Linda F. Malanson
(figure available in print form)
Monsoons are land and sea breezes on a very large scale. They are produced by changes in pressure systems. The Asian monsoon is the best known.
In winter dry, sinking air forms a large high pressure area over Asia. The weather is dry and winter monsoon winds blow away from the coasts.
In summer the land heats faster than the sea. The high-pressure systems weakens, and the winds reverse direction, bringing moist air from over the sea into the dry continent. The onset of the summer monsoon is usually sudden and dramatic, bringing heavy rains.
The following pictures show monsoons in Asia. In summer the land warms faster than the sea. Air pressure falls over central Asia and rises over the Indian Ocean. Moist air blows inland, rising as it reaches high ground and bringing heavy rains to India. This is the summer monsoon.
In winter the land cools rapidly, and sinking air forms a large but shallow area of high pressure over the continent. Air pressure is now higher over the land than over the sea, and northerly winds bring dry air to India. This is the winter monsoon. As mentioned previously, monsoons and El Nino are driven by the same process.
EXPERIMENT: MONSOON MUD MADNESS
(figure available in print form)
Monsoon rains increase the amounts of water flowing in rivers. In the lowlands, rivers flood surrounding farms. When the floods recede, tiny particles of silt that were in the water remain. Silt makes soil fertile, and farmers rely on the monsoons for this seasonal flooding. You can see how this flooding happens. TAKE CARE. THIS EXPERIMENT IS VERY MESSY, SO DO IT OUTSIDE TO KEEP YOUR SPACE CLEAN.
MATERIALS NEEDED:
1. large board
2. bricks
3. soil
4. water
5. spoon
DIRECTIONS:
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1. Raise one end of the board and support it with bricks at one end. Cover the board with about 5 c (2 in.) of soil and pack the soil down well.
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2. Use the spoon to make a hollow “lake” at the top of the board and a “river” channel down the board from the lake. About halfway from the bottom, make the channel meander gently from side to side.
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3. Pour water into the lake. When the river is flowing, pour faster until it breaks its banks, then stop. When it dries, where has the silt settled?