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Trade Winds

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Article Information
Category: Weather Weather
Content source: SKYbrary About SKYbrary
Content control: Royal Meteorological Society (RMetS) Royal Meteorological Society (RMetS)
WX
Tag(s) Climatic Phenomena

Definition

Trade Winds is defined as steady and persistent winds which blow on the Equatorial side of the subtropical high pressure systems in both Hemispheres.

Description

When air moves, the Coriolis force deflects air either to the right of air motion in the Northern Hemisphere and to the left of air motion in the Southern Hemisphere.

Where the Trade Winds from each hemisphere approach each other, the rising air creates instability which, depending on the strength of the winds, results in a line of cumulonimbus clouds. This line of weather is known as the Inter Tropical Convergence Zone (ITCZ). The Inter Tropical Convergence Zone (ITCZ) is generally located near the Equator year-round, except in the Indian Ocean and central Asian landmass in the Northern Hemisphere Summer.

Considering the air which moves from the subtropical high pressure systems at around 30 degrees latitude towards the ITCZ which is located near the Equator, and considering the effect of the Coriolis Force::

In the northern hemisphere the Trade Winds generally blow from the north east while in the southern hemisphere they blow from the south east. The direction of the winds is influenced by land masses so the Trade Winds tend to be more uniform over the oceans.

The weather associated with Trade Winds is generally fine for flying operations; scattered small cumulus and stratocumulus with a base of around three thousand feet . However, as the trade winds progress across the ocean, they gather moisture and the cloud becomes more developed, and so the western side of the tropical oceans is more cloudy than the eastern side. Where the Trade Winds pass over land, for example the Hawaiian Islands, showers may form as a result of orographic lift and instability caused by surface heating over the land.

The term “Trade Winds” is marine. Therefore the Trade Winds blow near the surface of the Earth. The winds in upper levels (circa 20000 – 30000 feet) over the Trade Wind level are generally called “Anti-Trade Winds”. These winds blow opposite to their surface-level counterparts (eg. NE trades → SW anti-trades)

The principle of the Monsoon stems in part from the effect of trade wind movement over land from water or water from land, and the location and displacement of the ITCZ.

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