When air is heated, air expands and rises.

heated ground surface

. . . . .

. . . .

. . . . .

. . . .

hot air expanses

As the air near the groundbecomes less dense / lighter ,

the air pressure becomes lower.

Low pressure near the ground

hot air rises

When air is cooled, air

contracts and sinks.

As the air near the groundbecomes denser / heavier ,

the air pressure becomes higher.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .cold air

contractscooled

ground surface

High pressure near the ground

cold air sinks

30°NCooler air contracts

and sinks

30°SCooler air contracts

and sinks

High pressure High pressure

Low pressure

0°Hot air expands an

d 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 seawind blows from sea to land as onshore windwinter :land is cooler : higher pressure over land sea is warmer : lower pressure over seawind blows from land to sea as offshore wind

21 Mar

22 Dec

23 Sept

22 June

23½°N 23½°S

0°

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).

JuneL

H

23½°Nonshore 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.

DecemberH

L

23½°N

NW monsoon

cold, dry (Pc)NE monsoon

NW monsoon

warm, moist (Tm)

The End