There are two general types of winds, local winds and global winds. Local winds blow from any direction and usually cover short distances. Global winds blow from a specific direction and almost always cover longer distances than local winds. Both local winds and global winds are caused by differences in air pressure due to unequal heating of the atmosphere. During the day, the air over a land area is often warmer than the air over a nearby lake or sea. The air is warmer because the land heats up faster than the water. As the warm air over the land rises, the cooler air over the sea moves inland o tale its place.
This flow of air from the sea to the land is called a sea breeze. During the night, the land cools off faster than the water. The air over the sea is now warmer than the air over the land. This warm air over the sea rises. The cooler air over the land moves to replace the rising warm air over the sea. A flow of air from the land to the sea is called a land breeze. A land breeze is also called an off-shore breeze. The name of a wind tells you from which direction the wind is blowing. A land breeze blows from the land to the sea. A sea breeze blows from the sea to the land.
Most local winds that we are familiar with are named according to the direction from which they are blowing. A major land and sea breeze is called a monsoon. A monsoon is a seasonal wind. During part of the year, a monsoon blows from the land to the ocean. During the rest of the year, it blows from ocean to the land. When a monsoon blows from the ocean to the land, it brings in warm, moist air. This results in a rainy season with warm temperatures and huge amounts of rain. The rainy season is important to many because it provides the water needed for farming.
Monsoon winds are very common in Asia. Unequal heating of the Earths surface also forms large global wind systems. In areas near the equator the sun is almost directly overhead for most of the year. The direct rays of the sun heat the Earths surface. The polar regions receive slanting rays from the sun. The slanting rays do not heat the Earths surface as rapidly as the direct rays do. So temperatures near the poles are lower than those near the equator. At the equator, the warm air rises and moves toward the poles. At the poles, the cooler air sinks and moves toward the equator.
This movement produces a global pattern of air circulation. Global winds do not move directly from north to south or from south to north. Because the Earth rotates, or spins on its axis, from west to east, the paths of the winds shift in relation to the Earths surface. All winds in the Northern Hemisphere curve to the right as thy move. In the Southern Hemisphere, winds curve to the left. This shift in wind direction is called Coriolis effect. The Coriolis effect is the apparent shift in the path of any fluid or object moving above the surface of the Earth due to the rotation of the Earth.
At any particular time or place local conditions may influence and change the wind pattern. At the Equator surface winds are quite calm. These winds are called the doldrums. A belt of air around the equator receives much of the suns radiant energy. The warm rising air produces a low pressure area that extends many kilometers north and south of the Equator. Cooler high pressure air would normaly flow into such an area creating winds. But the cooler air is warmed so rapidly near the Equator that the winds which form cannot move into the low pressure area. As a result any winds that do not form are weak.
About 30 degrees north and south of the equator the warm air rising from the quator cools and begins to sink. Here, the sky is usually clear. There are few clouds and little rainfall. At horse latitudes some of the sinking air travels back toward the equator. The rest of the sinking air continues to move toward the poles. The air moving back toward the equator forms a belt of warm steady winds. These winds are called trade winds. In the Northern Hemisphere, the Coriolis effect deflects the trade winds to the right. These winds called the northeast trades, blow from northeast to southwest.
In the Southern Hemisphere the trade winds are deflected to the left. The cool sinking air that continues to move toward the north and south poles is also influenced by the Coriolis effect. In the Northern Hemisphere the air is deflected to the right. In the Southern Hemisphere it is deflected to the left. So in both hemispheres the winds appear to travel west to east. These winds are called the prevailing westerlies. Unlike the trade winds the prevailing westerlies are often strong winds. In both hemispheres the westerlies start rising and cooling between fifty degrees and sixty degrees latitude as they approach the poles.
Here they meet xtremely cold air flowing towards the equator from the poles. This band of cold air is deflected west by the Coriolis effect. As a result the winds appear to travel from east to west and they are called the polar easterlies. The polar easterlies are cold but weak winds. Meteorologists and weather observers use a wind vane to determine the direction of the wind on the earths surface. A wind vane points in to the wind. An anemometer is used to measure wind speed. Wind speed is usually expressed in meters per second, miles per second or knots. One knot is equal to 1,850 meters per hour.
