AEROGRAPHER'S MATE 3 & 2 



First, observe the tropical cell of the 

 Northern Hemisphere which lies between the 

 Equator and 30°N lat. The air at the Equator 

 heats and rises. When it reaches the extremity 

 of the troposphere, it tends to flow toward the 

 North Pole. By the time the air has reached 

 30 °N lat, the Coriolis effect has deflected it 

 so much that it is moving eastward instead of 

 northward. This results in a piling up of air 

 near 30°N lat and a descending current of air 

 toward the surface which forms a belt of high 

 pressure. When the descending air reaches the 

 surface, part of it flows poleward to become 

 part of the midlatitude cell; the other part flows 

 toward the Equator, is deflected by the Coriolis 

 effect, and forms the northeast trades. 



The midlatitude cell is located between 30° 

 and 60°N lat. The air which comprises this 

 cell circulates poleward at the surface and 

 equatorward aloft with rising currents at 60° 

 (polar front) and descending currents at 30° 

 (high-pressure belt). However, in general, the 

 winds, both at the surface and aloft, blow from 

 the west. This is easily explained for the 

 surface wind by the Coriolis effect on the pole- 

 ward-moving surface air. The west wind aloft 

 is not so easily explained. Most authorities 

 agree that this wind is frictionally driven by 

 the west winds in the two adjacent cells. 



The polar cell lies between 60°N lat and the 

 North Pole. The circulation in this cell begins 

 with a flow of air at a high altitude toward the 

 pole. This flow cools and descends at the North 

 Pole and forms a high-pressure area in the 

 polar regions. After reaching the surface of the 

 earth, this air tends to flow equatorward and is 

 deflected by the Coriolis effect so that it moves 

 from the northeast. This air converges with 

 the poleward flow from the midlatitude cell 

 and is deflected upward with a portion circulating 

 poleward again and the remainder equatorward. 

 The outflow of air aloft between the polar and 

 midlatitude cells causes a semipermanent low- 

 pressure area at approximately 60°N lat and, 

 due to the discontinuity in temperature and 

 density of these two cells, the polar front develops 

 in this area. 



To complete the picture of the world's 

 general atmospheric circulation, it is necessary 

 that the prevailing winds and pressure belts be 

 associated with their corresponding pressure 

 belts along with some of the other basic 

 characteristics. 



World Winds 



Corresponding with the doldrums or the 

 intertropical convergence zone (ITCZ), there 

 would be a belt of relatively low pressure. 



The doldrums vary in position and tend to 

 move north and south of the Equator with the 

 sun, though more of the area is generally 

 located slightly to the north of the Equator. In 

 the region of the doldrums the temperatures 

 are high, and the wind is convergent (a net 

 inflow of air into the area), which results in 

 excessive precipitation. 



On the poleward side of the doldrums the 

 TRADE WINDS are found. Whenever the dol- 

 drums are absent in some part of the equatorial 

 region, the trade winds of the Northern and 

 Southern Hemisphere converge, causing heavy 

 rain squalls. A noticeable feature of the trade 

 wind belt is the regularity of these systems, 

 especially over the oceans. 



The wind blowing above and counter to the 

 trade wind is the ANTITRADE. 



Near 30°N and 30°S latitudes lie the sub- 

 tropical high-pressure belts. Winds are light 

 and variable. These areas are referred to as the 

 HORSE LATITUDES. Due to the descending air, 

 fair weather is often characteristic of this 

 region. The pressure decreases outward from 

 this area, both northward and southward. 



The prevailing westerlies, which are on the 

 poleward side of the trade winds, are persistent 

 throughout the midlatitudes. In the Northern 

 Hemisphere their direction at the surface is 

 from the southwest and in the Southern Hemi- 

 sphere from the northwest, due to the deflection 

 caused by the Coriolis effect as the air moves 

 poleward. 



Poleward of the prevailing westerlies, near 

 60°N and 60°S latitudes, lies the belt of low 

 pressure known as the polar front zone. Here 

 converging winds result in ascending air currents 

 and consequent poor weather. 



In the polar cells, high pressure exists 

 at low levels as the cold dense air sinks. 



Geostrophic and Gradient Winds 



On ' surface weather chart analysis, points 

 of equai pressure are connected by drawn 

 lines referred to as isobars, while in upper 

 air analysis, points of equal heights are con- 

 nected and called isoheights. 



The variation of these heights and pressures 

 from one locality to another is the initial 



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