Gration of the suns vertical rays and the subtropical

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gration of the sun’s vertical rays, and the subtropical highs of the Southern Hemisphere lie slightly north of their January locations. In examining pressure systems at Earth’s surface, we have seen that there are essentially seven belts of pressure (two polar highs, two subpolar lows, two subtropical highs, and one equatorial low), which are broken into cells of pressure in some places primarily because of the influence of certain large landmasses. We have also seen that these belts and cells vary in size, intensity, and location with the seasons and with the migration of the sun’s vertical rays over Earth’s surface. Since these global-scale pressure systems mi- grate by latitude with the position of the direct sun angle, they are sometimes referred to as semipermanent pressure systems because they are never permanently fixed in the same location. Global Surface Wind Systems The planetary, or global, wind system is a response to the global pressure patterns and also plays a role in the maintenance of those same pressures. This wind system, which is the major means of transport for energy and moisture through Earth’s atmosphere, can be examined in an idealized state. To do so, however, we must ignore the influences of landmasses and seasonal variations in solar energy. By assuming, for the sake of discussion, that Earth has a homogeneous surface and that there are no seasonal variations in the amount of solar energy received at different latitudes, we can examine a theoretical model of the atmosphere’s planetary circu- lation. Such an understanding will help explain specific features of climate such as the rain and snow of the Sierra Nevada and Cascade Mountains and the existence of arid regions farther to the east. It will also account for the movement of great surface currents in our oceans that are driven by this atmospheric engine. Idealized Model of Atmospheric Circulation Because winds are caused by pressure differences, various types of winds are associated with different kinds of pressure sys- tems. Therefore, a system of global winds can be demonstrated using the model of pressures that we previously developed (see again Fig. 5.10). The characteristics of convergence and divergence are very important to our understanding of global wind patterns. Surface air diverges from zones of high pressure and converges on areas of low pressure. We also know that, because of the pressure gradient, surface winds always blow from high pressure to low pressure. Knowing that surface winds originate in areas of high pres- sure and taking into account the global system of pressure cells, we can develop our model of the wind systems of the world ( Fig. 5.12). This model takes into account differential heating, Earth rotation, and atmospheric dynamics. Note that the winds do not blow in a straight north–south line. The variation is due of course to the Coriolis effect, which causes an apparent deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

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