Local Winds

LAND AND SEA BREEZES

These winds are common when there is an anticyclone with a light pressure gradient on a clear sunny day.

SEA BREEZE

During the day, the land heats up more quickly than the sea. The air in contact with the land heats up and rises by the process of convection which leads to a decrease in pressure at the surface and an increase in pressure at approximately 1000 — 2000 ft AGL.

This causes air at that height to move over the sea. Air then descends over the sea causing an increased pressure at the surface of the sea. Air then flows from the slightly higher pressure over the sea surface to the lower pressure over the land surface and creates the sea breeze.


Sea breeze circulation

Sea breezes are typically 10 to 15 kt in temperate latitudes and extend to about 10 nm either side of the coastline. In tropical areas, they can be slightly stronger and extend to 40 or 50 nm inland.

Initially, the wind will be at right angles to the coastline but as insolation increases throughout the day the wind will extend further from the coast and due to this longer fetch Coriolis effect comes into play. This causes a veering in the northern hemisphere and backing in the southern hemisphere.

LAND BREEZE

After sunset, the land starts to cool down much more rapidly than the sea. This leads to a reversal of the above situation. The sea surface experiences a lower pressure and the land a higher pressure. The wind now blows from the land to the sea.


Land breeze circulation

The temperature difference between land and sea is less at night so the land breeze is weaker than the sea breeze, typically half the speed (about 5 kt in temperate latitudes), and only extends to about 5 nm out to sea.

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