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What Is wind energy?
Air is a low-density fluid. Air in motion is wind and wind contains kinetic energy (in fact, wind energy is a form of motion energy, which is the movement of objects and substances from one place to another). Water is another, more dense fluid that also contains kinetic energy when it is in motion. Wind is fundamentally an indirect form of solar energy. About two per cent of the solar radiation that the earth receives is converted to wind energy in the atmosphere. Winds are created when air moves. When air is heated, it becomes lighter in weight and moves upwards. Cooler air, which is heavier and therefore closer to the ground, moves into the space left by the rising warmer air and the wind is formed.
However, the air in the atmosphere is not heated directly by the sun's rays. Heat from the sun is first absorbed by the earth’s surface and the heated surface warms the air above it. As the surface of our planet varies (land, water, desert, forests, etc.) the amount of absorbed heat varies. This creates temperature differences causing the air to move from one place to another place.
The amount of sunshine received in a region depends on its geographical location, the season and the time of day. For example, in any given year the tropical regions receive an excess of heat energy, while the polar regions don’t receive enough heat energy. However, the tropics do not get hotter from year to year nor do the poles get colder with time. This is because there is an exchange of heat – through wind breezes - across the latitudes. Breezes also occur between areas of land and water. Land and water along a coastline absorb heat differently (so do valleys and mountains). This creates the breezes.
Please refer to the following diagram about the formation of breezes along a coastline:
To enlarge animation, click here.
The land surface becomes hotter more quickly during the day than the water body of the earth. During the daytime, the sea’s temperature does not rise as rapidly as that of the land surface because much of the solar radiation is either absorbed by the water-body or consumed by evaporation. As a result, the lower layers of air above the land surface become warmer than the layers over the sea.
The heated air above the landmass expands, becomes lighter in weight, and rises while the comparatively cooler, heavier air over the water moves in to replace it. This process creates an on-shore or “sea” breeze. At night the breeze reverses direction. This is because at night the land surface cools more quickly than the water body, as the water holds heat from the sun longer and becomes the warmer surface. This in turn creates a warmer body of air above the water than over the landmass, causing a land or offshore breeze to form.
In the tropics the contrast between land and sea temperatures is quite marked. Consequently tropical sea breezes tend to be stronger. (Anyone who has experienced sea breezes up in the north of Western Australia will be aware of the force of winds in that region.)
A local and well-known example of a sea breeze is the refreshing wind welcomed by Perth people during the long, hot summer days. This is popularly known as the "Fremantle Doctor" and brings much relief to Perth people, particularly those working in the open, during the summer. This strong thermal breeze arrives in Perth most afternoons during the summer months. The good “Freo Doctor” reaches wind speeds up to 15 knots to 25 knots from October until March.
 Next Section >>The advantages of wind energy.
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