LAND AND SEA BREEZES AND MONSOONS
Land and sea breezes are, in fact, monsoons on a smaller scale. Both are basically caused by differential heating of land and sea, the former in a diurnal rhythm and the latter in a seasonal rhythm.
During the day, the land gets heated up much faster than the sea. Warm air rises, forming a region of local low pressure. The sea remains comparatively cool with a higher pressure, so a sea breeze blows in from sea to land. Its speed or strength is between 8-32 km.p.h. (5-20 m.p.h.) and it is generally stronger in tropical than temperate regions. Its influence does not normally exceed 24 km (15 miles) from the coast. It is most deeply felt when one stands facing the sea in a coastal resort.
At night, the reverse takes place. As the land cools down much faster than the sea, the cold and heavy air produces a region of local high pressure. The sea conserves its heat and remains quite warm. Its pressure is comparatively low. A land breeze thus blows out from land to sea. Fishermen in the tropics often take advantage of the outgoing land breeze and sail out with it. They return the next morning with the incoming sea breeze, complete with their catch.
Monsoons are caused in the same way. Seasonal changes in air pressure bring about a complete reversal in wind direction. In winter, the winds blow out from the high-pressure areas, but in summer they are drawn into the low-pressure areas.
Monsoon winds are most notably developed in Asia (China, Japan, South-East Asia and the Indian sub-continent). However, Northern Australia and the Guinea coast of West Africa also show the monsoon effect to a slight extent.
In January, an intense high-pressure area develops over Mongolia.
At the same time, an intense low-pressure area develops over North Australia (It is summer in the southern hemisphere) .
The continental high-pressure system intensifies to such a degree that it far exceeds the pressure of the sub-tropical high pressure zone. Thus winds blow out from Asia towards the Doldrums low-pressure area. However, as the continental low-pressure area over Australia is far more intense than that of the equatorial latitudes, this becomes the focus of the Asian air masses.
The actual direction of these winds varies. Over Japan, they are north-westerly. Over south China, the South China Sea and South-East Asia, the winds are north-easterly. On crossing the equator, the winds are deflected to the left (the effect of the earth’s rotation) to become the North-West Monsoon of Australia.
In July, the pressure and wind patterns are reversed. An intense low-pressure cell develops over the Asian land-mass. Conversely, high pressure develops over northern Australia.
Thus, air flows out from Australia across the equator towards the continental low of Asia. South-easterly winds from the continental high of Australia (where it is winter) blow towards the equator. These winds are drawn across the equator towards the Asian low-pressure system.
On crossing the equator, the south-east winds are deflected to the right to become the southwest winds. As they move northwards, they become part of the anticlockwise circulation around the Asian low and thus cross Japan and the China coast as the south-east monsoon.
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