The underlying cause of wind is unequal heating of Earth’s surface. In tropical regions, more solar radiation is received than is radiated back to space. In polar regions, the opposite is true: less solar energy is received than is lost. The atmosphere acts as a giant heat-transfer system, moving warm air poleward and cool air equatorward, in effect balancing temperature differences. On a smaller scale, but for the same reason, ocean currents also contribute to this global heat transfer. The general circulation of the atmosphere, called global circulation, is complex, and a great deal has yet to be explained. We can, however, develop a general understanding by first considering the global circulation that would occur on a nonrotating Earth having a uniform surface. We will then modify the generalization to fit observed patterns.

On a hypothetical nonrotating planet with a smooth surface of either all land or all water, two large thermally produced cells would form (▼). The heated equatorial air would rise until it reached the tropopause, which acts like a lid and deflects the air poleward. Eventually, this upper-level airflow would reach the poles, sink, spread out in all directions at the surface, and move back toward the equator. Once there, it would be reheated and start its journey over again. This hypothetical circulation system has upper-level air flowing poleward and surface air flowing equatorward.

If we add the effect of rotation—the Coriolis effect—this simple convection system will break down into smaller cells. The figure below (▼) illustrates the three pairs of cells proposed to carry on the task of heat redistribution on a rotating planet. The polar and tropical cells retain the characteristics of the thermally generated convection described earlier. The nature of the midlatitude circulation is more complex and will be discussed in more detail later in this chapter.

Idealized Global Circulation

Near the equator, the rising air is associated with the pressure zone known as the equatorial low. This region of ascending moist, hot air is marked by abundant precipitation. Because this region of low pressure is a zone where winds from the north and south converge, it is also referred to as the intertropical convergence zone (ITCZ). As the diverging upper-level flow from the equatorial low reaches 20° to 30° latitude, north or south, it sinks back toward the surface. This subsidence and associated adiabatic heating produce hot, arid conditions. The center of this zone of subsiding dry air is the subtropical high, which encircles the globe near 30° latitude, north and south (refer to ▲). The great deserts of Australia, Arabia, and Africa exist because of the stable, dry condition caused by the subtropical highs.

At the surface, airflow is outward from the center of the subtropical high. Some of the air travels equatorward and is deflected by the Coriolis effect, producing the reliable trade winds. The remainder travels poleward and is also deflected, generating the prevailing westerlies of the midlatitudes. As the westerlies move poleward, they encounter the cool polar easterlies in the region of the subpolar low. The interaction of these warm and cool winds produces the stormy belt known as the polar front. The source region for the variable polar easterlies is the polar high. Here, cold polar air is subsiding and spreading equatorward.

In summary, this simplified global circulation is dominated by four pressure zones. The subtropical and polar highs are areas of dry subsiding air that flows outward at the surface, producing the prevailing winds. The low-pressure zones of the equatorial and subpolar regions are associated with inward and upward airflow accompanied by clouds and precipitation.

Influence of Continents

Up to this point, we have described the surface pressure and associated winds as continuous belts around Earth. However, the only truly continuous pressure belt is the subpolar low in the Southern Hemisphere, where the ocean is uninterrupted by landmasses. At other latitudes, particularly in the Northern Hemisphere, where landmasses break up the ocean surface, large seasonal temperature differences disrupt the pattern. ▼ shows the resulting pressure and wind patterns for January and July. The circulation over the oceans is dominated by semipermanent cells of high pressure in the subtropics and cells of low pressure over the subpolar regions. The subtropical highs are responsible for the trade winds and westerlies, as mentioned earlier.

The large landmasses, on the other hand, particularly Asia, become cold in the winter and develop a seasonal high-pressure system from which surface flow is directed off the land (refer to ▲A). In the summer, the opposite occurs: The landmasses are heated and develop a low-pressure cell, which permits air to flow onto the land (refer to ▲B). These seasonal changes in wind direction are known as the monsoons. During warm months, areas in Asia, such as India, experience a flow of warm, water-laden air from the Indian Ocean, which produces the rainy summer monsoon. The winter monsoon is dominated by dry continental air. A similar situation exists, but to a lesser extent, over North America with monsoons impacting areas of Mexico and the southwest United States in the months of June to September.

In summary, the general circulation is produced by semipermanent cells of high and low pressure over the oceans and is complicated by seasonal pressure changes over land.

The Westerlies

The circulation in the midlatitudes, the zone of the westerlies, is complex and does not fit the convection system proposed for the tropics. Between 30° and 60° latitude, the general west-to-east flow is interrupted by the migration of cyclones and anticyclones. In the Northern Hemisphere, these cells move from west to east around the globe, creating an anticyclonic (clockwise) flow or a cyclonic (counterclockwise) flow in their area of influence. A close correlation exists between the paths taken by these surface pressure systems and the position of the upper-level airflow, indicating that the upper air steers the movement of cyclonic and anticyclonic systems.

Among the most obvious features of the flow aloft are the seasonal changes. The steep temperature gradient across the middle latitudes in the winter months corresponds to a stronger flow aloft. In addition, the polar jet stream fluctuates seasonally such that its average position migrates southward with the approach of winter and northward as summer nears. By midwinter, the jet core may penetrate as far south as central Florida.

Because the paths of low-pressure centers are guided by the flow aloft, we can expect the U.S. southern states to experience more of their stormy weather in the winter season. During the hot summer months, the storm track is across the northern states, and some cyclones never leave Canada. The northerly storm track associated with summer also applies to Pacific storms, which move toward Alaska during the warm months, thus producing an extended dry season for much of the west coast. The number of cyclones generated is seasonal as well, with the largest number occurring in the cooler months, when the temperature gradients are greatest. This fact is in agreement with the role of cyclonic storms in the distribution of heat across the midlatitudes.

• If Earth’s surface were uniform, four belts of pressure oriented east to west would exist in each hemisphere. Beginning at the equator, the four belts would be the (1) equatorial low, also referred to as the intertropical convergence zone (ITCZ), (2) subtropical high at about 25° to 35° on either side of the equator, (3) subpolar low, situated at about 50° to 60° latitude, and (4) polar high, near Earth’s poles.

• Particularly in the Northern Hemisphere, large seasonal temperature differences over continents disrupt the idealized, or zonal, global patterns of pressure and wind. In winter, large, cold landmasses develop a seasonal high-pressure system from which surface airflow is directed off the land. In summer, landmasses are heated, and a low-pressure system develops over them, which permits air to flow onto the land. These seasonal changes in wind direction are known as monsoons.

• In the middle latitudes, between 30° and 60° latitude, the general west-to-east flow of the westerlies is interrupted by the migration of cyclones and anticyclones. The paths taken by these cyclonic and anticyclonic systems are closely correlated to upper-level airflow and the polar jet stream. The average position of the polar jet stream, and hence the paths followed by cyclones, migrates southward with the approach of winter and northward as summer nears.

• equatorial low: A belt of low pressure that lies near the equator and between the subtropical highs.

• intertropical convergence zone (ITCZ): The zone of general convergence between the Northern and Southern Hemisphere trade winds in the area of the equatorial low.

• monsoons: Seasonal reversals of wind direction associated with large continents, especially Asia. In winter, the wind blows from land to sea creating dry continental air; in summer, the wind blows from sea to land bringing rainy conditions.

• polar easterlies: In the global pattern of prevailing winds, winds that blow from the polar high toward the subpolar low.

• polar front: The stormy frontal zone separating air masses of polar origin from air masses of tropical origin.

• polar high: Areas of high atmospheric pressure due to descending cold air near the North and South Poles; the source region of polar easterlies.

• subpolar low: Low pressure located at about the latitudes of the Arctic and Antarctic Circles. In the Northern Hemisphere the low takes the form of individual oceanic cells; in the Southern Hemisphere there is a deep and continuous trough of low pressure.

• subtropical high: High pressure belt located at about 23.5 degrees North and South latitude.  The location of the subtropical high coincides with some of Earth's major deserts, including the Sahara, Arabian, Kalihari, and the Australian Outback.

• trade winds: Two belts of winds that blow almost constantly from easterly directions and are located on the equatorward sides of the subtropical highs.

• westerlies: Prevailing winds with a west-to-east motion that characterizes the regions on the poleward side of the subtropical highs.