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Mean January prevailing surface winds and

centers of atmospheric pressure, 1959-1997.

The red line on this image represents the

intertropical convergence zone (ITCZ).

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Mean July prevailing surface winds and centers

of atmospheric pressure, 1959-1997. The red

line on this image represents the intertropical

convergence zone (ITCZ).

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Monsoon clouds over Lucknow , India

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Equator 

ITCZ

South westerly winds

over Sri Lanka

South West monsoon (Summer monsoon)

May to August

areas of the northern and

central Indian subcontinent

heats up considerably

during the hot summers,

which causes a low

pressure area over the

northern and central

Indian subcontinent. To fill

this void, the moisture-

laden winds from the

Indian Ocean rush in to the

subcontinent.

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Equator 

 North Easterly winds

over Sri Lanka

 North East monsoon

(Winter monsoon)

 November to February

ITCZ

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Pressure pattern (winter)

Siberian high (large high pressure system)

H

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L

Pressure pattern (summer)

L

Surface pattern over Asia dominated by low

 pressure over southern and eastern Asia

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Large high pressure system over Siberia (Siberian High)

Produces diverging clockwise flow that spreads cold, dry air 

over large portions of Asia

Winter is the dry season over Asia

Surface pressure over Asia

Winter 

Summer 

Warm moist air flows into Asia from the Indian and Pacific oceans

Air rises in low pressure systems and as it flows up mountain

ranges like the Himalayas, producing clouds and precipitation

Dominated by low pressure over southern and eastern Asia

Summer is the rainy season over much of Asia

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Equator 

Afternoon thunderstorms

Inter monsoon period

ITCZ

 November to MarchMarch to AprilOctober to November 

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Topography of Sri Lanka

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Flow over a mountain

Warm dry air 

Cold moist air 

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R ainfall in Sri Lanka

SW monsoon brings more

rain than NE monsoon

SW air contains more moisture

than NE

SW air travels over the ocean

for a longer period and collects

more moisture

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Monsoon weather linked to seasonal shift in general circulation, particularly

the N S shift of ITCZ

Winds cover sufficiently long trajectories and persists long enough to be

influenced by the Coriolis effect

When humid air flows over land, gets heated and rises bringing

condensation, clouds and rain

Also lifting of air due to mountains

Also linked to seasonal difference in the heating of the land and sea

seasonal changes in atmospheric circulation

and precipitation

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divided into a warm and wet summer

monsoon and a cold and dry winter monsoon.

The East Asian monsoon

carries moist air from the Indian Ocean and

Pacific Ocean to East Asia

affects approximately one-third of the global

population, and the countries China, Japan,

North Korea, South Korea, Taiwan,

driven by temperature differences between

the Asian continent and the Pacific Ocean.

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North American Monsoon

Although North America is not as large as Asia, it

is large enough to produce a monsoon over

the southwest.

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The average sea levelpressure over the

southwestern portion

of the U.S. is generally

dominated by high

pressure in January.

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Low pressure develops

at the surface in July.

If the low pressure is

strong enough, it is able

to pull in moist air from

the Gulf of California

into Arizona and

summertime

thunderstorms can

develop.

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Winter monsoon

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Summer monsoon

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The wind flows towards a mountain and

produces a first oscillation (A). A second wave

occurs further away and higher. The lenticular

clouds form at the peak of the waves (B).

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Lenticular clouds

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