tracks of cyclones and block the passage of 

 air masses in the lower levels. If the pressure 

 gradients are strong enough to force the air 

 masses over the mountains, the forced ascent 

 and descent will modify the air masses to a 

 great extent, thus modifying the clims.te on 

 both the windward and leeward sides. 



The orientation of the mountain range may 

 block certain air masses and keep them from 

 getting to the lee side of the mountains. For 

 example, the Himalayas and the Alps, with an 

 east-west orientation, prevent fresh polar air 

 masses from advancing southward. Therefore, 

 the climates of India and Italy are warmer in 

 winter than other locations of the same latitude. 

 The coastal ranges of mountains in North 

 America, running in a north-south line, prevent 

 the passage of unmodified maritime air masses 

 to the lee side. 



Probably the most noted effect of mountains 

 is the distribution of precipitation. The values, 

 level for level, are much higher on the wind- 

 ward side. 



In regions where a westerly circulation is 

 predominant, the amounts of precipitation 

 increase more or less uniformly up to the 



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