Trade Winds

The trade winds are a large-scale component of Earth circulation, occupying most of the tropics, straddling the equator between approximately latitude 30 degrees N and latitude 30 degrees S, with a seasonal shift of the entire trade wind belt system about 5 degrees of latitude northward during summer (July) and southward during winter (December).

In the Northern Hemisphere, warm equatorial air rises and flows north toward the pole, the Coriolis effect (caused by the Earth’s rotation) deflects the current, and as the air cools, it descends, blowing southwestward from the northeast. In the Southern Hemisphere, warm equatorial air rises and flows south toward the pole, the Coriolis effect deflects the current, and as the air cools, it descends, blowing northwestward from the southeast. The rising air is associated with deep atmospheric convection, heavy precipitation, and weak wind speeds, with an influence on global weather patterns. Air heated by the sun rises and releases moisture through rain and thunderstorms.

Once the air cools, it descends as drier air. In the equatorial low, the air rises and travels aloft to the subtropical highs, where it then sinks. Mariners called these reliable wind currents for sailing the trade winds, or westerlies. The name trade winds comes from an old sailing term meaning that the winds could be counted on to blow steadily from the same direction at a constant speed. The trade winds, or easterlies, carried air from east to west at low latitudes and are less regular over land areas than they are over the oceans. The trade winds meet at the Intertropical Convergence Zone. The doldrums (downward branch of the Hadley Cell, named for George Hadley, whose 1735 paper linked rising air and the Earth’s rotation in causing the trade winds) are the calm winds at the Intertropical Convergence Zone in the area between latitude 5 degrees N and latitude 5 degrees S, where a sailing ship might not move because of the calm winds. In satellite imaging, the Intertropical Convergence Zone appears as a band of clouds. The strength and position of the Intertropical Convergence Zone influences tropical and global weather patterns.

Air temperature differences across the Earth’s surface (both land and water) create winds, with warm air being lighter than cold air. Near the equator, the sun heats the sea surface, causing the warm air at the surface to rise and be replaced by the trade winds blowing from subtropical high pressure systems into equatorial low-pressure troughs. The trade winds blow steadily for days and are among the most consistent on Earth. When trade winds move over warm tropical waters, they pick up moisture and bring heavy rainfall to the windward-facing slopes of mountainous areas, contrasting with the downward motion of dry air that creates desert areas on land. Because the area of Earth between the Tropic of Cancer and Tropic of Capricorn, lying at approximately 23 degrees latitude on either side of the equator, receives more solar heat than the rest of the Earth, the warm air creates clouds and rain, with thundershowers there almost every day.

The influence of the trade winds on weather and climate is seen with El Niño, La Niña, and the development of hurricanes and cyclones. The differences in pressure and temperature between the two sides of the Pacific are caused by the trade winds; air blowing from east to west pushes water, making the sea level higher in the western Pacific, and makes cold water rise toward the surface, making the eastern Pacific approximately 14 degrees F (7.7 degrees C) cooler than the western Pacific. During El Niño years, the eastern Pacific sea surface is warmer, and the Intertropical Convergence Zone is closer to the equator, causing rainfall over the Pacific. The warm surface temperature is associated with reversed air pressure patterns and decreasing strength of trade winds, so more water stays in the eastern Pacific off the coast of South America. With the rain pattern shift eastward, the western Pacific can become drier over India and much of southeast Asia. A similar pattern sets up in the Atlantic, resulting in extreme drought in the eastern United States and reduced tropical storm development in the Atlantic Ocean.

During La Niña years, the trade winds are stronger than normal, causing more cold water to rise to the ocean surface. The cooler surface temperature is associated with a rain pattern shift westward. The eastern areas thus become drier, with an increased probability of flooding from monsoons in both India and much of southeast Asia. Hurricanes (Atlantic) and cyclones (Indian Ocean) are tropical storms of low-pressure cells. Formation of hurricanes in the Atlantic comes from solar heating of water off the West African coast along the Intertropical Convergence Zone, with high cumulus cloud formation in the lowpressure area along the edge. These systems are pushed westward by the trade winds, and the rotation is set in motion by the Coriolis effect. A similar pattern sets up in the Pacific, causing cyclones.