The trade winds are the prevailing pattern of easterly surface winds found in the tropics,
within the lower portion of the Earth's atmosphere, in the lower section of the troposphere near
the Earth's equator. The trade winds blow predominantly from the northeast in the Northern
Hemisphere and from the southeast in the Southern Hemisphere, strengthening during the winter
and when the Arctic oscillation is in its warm phase. Historically, the trade winds
have been used by captains of sailing ships to cross the world's oceans for centuries,
and enabled European empire expansion into the Americas and trade routes to become established
across the Atlantic and Pacific oceans. In meteorology, the trade winds act as the
steering flow for tropical storms that form over the Atlantic, Pacific, and southern Indian
Oceans and make landfall in North America, Southeast Asia, and Madagascar and eastern
Africa, respectively. Trade winds also transport African dust westward across the Atlantic
Ocean into the Caribbean Sea, as well as portions of southeastern North America. Shallow cumulus
clouds are seen within trade wind regimes, and are capped from becoming taller by a trade
wind inversion, which is caused by descending air aloft from within the subtropical ridge.
The weaker the trade winds become, the more rainfall can be expected in the neighboring
landmasses.
History
The term trade winds originally derives from the early fourteenth century late Middle English
word 'trade' meaning "path" or "track." The Portuguese recognized the importance of the
trade winds in navigation in both the north and south Atlantic ocean as early as the 15th
century. They learned, in order to reach South Africa, they needed to go far out in the ocean,
head for Brazil and around 30°S go east again. Following the African coast southbound means
upwind in the Southern hemisphere. In the Pacific ocean, the full wind circulation,
which included both the trade wind easterlies and higher-latitude Westerlies, was un-known
until Andres de Urdaneta's voyage in 1565. The captain of a sailing ship seeks a course
along which the winds can be expected to blow in the direction of travel. During the Age
of Sail the pattern of prevailing winds made various points of the globe easy or difficult
to access, and therefore had a direct impact on European empire-building and thus on modern
political geography. For example, Manila galleons could not sail into the wind at all.
By the 18th century the importance of the trade winds to England's merchant fleet for
crossing the Atlantic Ocean had led both the general public and etymologists to identify
the name with a later meaning of 'trade', "(foreign) commerce". Between 1847 and 1849,
Matthew Fontaine Maury collected enough information to create wind and current charts for the
world's oceans. The French word, also a women's name, Alizé
or Alizée (French pronunciation: ​[a.li.ze], means trade winds. Examples are: Alizée and
Alizé Cornet. Cause
As part of the Hadley cell circulation, surface air flows toward the equator while the flow
aloft is towards the poles. A low-pressure area of calm, light variable winds near the
equator is known as the doldrums, equatorial trough, intertropical front, or the Intertropical
Convergence Zone. When located within a monsoon region, this zone of low pressure and wind
convergence is also known as the monsoon trough. Around 30° in both hemispheres air begins
to descend toward the surface in subtropical high-pressure belts known as subtropical ridges.
The subsident air is relatively dry because, as it descends, the temperature increases
but the absolute humidity remains constant, which lowers the relative humidity of the
air mass. This warm, dry air is known as a superior air mass and normally resides above
a maritime tropical air mass. An increase of temperature with height is known as a temperature
inversion. When it occurs within a trade wind regime, it is known as a trade wind inversion.
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. These winds blow predominantly
from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.
Because winds are named for the direction from which the wind is blowing, these winds
are called the northeasterly trade winds in the Northern Hemisphere and the southeasterly
trade winds in the Southern Hemisphere. The trade winds meet at the doldrums.
As they blow across tropical regions, air masses heat up over lower latitudes due to
more direct sunlight. Those that develop over land are drier and hotter than those that
develop over oceans, and travel northward on the western periphery of the subtropical
ridge. Maritime tropical air masses are sometimes referred to as trade air masses. The one region
of the Earth which has an absence of trade winds is the north Indian ocean.
Weather effects
Clouds which form above regions within trade wind regimes are typically composed of cumulus
which extend no more than 4 kilometres in height, and are capped from being taller by
the trade wind inversion. Trade winds originate more from the direction of the poles during
the cold season, and are stronger in the winter than the summer. As an example, the windy
season in the Guianas, which lie at low latitudes in South America, occurs between January and
April. When the phase of the Arctic oscillation is warm, trade winds are stronger within the
tropics. The cold phase of the AO leads to weaker trade winds. When the trade winds are
weaker, more extensive areas of rain fall upon landmasses within the tropics, such as
Central America. During mid-summer in the Northern Hemisphere,
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 travels 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. Over 50% of the African dust that reaches the United States affects Florida.
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. Dust events have been linked to a decline in the health of coral reefs across the Caribbean
and Florida, primarily since the 1970s. See also
Intertropical Convergence Zone References