Thursday 15 August 2013

Typhoon Utor hits south China.

Typhoon Utor made landfall at about 3.50 pm local time (7.50 am GMT) on Wednesday 14 August 2013, near the city of Yangjiang in western Guangdong Province, bringing widespread flooding and winds of up to 150 kph. One person is said to have drowned as the Quehua River breached its banks, and a container ship capsized and sank off the coast of Hong Hong, though all crew members are reported to have been evacuated safely.

The bulk carrier Trans Summer sinking off the coat of Hong Kong. Hong Kong Government Flying Service.

The typhoon had already swept across Luzon Island in the Philippines, where it is known to have caused eight deaths; two of whom drowned at sea, one was killed in a landslide and the remainder caught in flash floods on land. The death toll is likely to climb in both China and the Philippines as cleanup efforts continue.

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.

Typhoon Utor has been described as the worst storm to hit the Philippines so far this year, with sustained winds of around 140 kph and gusts of up to 170 kph. However the country seems to have escaped the widespread flooding associated with many such events. 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.

Damage on Luzon Island in the wake of Typhoon Utor. AFP.

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