Monday 12 August 2013

At least one person dead as Typhoon Utor sweeps across Luzon Island.

Typhoon Utor swept across Luzon Island in the Philippines on the afternoon of Monday 12 August 2012, leaving one person dead and dozens more missing. The dead person has been described as a 22-year-old man from Benguet Province, who was hit by a landslide while trying to clear a storm canal and died on the way to hospital. Most of the missing people are fishermen from Pangasinan, Catanduanes and Camarines Norte Provinces, while a woman who had refused to be evacuated was reportedly swept away by a swollen river in Isabela Province and her fate is unknown.

Rainfall in Manilla due to Typhoon Utor. Jay Directo/AFP.

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 path of Typhoon Utor to 12.00 noon GMT on 12 August 2013 (thick line), and the predicted future path of the storm (thin line). Tropical Storm Risk.

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.

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