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Earth Sense: Level land areas are vulnerable to floods
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Floodwaters in southeastern China are receding, but the trouble has moved farther north. Now the disasters have hit Henan and Hebei provinces, including Beijing, the country’s capital.

In Google Earth, when the “weather” checkbox is on, you can see a huge band of rainstorms extending from the Bay of Bengal, 2,000 miles to the southwest, into northeastern China. This effect occurs just about every summer.

Much of China is part of the monsoon system that brings tremendous amounts of rain to eastern Asia. This year, though, the monsoon has been unusually active. It combines with airflow from the Pacific Ocean to form what’s called the “Mei-Yu front.”

This time of the year, eastern and southeastern China are open to strong air currents from the warm Pacific. They form a so-called tropical easterly jet.

Heavy monsoon rains are normally no threat to Southeast Asia. India and Pakistan receive rain totaling in the hundreds of inches every summer. Agriculture in those countries even depends on the summer rains, because winter is dry season.

But the flat, relatively level land areas of eastern China are vulnerable to the area floods that the Mei-Yu has been producing this year. BBC News reported 114 people killed as of last week, 111 missing and 53,000 homes destroyed.

Why don’t we have a monsoon in the Southeastern U.S.? It’s because this region lacks the enormous chunk of rock that forms the Tibetan Plateau, serving as a heat source in the summer.

In Asia, the big airflow controller, the jet stream, is far north at the edge of Russia. Every major ocean brings warm, humid air to the eastern side of each continent. Combining with the heat from the Plateau, and air invading from the Bay of Bengal, this is the source of the enormous downpours that have plagued China for almost two months now.

Over here, it’s easy to notice that we are receiving warm moisture from our own east coast also. But the Appalachians aren’t nearly as big a player as the Tibetan Plateau, and they aren’t a great heat source.

Ignoring tropical storms, which are caused by different factors, our summer weather brings mainly isolated thunderstorms. Some are severe, but all of them supply badly needed rain.

In North Georgia, we’ve had our share of flash flooding. But our part of the state lacks the level plains that make devastating area floods possible.

Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at gainesvilletimes.com.

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