Air over the equator is heated strongly by contact with the warm surface. The warm air rises all the way to the tropopause. There, trapped beneath the tropopause, it moves away from the equator. Cooler air flows toward the equator at low level, to take its place.
Moving air tends to turn in a circle. This is called vorticity and it is why the water usually forms a spiral, or vortex, when it flows out of a bathtub. As the cool air moves toward the equator its vorticity swings it to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The resulting surface winds, shown in the diagram, blow from the northeast on the northern side of the equator and from the southeast on the southern side. These are the trade winds. They are the most dependable winds on Earth.
The trade winds were well known to mariners in the days of sailing ships, and in the 17 th and 18th centuries scientists tried to discover why they are so reliable. Their calculations led them to propose what we now
call the general circulation of the atmosphere (see the sidebar "George Hadley and Hadley cells" below). This is a description of the way air moves. Its modern form, the three-cell model, is greatly simplified and quite often the air is not moving in the way it describes. Nevertheless, it provides a broadly accurate picture of the atmosphere.
Hadley believed there was just one cell in each hemisphere. In fact there are three. What he thought was a single cell covering an entire hemisphere affects only the Tropics.
Air rises over the equator. It is very moist, because oceans cover most of the equatorial region. As it rises the air cools and its water vapor condenses. This produces the heavy rainfall of the Tropics. It also releases
When European ships began venturing far from their home ports, into the Tropics and across the equator, sailors learned that the trade winds are very dependable in both strength and direction. They made use of them and by the end of the 16th century their existence was well known. Many years passed, however, before anyone knew why the trade winds blow so reliably. Like many scientific explanations, this one developed in stages.
Edmond Halley (1656-1742), an English astronomer, was the first person to offer an explanation. In 1686 he suggested that air is heated more strongly at the equator than anywhere else. The warm equatorial air rises, cold air flows in near the surface from either side to replace it, and this in-flowing air forms the trade winds. If this were so, however, the trades on either side of the equator would flow from due north and south. In fact, they flow from the northeast and southeast.
There the matter rested until 1735. In that year George Hadley (1685-1768), an English meteorologist, proposed a modification of the Halley theory. Hadley agreed that warm equatorial air rises and is replaced at the surface, but he suggested that the rotation of the Earth from west to east swings the moving air, making the winds blow from the northeast and southeast.
Hadley was right about what happened, but not about the reason for it. This was discovered in 1856 by the American meteorologist William Ferrel (1817-91), who said the swing is due to the tendency of moving air to rotate about its own axis, like coffee stirred in a cup.
In accounting for the trade winds, Hadley had proposed a general explanation for the way heat is transported away from the equator. He suggested that the warm equatorial air moves at a great height all the way to the poles, where it descends. This vertical movement in a fluid, driven by heating from below, is called a convection cell and the cell Hadley described is known as a Hadley cell.
The rotation of the Earth prevents a single, huge Hadley cell from forming. What really happens is more complicated. In various equatorial regions, warm air rises to a height of about 10 miles (16 km), moves away from the equator, cools, and descends between latitudes 25° and 30° N and S. These are the Hadley cells. When it reaches the surface in the Tropics, some of the air flows back toward the equator and some flows away from the equator.
Over the poles, cold air descends and flows away from the poles at low level. At about latitude 50° it meets air flowing away from the equatorial Hadley cells. Where the two types of air meet is called the polar front. Air rises again at the polar front. Some flows toward the pole, completing a high-latitude cell, and some flows toward the equator until it meets the descending air of the Hadley cell, which it joins.
There are three sets of cells in each hemisphere. This is called the three-cell model of atmospheric circulation, by which warm air moves away from the equator and cool air moves toward the equator.
latent heat (see the box "Latent heat and dew point" on page 32). The latent heat warms the surrounding air, causing it to continue rising.
When air and water move away from the equator they are deflected by the Coriolis effect, often abbreviated as CorF because it was once thought to be a force rather than a simple consequence of the rotation of the Earth (see the sidebar "The Coriolis effect" on page 16). The magnitude of the CorF is zero at the equator and reaches a maximum at the North and South Poles. Air that rises over the equator is unable to cross the tropopause, so the constant stream of rising air pushes it away from the equator, heading due north and south. It then becomes subject to the CorF. This deflects it, so by the time it reaches about latitude 25° N and S it is moving more from west to east than toward the pole. At the same time, the warm air is radiating its heat into space and cooling. As its temperature decreases, the density of the air increases. The two effects combine to "pile up" increasingly dense air. Eventually the air is denser than the air beneath it, and it subsides all the way to the surface. This happens in latitudes 25°-30° in both hemispheres.
Descending air is compressed and warms adiabatically (see the sidebar "Adiabatic cooling and warming" on page 34). The air lost most of its moisture during its ascent, when its temperature was decreasing. As it descends and warms, its capacity for holding moisture increases and its relative humidity—the amount of water it carries as a percentage of the amount needed to saturate it—falls. By the time it reaches the surface, the air is hot and very dry. The subsiding air produces several regions of high surface pressure, with air flowing out of them. This prevents moist air from entering and ensures that the affected regions have a dry climate. These regions comprise the belts of subtropical deserts that girdle the Earth in both hemispheres. They include the Sonoran, Sahara, Arabian, Middle Eastern, and Thar Deserts in the Northern Hemisphere and the Kalahari and Australian Deserts in the Southern Hemisphere.
Air flows away from the subtropical high-pressure regions in both directions: toward the equator and toward the pole. The air flowing toward the equator becomes the easterly trade winds and air flowing away from the equator becomes the mid-latitude westerly winds.
This equatorial and tropical circulation comprises the Hadley cells. There are several of these in each hemisphere—usually about five in winter and four in summer. They coincide with subtropical high-pressure regions that are separated by low-pressure regions.
The trade winds from the north and south meet at the Intertropical Convergence Zone (ITCZ). This is a region of low surface pressure, and it contains areas where the winds are light and often do not blow at all. These regions are called the doldrums.
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