PREVAILING EASTERLY WINDS

Early commerce to the Americas relied on a phenomena we call the ‘Trade Winds’. These are prevailing easterly winds that encircle the earth near the equator. They flow in both hemispheres between about 30ºN and 30ºS.

Trade winds are steady and predictable surface winds that rarely exceed 15 knots at the surface but can extend up to 10,000 ft. They blow from the subtropical highs to the equatorial low (ITCZ), i.e., blowing from the northeast in the northern hemisphere and from the southeast in the southern hemisphere.

As part of the Hadley Cell, surface air flows toward the equator. This, in combination with the coriolis effect, causes the prevailing winds to slant and move from east-to-west.

Since winds are named for the direction ‘from which they flow’, these are called the ‘northeasterly trade winds in the northern hemisphere and the ‘southeasterly trade winds’ in the southern hemisphere.

Trade winds of both hemispheres meet near the equator in an area of calm, light variable winds known as ‘the doldrums’.
Weaker trade winds lead to move rainfall around neighbouring landmasses.

Around 30ºN/S latitude air descends toward the surface in the subtropical high-pressure belts, also known as ‘subtropical ridges’. The sinking air is relatively dry because as it descends its temperature increases while moisture content remains constent. This lowers the relative humidity of the air mass.

The warm, dry air normally resides above a warm and moist maritime tropical air mass, creating a temperature inversion (temperature increase with increasing altitude). When this occurs within a trade wind regime it is known as a ‘trade wind inversion’.

Shallow cumulus clouds seen within the trade wind regime are capped from becoming taller by this inversion. In the fourteenth century the trade winds (Origin of Trade: Track/Route) helped ships navigate from the Atlantic east-to-west. The Portuguese recognised how important this was in navigation in both the north and south Atlantic ocean.

Near 30ºN/S, sinking air creates an area of high pressure with weak winds. We call this region the ‘horse latitudes’. According to legend, the name was given to this area when sailing ships carried horses from Europe to America. When ships were stuck in the windless area, horses were sometimes thrown overboard in order to save drinking water for the sailors.
Apart from trade winds, other easterly winds exist on earth.

These are the;

• Polar Easterlies,
• Polar Night Jet,
• Tropical Easterly Jet,
• African Easterly Jet, &
• Somali Jet

Polar Easterlies are prevailing winds blowing from east-to-west at latitude 60–90ºN/S. The dry, cold, and often weak irregular surface winds created by the polar high pressure are deflected by the Coriolis Force

The Polar Night Jet is a winter feature in the stratosphere acting as a high-level area of low pressure centred over the pole. A circling Polar Vortex is created by the thermal gradient between the poles and equator.

The Equatorial Easterly Jet develops in the summer months due to intense heating of the Tibetan Plateau. With cooler sea to the south, the usual north-south temperature gradient is reversed and so the jet flows from east-to-west, with wind speeds of up to 170mph recorded.

The West African Jet forms due to the temperature contrast between the hot Sahara Desert and the cooler Gulf of Guinea. It varies seasonally between 5ºN in January and north-west Africa in August. Although relatively weak, seldom exceeding 30mph, it plays a crucial role in the West African Monsoon.

One of the strongest and most sustained low-level wind systems on earth is the Somali Jet with wind speeds exceeding 100mph in July-August and plays a part of the larger Asian summer monsoon circular pattern. Surges in the Somali Jet are associated with particular heavy monsoon rain for 2–3 days after the surge.

Prevailing easterlies also transports Saharan Dust westward across the Atlantic Ocean into the Caribbean Sea and portions of southeastern North America.

Trade Winds steer Tropical Storms that form over the Atlantic, Pacific and southern Indian Oceans, making landfall in North America, Southeast Asia, Madagascar and eastern Africa.