Monday 23 September 2013

Typhoon Usagi kills at least 25 in Guangdong Province, China.

Typhoon Usagi arrived in Guangdong Province in south China, brining with it winds of up to 165 km per hour as well as a massive storm swell and heavy rains, which have led to widespread flooding on Sunday 22 September 2013. At least 25 people are known to have died, including 13 in the city of Shànwěi in eastern Guangdong Province. In addition around 7000 homes are thought to have been destroyed by the storm, and 80 000 people have been evacuated from their homes in low lying areas of Fujian Province. Transport networks have been severely disrupted across much of southern China. The typhoon has already caused widespread flooding and a number of fatalities in Taiwan and the Philippines, where it is called Typhoon Odette.

Boats in Shànwěi Harbour on Sunday 23 September 2013. Sina English.

Tropical storms are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere.These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.

The low pressure above tropical storms causes water to rise there by ~1 cm for every millibar drop in pressure, leading to a storm surge that can overwhelm low-lying coastal areas, while at the same time the heat leads to high levels of evaporation from the sea - and subsequently high levels of rainfall. This can cause additional flooding on land, as well as landslides, which are are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall.

A mudslide triggered by Typhoon Usagi in Taiwan. CNN.

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