Wednesday 16 October 2013

At least 17 dead as Typhoon Wipha hits Japan.

Seventeen people are known to have died and several more are missing after Typhoon Wipha made landfall in Japan on Wednesday 15 October 2013. The majority of the deaths occurred on Izu Oshima Island, a popular tourist resort 120 km to the south of Tokyo, where 122 mm of rain fell within an hour, triggering a widespread flooding and a series of landslides. Landslides 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.

Rescue workers searching through the remains of a house destroyed by a mudslide associated with Typhoon Wipha at Oshima on Izu Oshima Island. Kyodo News/AP.

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. 

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