Trade wind

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The trade winds (also called trades) are the prevailing pattern of easterly surface winds found in the tropics near the Earth's equator^[1]. The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. The trade winds act as the steering flow for tropical storms that form over the Atlantic, Pacific, and Indian Oceans that make landfall in North America, Southeast Asia, and India, respectively. Trade winds also steer African dust westward across the Atlantic ocean into the Caribbean sea, as well as portions of southeast North America.

Contents 1 Etymology 2 Origin of the winds 3 Impact on Saharan dust migration 4 See also 5 References

[edit] Etymology

Their name originally derives from the late Middle English word 'trade' (borrowed from Middle Low German, and cognate with English 'tread'), meaning "path" or "track," and thence the obsolete nautical phrase "the wind blows trade," that is to say, on a consistent track. However, by the 18th century, because of the importance of these winds to England's merchant fleet crossing the Atlantic Ocean, both etymologists and the general public had come to identify them with a later meaning of 'trade', "(foreign) commerce".^[2] For relation to European shipping and exploration see Winds in the Age of Sail.

[edit] Origin of the winds

The surface air flows toward the equator while the flow aloft is poleward. A low-pressure area of calm, light variable winds near the equator is known to mariners as the doldrums and climatologists as the Intertropical Convergence Zone. Around 30° in both hemispheres air begins to descend toward the surface in subtropical high-pressure belts. The sinking air is relatively dry because its moisture has already been released near the equator.

The surface air that flows from these subtropical high-pressure belts toward the Equator is deflected toward the west in both hemispheres by the Coriolis effect. Because winds are named for the direction from which the wind is blowing, these winds are called the northeast trade winds in the Northern Hemisphere and the southeast trade winds in the Southern Hemisphere. The trade winds meet at the doldrums. Surface winds known as "westerlies" flow from the Horse Latitudes toward the poles. The "westerlies" meet "easterlies" from the polar highs at about 50-60° N. and S.

Among the most well known trade winds is the alizé (sometimes alize), a steady, mild northeasterly wind which blows across central Africa and the Caribbean. It brings cool temperatures between November and February.

Trade winds are known to blow across Madagascar and other regions in the area. They are usually strongest between April and October, but they blow all year long.

[edit] Impact on Saharan dust migration

During mid-summer (July), the westward-moving trade winds south of the northward-moving subtropical ridge expand northwestward from the Caribbean sea into southeastern North America. When dust from the Sahara moving around the southern periphery of the ridge moves over land, rainfall is suppressed and the sky changes from a blue to a white appearance which leads to an increase in red sunsets. Its presence negatively impacts air quality by adding to the count of airborne particulates.^[3] Over 50% of the African dust that reaches the United States affects Florida.^[4] Since 1970, dust outbreaks have worsened due to periods of drought in Africa. There is a large variability in the dust transport to the Caribbean and Florida from year to year.^[5] Dust events have been linked to a decline in the health of coral reefs across the Caribbean and Florida, primarily since the 1970s.^[6]

[edit] See also

• Hadley cell
• Intertropical Convergence Zone

[edit] References