Atmospheric circulation

Atmospheric circulation is the large-scale movement of air that distributes heat from tropical to polar latitudes across the surface of the earth. The global wind patterns are guided by three distinct convection cells—known as the Hadley cell, Ferrel cell, and Polar cell—that transport heat by circulating air at various latitudes and that extend from the earth’s surface to the upper boundary of the troposphere. The troposphere is the lowest layer of the atmosphere and extends from the earth’s surface upward to approximately 15 kilometers (9.3 miles) above the surface, where it is separated from the stratosphere by an area of temperature inversion known as the tropopause. Because the troposphere is where nearly all of the world’s weather conditions originate, it is important for scientists to understand atmospheric circulation patterns in order to more accurately predict climate conditions that can affect everything from crop production to transportation safety.

Background

English meteorologist George Hadley (1685–1768) was interested in finding out why sailors encountered westerly winds at the midlatitudes and easterly winds, known as the trade winds, closer to the equator. In 1735, Hadley described atmospheric circulation as a massive version of a huge sea breeze in which warm air rises over the equator and sinks over the poles and is moved directionally along latitudinal lines as a result of the rotation of the earth. His is regarded as the first attempt to describe how weather patterns combine and interact to produce a general circulation of the atmosphere. In recognition, the largest of the three convention cells was named after Hadley.

The Hadley cell lies nearest to the equator, stretching north and south from the equatorial line to approximately 30 degrees latitude. Within the Hadley cell, warm air rises from along the equator and flows toward the poles within the troposphere before cooling and descending in the subtropics. Near the surface, trade winds blow toward the equator in a westward direction and often develop into thunderstorms as they rise near the equator, in what is called the Inter-Tropical Convergence Zone. The rising warm air from the equator circulates toward higher latitudes and then sinks at approximately 30 degrees latitude, creating high-pressure regions over the world’s subtropical oceans and deserts.

The Ferrel cell, named in honor of nineteenth-century American meteorologist William Ferrel (1817–1891), represents the midlatitude segment of the earth’s atmospheric circulation, ranging between 30 and 60 degrees north and south latitude. Air circulation in the Ferrell cell is opposite the flow in the Hadley cell. In the Ferrel cell, air near the surface flows toward the poles in an eastward direction, and air at higher altitudes flows toward the equator in a westward direction. The prevailing winds in this cell, known as the westerlies because they originate in the west and flow eastward, are more susceptible to passing weather systems—particularly subtropical highs—than the prevailing winds in the Hadley and Polar cells and can change direction abruptly.

The Polar cell lies at the farthest distance from the earth’s equator, extending from 60 degrees latitude to the North and South Poles. These are the smallest and weakest of the atmospheric circulation cells. Air in the Polar cell rises at lower latitudes and moves toward the poles through the troposphere. When this circulating air reaches the pole, it has cooled significantly and descends, traveling along the surface back toward the equator in a westward direction. The prevailing winds in this cell are known as the polar easterlies.

Overview

Atmospheric circulation occurs when pressurized air moves around the globe in convection cells, with warmer, denser air rising from the surface and cooler, less dense air descending from the troposphere. Air circulation is also driven by movement from dense, high-pressure areas to low-pressure areas.

The vertical and horizontal air movements come together to influence climate and weather conditions in the various parts of the world. For example, land is heated more quickly than water during the daytime hours due to the differences in the specific heat capacity of land and water. Therefore, the air above the land becomes warmer and rises (vertical movement), adding to the atmospheric pressure. Horizontal air flow then moves the pressurized air into lower-pressure areas over the sea, creating less air mass over the land. The cycle perpetuates when the pressurized air over the sea makes its way to the lower-pressure atmosphere near the land, where it heats up again and continues the rotation. The flow is reversed in the evening—land loses heat more quickly than water—creating an opposite current of circulating air.

Air moves through the atmosphere under the influence of pressure gradients that propel it from high-pressure areas to low-pressure areas. Horizontal winds that travel long distances appear to follow curved trajectories because of the eastward rotation of the earth. The specific arc is a result of air’s speed of movement and its latitude. For example, a mass of air that is flowing from the equator toward the pole appears to be deflected because the air is moving faster to the east at the equator than its destination at the pole. This is because a stationary object at the equator completes a path of approximately 40,000 kilometers in one day because of the earth’s rotation, while an object located at 60 degrees latitude travels only half that distance in the same time. This force is known as the Coriolis effect in honor of the French physicist Gustave-Gaspard Coriolis, who described the phenomenon in 1835.

Knowing how air pressure—as well as the earth’s natural forces—relates to air movement is essential for making predictions and preparations about regional climates, ocean currents, storm systems, and wind behavior that can impact the safety, well-being, and livelihoods of people all over the world.

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