Prevailing winds

The term prevailing winds refers to the winds that most commonly move across an area. They are named for the direction of their origin.

Wind is caused by differences in atmospheric pressure. Some winds are very reliable. On the seas, for example, sailors needed to know where prevailing winds were located and in which direction they flowed so ships could cross vast oceans by sail. Common winds on Earth include the prevailing westerlies, polar easterlies, trades, antitrades, and Roaring Forties.

Trade winds have been vital throughout human history. Christopher Columbus, for example, used them to carry his ships across the Atlantic Ocean from Europe to the Americas. These winds are the steadiest on Earth. Columbus harnessed the power of the prevailing westerlies for his voyage home.

Overview

The atmosphere is composed of layers. Beginning at Earth's surface, these layers are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Atmospheric pressure is high when a great mass of air is above us. Air moves to fill in low pressure areas.

Wind development is strongly tied to temperature. Heated air rises, leaving behind areas of low pressure and creating cycles of air, or cells, moving from high pressure areas to low pressure areas.

The most direct rays of the sun hit Earth at the equator, which is 0 degrees latitude. Because the poles receive less sun, the air there remains colder and sinks, leading to high pressure. Heated equatorial air rises and moves outward. About thirty degrees north and south, this warm air starts to cool and begins losing altitude. Most of the air cycles back to the equator, but the rest continues to move away.

These cells are trade winds, which earned this name because nautical traders relied on them to travel between Europe and the Americas. In the north they move northeast to southwest; in the south they move southeast to northwest. Antitrade winds flow over the trade winds and travel in the opposite direction.

Because Earth is rotating from west to east, winds of the equatorial cell do not travel directly north or south. Instead, they veer slightly because the equator has a greater distance to travel in its rotation than the poles do. In the Northern Hemisphere, wind traveling south appears to bend to the right. In the Southern Hemisphere, such currents appear to bend to the left. This is called the Coriolis effect.

Air moving from the poles toward the equator affects the cooling equatorial air as it falls to Earth. Because of the Coriolis effect, weather patterns such as storms in the Northern Hemisphere swirl counterclockwise, while in the Southern Hemisphere they swirl clockwise.

Prevailing westerlies are generally located between thirty and sixty degrees latitude in the Northern and Southern Hemispheres. Polar easterlies occur at about sixty degrees latitude in both the Northern and Southern Hemispheres.

The factors that create and affect wind are constantly changing. The effect of the sun, for example, varies with the seasons. Subtle changes in air pressure affect air currents.

Jet streams greatly influence wind and weather, although these influences are not experienced directly at the surface. A jet stream is a swift current of air that moves through the tropopause, which is the boundary between the troposphere and the stratosphere. It averages about 110 miles per hour, and frequently bends and twists. Usually a polar-front jet stream and a subtropical jet stream are simultaneously active in both the Northern and Southern Hemispheres.

A cell over the Pacific Ocean known as the Walker Circulation also strongly influences weather. The air current flows east to west near the surface (the trade winds), rises in the west, and flows east at high altitudes before falling again. This cycle continues because water in the western Pacific, known as the Pacific Warm Pool, warms the air and causes it to rise. At the eastern end of the loop, the water is cooler, so the air falls. This cell is one of many—the winds of each constantly meet and deflect one another all around the globe.

Evidence indicates some prevailing winds are being affected by global climate change. Scientists using sea-level atmospheric pressure readings from the middle of the nineteenth century to modern times have created computer models of the Walker Circulation over a 150-year period. They found that these trade winds have weakened by about 3.5 percent over this time. Many natural factors, including volcanic eruptions, affect the Walker Circulation; however, no natural events or conditions could completely account for this change. The only explanation the researchers could find for these changes was human activity. They believe rising temperatures increase the rate at which the atmosphere absorbs water vapor; this air becomes increasingly heavy and moves more slowly. The researchers used their study and other research to predict another decrease of 10 percent in the Walker Circulation by the end of the twenty-first century.

Other researchers studying the Roaring Forties, the trade winds of the Southern Hemisphere, have found global climate change appears to be making wind stronger. According to a study published in the Washington Post in 2023, because of climate change, wind speeds are increasing 2.58 percent per decade. The flow of air also has shifted closer to the South Pole, frequently diverting much-needed rainfall away from farms in southern Australia.

Bibliography

Cappucci, Matthew. "Climate Change Is Increasing Thunderstorm Winds, Study Finds." The Washington Post, 2 Nov. 2023, www.washingtonpost.com/weather/2023/11/02/climate-change-damaging-thunderstorm-winds/. Accessed 4 Nov. 2024.

"Coriolis Effect." National Geographic Education, 19 Oct. 2023, education.nationalgeographic.org/resource/coriolis-effect-1/. Accessed 4 Nov. 2024.

Hannam, Peter. "Wilder Winds, Less Rain, as Roaring Forties Become Furious Fifties." Sydney Morning Herald. Fairfax Media. 11 May 2014, www.smh.com.au/environment/climate-change/wilder-winds-less-rain-as-roaring-forties-become-furious-fifties-20140511-zr9b1.html>. Accessed 4 Nov. 2024.

"Prevailing Winds." National Geographic Education, 19 Oct. 2023, education.nationalgeographic.org/resource/prevailing-winds/. Accessed 4 Nov. 2024.