Monsoons, Rain Shadows and Deserts

Mission Brief #9

In one year Cherrapunji received more rain than London gets in four decades, while parts of the Atacama can go years with none at all.

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Some of the world's sharpest climate contrasts are created by wind and mountains. A monsoon is a seasonal reversal of winds: in South Asia, land heats faster than the ocean in summer, drawing moist air inland and bringing heavy rains, mainly from June to September, while dry winds blow outward in winter. Hundreds of millions of farmers depend on the timing of that rain, so a late or weak monsoon can damage harvests and economies.

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Mountains create rain shadows: moist air rising over a range cools and drops rain on the windward side, then descends and warms on the leeward side, leaving it dry. Deserts form for several reasons, including sinking air in the subtropics, rain shadows, distance from the sea and cold ocean currents. Chile's Atacama combines several of these causes, making it the driest non-polar desert on Earth.

Key facts

Remember these

1India's southwest monsoon, roughly June to September, delivers most of the country's annual rainfall.
2Cherrapunji in northeast India recorded about 26,470 mm of rain in the 12 months from August 1860 to July 1861.
3Parts of the Atacama Desert have gone years without measurable rainfall.
4The world's largest hot desert, the Sahara, lies beneath the subtropical belt of sinking, drying air.

Timeline

When it happened

1686
Edmond Halley publishes an explanation of trade winds and monsoons as driven by solar heating.
1861
Cherrapunji completes a record-breaking 12-month rainfall total.
1875
The India Meteorological Department is founded, partly to forecast the monsoon.
2003
Scientists report that Atacama soils are so dry they resemble the soils of Mars.

Places to know

Where it happened

Cherrapunji (Sohra), India
25.27° N, 91.73° E
Atacama Desert, Chile
24.50° S, 69.25° W

Then & Now

Each year forecasters announce the monsoon's arrival in Kerala, and farmers and markets watch it closely.

Mind = blown

NASA has tested Mars rover equipment in the Atacama because its dry soils resemble conditions on Mars.

Question 1 of 3

What causes the South Asian summer monsoon?

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Question 2 of 3

Why is the leeward side of a mountain range often dry?

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Question 3 of 3

Why might a weak monsoon year be described as an economic as well as a weather event?

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You got 0 of 3 right.

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