The trade winds or easterlies are the permanent east-to-west prevailing
winds that flow in the Earth's equatorial region. The trade winds blow
mainly 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. Trade winds have been
used by captains of sailing ships to cross the world's oceans for
centuries. They enabled colonial expansion into the Americas, and trade
routes to become established across the Atlantic Ocean and the Pacific
Ocean.
In meteorology, they 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 East Africa.
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.
The trade winds also transport nitrate- and phosphate-rich Saharan dust to
all Latin America, the Caribbean Sea, and to parts of southeastern and
southwestern North America. Sahara dust is on occasion present in sunsets
across Florida. When dust from the Sahara 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.[1]
The term originally derives from the early fourteenth century sense of
trade (in late Middle English) still often meaning "path" or "track".[2]
The Portuguese recognized the importance of the trade winds (then the
volta do mar, meaning in Portuguese "turn of the sea" but also "return
from the sea") in navigation in both the north and south Atlantic Ocean as
early as the 15th century.[3] From West Africa, the Portuguese had to sail
away from continental Africa, that is, to west and northwest. They could
then turn northeast, to the area around the Azores islands, and finally
east to mainland Europe. They also learned that to reach South Africa,
they needed to go far out in the ocean, head for Brazil, and around 30°S
go east again. (This is because following the African coast southbound
means sailing 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 unknown to Europeans until Andres de
Urdaneta's voyage in 1565.[4]