LAND AND SEA BREEZES
The regular alternation of land and sea breezes is a well-known feature of most tropical and sub-tropical coasts and large islands.
These breezes also occur at times in temperate latitudes in fine weather in the summer, though they are here much weaker and less well marked than is the case in lower latitudes.
The cause of these breezes is the unequal heating and cooling of the land and sea. By day the surface of the land rapidly acquires heat from the sun whereas the sea temperature remains virtually unaffected.
The heat of the land is communicated to the air in contact with it, which expands and rises. Air from over the sea flows in to take its place, producing an onshore wind known as a sea breeze.
By night, land rapidly loses heat by radiation and becomes much colder than the adjacent sea. The air over the land is chilled, becomes denser and heavier and flows out to sea under the influence of gravity, producing an offshore wind known as a land breeze.
Sea breezes usually set in late in the forenoon and reach maximum strength, about force 4 (occasionally they reach force 5 or even 6), around 1400. They die away around sunset.
Land breezes are usually less well-marked and weaker than sea breezes. The effect of these breezes may be to deviate the prevailing wind, reinforce it, neutralize it or even reverse it.
The following factors favour the formation of well-marked land and sea breezes:
- A dry desert coast as opposed to forests or swamps.
- High ground near the coast.
- A weak prevailing wind.
- A clear or partly cloudy sky.
A cold current along the coast also has the effect of favouring the establishment of a well-marked sea breeze.
Small islands less than 5 to 10 miles in diameter will not usually produce land and sea breezes.
As the land is heated during the daytime the air over it will be heated by conduction.
This heating causes a decrease in the density of the air, and the pressure falls.
The sea temperature remains more or less the same and the pressure over it is high compared with that over the land. The pressure gradient is sufficient for air to flow from over the sea to the land; this is the sea breeze.
The sea breeze (about force 3 – 4) sets in during the morning, reaches its maximum strength about 1400 hours and then dies away towards sunset.
After sunset the land cools rapidly and the air above it also cools and its density increases giving rise to an increase in pressure.
The pressure over the sea is now low compared with that over the land. The pressure gradient causes air to flow from the land to the sea; this is the LAND BREEZE.
The land breeze (generally very light compared to sea breeze) sets in shortly after sunset and continues until dawn.