Page 189 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 6 atmospheric and Oceanic Circulations 153
characteristics of these pressure areas. We now ex­ amine each principal pressure region and its as­ sociated winds, all illustrated in Geosystems in
Action, Figure GIA 6.
L
Converging winds
H
Diverging winds
Equatorial Low or ITCZ: Warm and Rainy Constant high Sun altitude and consistent daylength (12 hours a day, year­round) make large amounts of energy available in the equato­ rial region throughout the year. The warming associated with these energy surpluses cre­ ates lighter, less­dense, as­ cending air, with surface winds converging along the entire extent of the low­pressure trough. This converging air is extremely moist and full of latent heat energy. As the air rises, it expands and cools, producing condensation; consequently, rainfall is heavy throughout this zone (condensation and precipitation are discussed in Chapter 7). Vertical cloud columns frequently reach
the tropopause, in thunderous strength and intensity. The equatorial low, or equatorial trough, forms the intertropical convergence zone (ITCZ), which is identi­ fied by bands of clouds along the equator and is noted on Figure 6.10 and Figure GIA 6.1a as a dashed line. In January, the zone crosses northern Australia and dips
southward in eastern Africa and South America.
Figure GIA 6.2 shows the band of precipitation asso­ ciated with the ITCZ on January and July satellite images; precipitation forms an elongated, undulating narrow band that is consistent over the oceans and only slightly interrupted over land surfaces. Note the position of the ITCZ in Figure 6.10 and compare this with the precipi­ tation pattern captured by the TRMM (Tropical Rainfall
Measuring Mission) sensors in Figure GIA 6.2.
Trade Winds The winds converging at the equatorial low are known generally as the trade winds, or trades. Northeast trade winds blow in the Northern Hemisphere and southeast trade winds in the Southern Hemisphere. The trade winds were named during the era of sailing ships that carried merchandise for trade across the seas. These are the most consistent winds on Earth.
Figure GIA 6.1b shows circulation cells, called Hadley cells, in each hemisphere that begin with winds rising along the ITCZ. These cells were named for the eighteenth­century English scientist who described the trade winds. Within these cells, air moves northward and southward into the subtropics, descending to the surface and returning to the ITCZ as the trade winds. The sym­ metry of this circulation pattern in the two hemispheres is greatest near the equinoxes of each year.
▲Figure 6.9 High- and low-pressure cells and associated wind movement. Side view of high- and low-pressure cells over the United States. note surface winds spiraling clockwise out of the high-pressure area toward the low pressure, where winds spiral counterclockwise into the low.
are suggested by the isobars. The high­ and low­pressure areas of Earth’s primary circulation appear on these maps as cells or uneven belts of similar pressure that are interrupted by landmasses. Between these areas flow the primary winds. The highs and lows of Earth’s secondary circulation form within these primary pressure areas, ranging in size from a few hundred to a few thousand kilometres in diameter and hundreds to thousands of metres in height. The systems of secondary circulation seasonally migrate to produce changing weather patterns in the regions over which they pass.
Four broad primary pressure areas cover the North­ ern Hemisphere, and a similar set exists in the Southern Hemisphere. In each hemisphere, two of the pressure areas are stimulated by thermal (temperature) factors. These are the equatorial low (marked by the ITCZ line on the maps) and the weak polar highs at the North and South poles (not shown, as the maps are cut off at 80° N and 80° S). Remember from our discussion of pressure, density, and temperature earlier in the chapter that warmer air is less dense and exerts less pressure. The warm, light air in the equatorial region is associated with low pressure, while the cold, dense air in the polar re­ gions is associated with high pressure. The other two pressure areas—the subtropical highs (marked with an H on the map) and subpolar lows (marked with an L)— are formed by dynamic (mechanical) factors. Remember in our discussion of pressure gradients that converging, rising air is associated with low pressure, whereas sub­ siding, diverging air is associated with high pressure— these are dynamic factors because they result from the physical displacement of air. Table 6.1 summarizes the