"Prevailing westerlies" describes the winds that blow in the area of the globe between 30 and 50 degrees latitude in both the Northern and Southern Hemispheres. The prevailing westerlies blow from the direction of the west toward the east, and they combined with the trade winds to help sailing ships make the round trip from Europe in earlier times.
The westerly winds are just one type of wind that exists in the different latitudes around the world. At the equator, the prevailing winds are known as the doldrums, which can be almost nonexistent at times. The trade winds occupy the latitudes on either side of the equator to 30 degrees north and south and blow from the east. Then come the prevailing westerlies, with the polar easterlies found from the border of the west winds all the way to 0 degrees latitude north and south.
Air pressure is responsible for these various winds and the directions they come from. Heat at the equator, caused by the direct rays from the sun, make the air rise and form an area of low pressure around the globe. As this air rises, it goes north and south until it cools, making regions of high pressure. Once this air starts to move, it blows from the east as the trade winds and from the west as the prevailing westerlies.
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The "Roaring Forties" is the nickname that the particularly strong westerly winds in the band between 40 and 50 degrees southern latitude. These winds, which are stronger than those in the same latitudes to the north, played a major role in shipping in the time period from the 1600s through the 1800s. Since there are far fewer land masses in the southern oceans to slow these winds down, they were especially useful in helping ships get from the Indian Ocean back to Europe via an eastern route.
European ships that were making their return trip from South America and Central America or the southern coast of America used to sail north and turn to the east when they got off the coast of Virginia. This would place them into the prevailing westerlies and put the wind at their back as they sailed back to Europe. Conversely, those leaving Europe and heading to the Far East would go south around the bottom of Africa and then use the west winds to make it to their destination.
The prevailing westerlies bring warm weather to the western coasts of continents, more so in the Southern Hemisphere. They have lost their importance in shipping with the advent of steam ships but are still used by yacht races, with the southern westerlies still the fastest way around the world for a sailing vessel.