Sunday 13 October 2013

At least thirteen dead as Typhoon Nari sweeps across the Philippines.

Thirteen people are known to have died and at least three more are still missing after Typhoon Nari swept across Luzon Island, making landfall on late on Friday 11 October 2013, and bringing with it widespread flooding and winds of up to 150 km per hour. The storm also inundated large areas of agricultural land ruining unharvested crops as well as destroying several thousand homes and leaving around 2 million people without electricity. The storm is currently moving across the South China Sea towards Vietnam, where it is feared it will create further problems.

Residents of San Miguel town in Bulacan Province, the Philippines, starting to clear damaged homes after the passage of Typhoon Nari. 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, 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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