Monday 16 September 2013

One person confirmed dead and two more missing as Typhoon Man-yi hits Japan.

A 71 year old woman has been confirmed dead after a landslip hit her home in Ritto City in Shiga Prefecture, Japan, on Monday 16 September 2013 following heavy rains associated with Typhoon Man-yi. Landslides 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. Two more people are missing in Fukui and Hyogo Prefectures, where there has been extensive flooding, and around 200 000 people have been evacuated from their homes across the Kansai Region of south-central Honshū.

Flooding along the River Katsura in Kyoto, Japan, following the arrival of Typhoon Man-yi in the area. 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. This can cause additional flooding on land, as well as landslides.

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