Wind Currents
The Wind

Winds are formed when parts of the Earth receive different amounts of heat energy from the sun. Simple. Winds can form over fairly small areas, or over large areas. There are two main examples of winds that form due to these variations in the amount of radiant energy received from the sun.



Differences in the heating and cooling rates of land and water will affect the local circulation of air in an area. Land and water temperatures rise and fall at different rates because the land tends to absorb heat more quickly, and loses heat more quickly, than does water. During the day, dense, cool air from over the water flows inland and forces the warms air over the land aloft. This small scale circulation is called a SEA BREEZE. At night, the land is cooler than the water because the land has given its heat to the atmosphere faster than the water did. The cool air flows over the warmer water and forces the warm air there aloft. This type of circulation is called a LAND BREEZE (SPASEBA!!). The thing to keep in mind here is: Warm air will always rise, and the cooler air nearby will slide over to take its place. The other thing to remember is that A WIND IS NAMED FOR THE DIRECTION THAT IT COMES FROM. This means that a Land Breeze comes FROM THE LAND, while a sea breeze, comes FROM THE SEA, or water. If you keep these two facts straight, you'll never mix up a land breeze with a sea breeze.


Global Wind Systems

Victor Starr, an American Meteorologist, designed a model of the major wind systems on Earth that are cause by the uneven heating of Earth's surface. Starr used a shallow pan of water with ice at the center. He heated the outer rim of the pan to set up convection currents,(currents formed when hot things rise and cooler things sink). When the convection system was well established, Starr rotated the pan. As the pan rotated, the water broke up into eddies, circular movements that form within the main current. The eddies moved in the belt halfway between the warm outer rim and the cold center. Starr's model suggested that the major wind patterns are controlled by a combination of unequal heating and Earth's rotation.

The major wind systems of the Earth are similar to Starr's model. The DOLDRUMS are a windless zone at the equator. Here the air seems to be motionless, but actually it is being forced aloft. Vessels were often stranded in this zone during the heyday of sailing. This zone is like the rim of the pan in Starr's model. At about 30 degrees North and South latitude, this rising air descends back to Earth's surface, having cooled in Earth's atmosphere. This area is also known for calm surface winds. It is called the HORSE LATITUDES. In days past, many a sailing vessel foundered in this area, resorting to throwing valuable cargo overboard in an effort to lighten the load. Unfortunately, some of the discarded cargo were horses, a valuable commodity. This is how the horse latitudes got their name. The TRADE WINDS blow towards the equator and help force the air of the doldrums to rise. In the Northern Hemisphere, the winds blow from the northeast to the southwest. Remember that A WIND IS NAMED FOR THE DIRECTION THAT IT COMES FROM. So the trade winds in the northern hemisphere are called the Northeast Trade winds. The trade winds provided a busy route for ships sailing between Europe and the Americas. Today, pilots still take advantage of the trade winds while flying west. The PREVAILING WESTERLIES blow from the southwest to the northeast in the zone between 30 and 60 degrees latitude in the Northern Hemisphere. This zone is similar to the pattern of eddies that form midway between the warm rim and the cold center of Starr's model. Westerlies are the winds responsible for the movement of weather across the United States and Canada. In the Southern Hemisphere, the prevailing westerlies blow from the northwest to the southeast in this latitude zone. The POLAR EASTERLIES are cold, dry, dense air masses that descend tot he Earth at the poles and flow outward. Polar zones are relatively small, but these winds contribute large amounts of energy to the zone of the westerlies. The cold polar air displaces the warm air of the westerlies and forces it to rise at about 60 degrees latitude, North and South. These rising air currents join the pole ward moving currents from the equator and eventually become polar winds again.

Remember that all global wind systems' circulation if affected by the rotation of the Earth. This rotation causes the winds to be deflected to the west. This movement is caused by the Coriolis Effect. The coriolis effect could be defined as the apparent shift in the path of a fluid or object moving over Earth's surface, caused by the rotation of the Earth. This apparent force causes moving objects near Earth's surface to be turned westward from their original paths.