AEROGRAPHER'S MATE 3 & 2 



direct rays of the sun hit the earth near the 

 Equator and cause a net gain of heat. The air 

 at the Equator heats, rises, and flows in the 

 upper atmosphere toward both poles. Upon 

 reaching the poles, it cools and sinks back 

 toward the earth, where it tends to flow along 

 the surface of the earth back to the Equator. 

 Simple circulation of the atmosphere would 

 occur as described above if it were not for 

 the following factors: 



1. The earth rotates, resulting in an apparent 

 force known as the Coriolis (or deflecting) 

 effect described in chapter 12 and also resulting 

 in constant change of the area being heated. 



2. The earth is covered by irregular land 

 and water surfaces. 



Regions under the direct rays of the sun 

 receive more heat per unit time than those 

 under oblique rays. The heat brought by the 

 slanting rays of early morning may be com- 

 pared with the heat that is caused by the slanting 

 rays of winter. The heat which is due to the 

 more direct rays of midday may be compared 

 with the heat resulting from the more direct 

 rays of summer. 



The length of day, like the angle of the sun's 

 rays, influences the temperature. The length 

 of day varies with the latitude and the season. 

 Near the Equator there are about 12 hours of 

 daylight every day in the year, and the sun at 

 noonday is always high in the sky (giving nearly 

 direct rays). Consequently, equatorial regions 

 have no pronounced seasonal temperature 

 changes. 



During the summer in the Northern Hemi- 

 sphere, all places north of the Equator have 

 more than 12 hours of daylight. During the 

 winter the situation is reversed, latitude north 

 of the Equator having less than 12 hours of 

 daylight. 



Large seasonal variation in the length of the 

 day and the seasonal difference in the angle 

 at which the sun's rays reach the earth's sur- 

 face cause seasonal temperature differences in 

 middle and high latitudes. 



The weak temperature gradient in the sub- 

 tropical areas and the steeper gradient poleward 

 can be seen in figure 13-1. Note also how much 

 steeper the gradient is poleward in the winter 

 season of each hemisphere than it is in the 

 summer season. 



PRESSURE OVER THE GLOBE 



In the previous chapter it was stated that 

 an increase in temperature causes air to expand 

 and lowers its pressure and density, and vice 

 versa. The unequal heating of the earth's sur- 

 face due to its tilt, rotation, and differential 

 insolation, results in the wide distribution of 

 pressure over the earth's surface. In figure 

 13-2, note that a low-pressure area lies along 

 the doldrums in the equatorial region. This is 

 due to the higher temperatures maintained 

 throughout the year in this region. At the poles, 

 permanent high-pressure areas remain near the 

 surface because of the low temperatures in this 

 area throughout the entire year. The subtropical 

 high-pressure areas at 30°N and S lat are 

 caused mainly by the "piling up" of air in 

 these regions. There are other areas which are 

 dominated by relatively high or low pressures 

 during certain seasons of the year. 



ELEMENTS OF CIRCULATION 



It has previously been shown how temperature 

 differences cause pressure differences. Pressure 

 differences in turn cause air movements. This 

 section and the following sections of the chapter 

 are designed to show Aerographer's Mates how 

 the air movements work and how they evolve 

 into the various circulations — general, second- 

 ary, and tertiary. 



Static Earth 



If the earth were a nonrotating sphere com- 

 posed of a uniform surface, the atmospheric 

 circulation would be relatively simple. The air 

 at the Equator would be heated and become 

 less dense, causing it to rise and expand. Due 

 to less insolation at the poles, the air would be 

 cooled and become denser, causing it to 

 descend. Therefore, if the earth were static, 

 the flow of air would be a simple circulation 

 from the poles to the Equator at the surface, 

 and from the Equator to the poles aloft. (Refer 

 to chapter 12, fig. 12-6.) 



Rotating Nonuniform Earth 



The earth is neither static nor uniform; 

 consequently, the basic wind pattern is con- 

 siderably different from the simple one described 

 above. 



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