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Offshore and onshore winds are generated along the shores of large lakes, such as the Great Lakes of North America, and along the coastlines of the world’s oceans. Offshore and onshore winds blow regularly, nearly every day of the year. They are produced by the differential heating of land and water, caused by solar energy.

Here’s how this happens: As shown in Figure 2.1a, sunlight shining on the Earth’s surface heats the land and water simultaneously. As the water and adjoining land begin to warm, they radiate some of the heat (infrared radiation) into the atmosphere. This heat, in turn, warms the air above them. When air is heated it expands, and as it expands it becomes less dense and rises. The upward movement of air is called a thermal or updraft.

Although water and land both heat up when warmed by the sun, land masses warm more rapidly than neighboring bodies of water. Because air over land heats up more quickly than air over water, air pressure over land is lower than over neighboring surface waters. As warm air rises over land, cooler, high pressure air moves in to fill the void, resulting in a steady breeze known as onshore wind.

At night, the winds blow in the opposite direction — from land to water — as illustrated in Figure 2.1b. These are known as offshore breezes or offshore winds.

Like onshore winds that occur during the day, offshore winds are created by differences in air pressure between the air over land and neighboring water bodies. Here’s what happens: after sunset, the land and the ocean both begin to cool. Land, however, cools more rapidly than water. Because the water cools more slowly, air above it is warmer. Warm air expands and rises. Cooler high pressure air flows from the land to the water at night (Figure 2.1b). The result is an offshore breeze: steady winds that flow from land to water.

Fig. 2.1a and 2.1b: Onshore and offshore breezes. Onshore (a) and offshore (b) breezes occur along the coastlines of major lakes and oceans.

Offshore and onshore breezes operate day in and day out on sunny days, providing a steady supply of wind energy. Because offshore and onshore winds are fairly reliable, coastal regions of the world are often ideal locations for small (and large) wind turbines.

Coastal winds are more consistent than winds over the interior of continents and also tend to be more powerful because of the relatively smooth and unobstructed surface of open waters. That is to say, wind moves rapidly over water because lakes and coastal waters provide very little resistance to its flow, unlike forests or cities and suburbs, which dramatically lower surface wind speeds.