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Secondary Winds, also called Periodic Winds, change their direction periodically, with the change in season, and blow at certain times of the day in a certain direction or a particular season of the year.
While Planetary or the Prevailing Winds constantly blow in the same direction all around the world, Periodic Winds blow at regular intervals, change their directions, and are not continuous. In comparison to the Planetary Winds they blow at a much smaller scale. Apart from Land and Sea Breezes, Mountain and Valley Breezes, Monsoons are the best example of the Periodic Winds.
Since the specific heat of water is very high compared with that of land, the land and sea absorb and transfer heat differently. During the day the land gets heated more quickly and becomes warmer than the sea. As a result, the warm air near the surface expands, becomes lighter and finally ascends, causing a low pressure and the cool heavy air from the sea moves in to take its place. This inward flow of the wind from sea to land is known as Sea Wind. The sea wind is most regular when the pressure gradient from the sea to the land is created and the sky is clear. The strength of the sea breeze depends on the topography of the coast and the region. Sea breeze is usually cool and refreshing. On hot summer days, it moderates the weather of the coastal area.
During the night, the reversal of condition takes place, when due to rapid radiation, the land masses cool quicker than sea. It results in high pressure over the land and low pressure over the sea. As a result, in calm and cloudless weather, cooler and heavier air blows from the land towards the sea.
The land and sea winds thus maintain air circulation in the coastal areas and have a moderating affect on the temperatures of the region. The general effect of the contrast in heating of land and water is to produce cooler winters and warmer summers in the centres of continents than along the coasts.
