Monday 30 September 2013

Around 75 missing as Typhoon Wutip sinks Chinese fishing vessels.

Two Chinese fishing vessels are known to have sunk and a third is missing after they encountered high winds associated with Typhoon Wutip close to the Paracel islands, around 330 km south of the Chinese island of Hainan, on Sunday 29 September 2013. The ships were part of a fleet of five vessels with a combined crew of about 150 people. A rescue attempt is underway, but only 14 survivors have been found so far. Typhoon Wutip was a Category 2 storm, with sustained winds of over 96 km per hour, though it has lost some energy as it moves towards landfall in Vietnam.

The approximate location of the area where the Chinese vessels were lost. Google Maps.

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.

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