When air is heated, air expands and rises. heated ground surface hot air expanses As the air near the ground becomes less dense / lighter, the air pressure becomes lower. L ow pressure near the ground hot air rises

When air is cooled, air contracts and sinks. As the air near the ground becomes denser / heavier, the air pressure becomes higher cold air contracts cooled ground surface High pressure near the ground cold air sinks

30°N Cooler air contracts and sinks 30°S Cooler air contracts and sinks High pressure Low pressure 0° Hot air expands and rises

1. Pressure gradient force Wind blows from high pressure areas to low pressure areas. 2. Coriolis force (created by the rotation of earth) So, winds are deflected to the right in the northern hemisphere and to the left in the southern hemisphere.

The word monsoon is derived from the Arabic word for “a season”. It refers to the seasonal reversal of wind direction. Its results from the different rate of heating and cooling between land and sea, and the pressure difference between large land masses in northern and southern hemisphere in various seasons. Land heats up and cools down more quickly than the land. summer:land is hotter: lower pressure over land sea is cooler : higher pressure over sea wind blows from sea to land as onshore wind winter :land is cooler : higher pressure over land sea is warmer : lower pressure over sea wind blows from land to sea as offshore wind

21 Mar 22 Dec 23 Sept 22 June 23½°N 23½°S 0° Earth’s Revolution In winter solstice, overhead sun lies on Tropic of Capricorn In summer solstice, overhead sun lies on Tropic of Cancer In vernal & autumnal equinoxes, overhead sun lies on the equator

On the Asian continent, since South China Sea heats up slowly, pressure is relatively higher than that of the Mainland. So, winds blow onshore from the sea to Hong Kong and S. China as onshore SE monsoon. It is winter in Australia. High pressure develops over Australia. Yet, it is summer in China. The land is intensely heated. So, low pressure develops over the Mainland. Since winds blow from high pressure areas to low pressure areas, wind blows from Australia to the mainland China. According to Coriolis force, winds are deflected to the left in the S. hemisphere. So, in N. Australia, winds blow offshore in a northwesterly direction. After crossing the equator, winds are deflected to the right in the N. hemisphere. They blow onshore as SW monsoon in India. In June, the overhead sun lies on Tropic of Cancer (23½°N).

June L H 23½°N onshore SE monsoon onshore SW monsoon offshore SE monsoon warm, moist (Tm) cold, dry (Pc)

It is winter in China. The land is intensely cooled. High pressure develops over China. Yet, it is summer in Australia. So, low pressure develops over Australia. Since winds blow from high pressure areas to low pressure areas, wind blows from China to Australia. According to Coriolis force, winds are deflected to the right in the N. hemisphere. So, in S. China, winds blow offshore in a northwesterly direction. In India, winds blow offshore in a northeasterly direction. In December, the overhead sun lies on Tropic of Capricorn (23½°S).After crossing the equator, winds are deflected to the left in the S. hemisphere. They blow onshore as NW monsoon in Australia.

December H L 23½°N NW monsoon cold, dry (Pc) NE monsoon NW monsoon warm, moist (Tm)

The End