Tuesday 4 December 2012

Typhoon Bopha sweeps across the southern Philippines.

Typhoon Bopha made landfall on Mindanao early in the morning of Tuesday 4 December 2012, bringing with it widespread flooding, wind-damage and landslides. Authorities have reported at least one casualty so far, with more loss of life feared in remote areas. The storm has already swept across Micronesia and Palau, growing from a tropical depression to a Super-Typhoon shortly before hitting Mindanao, though it has since lost energy over land and been downgraded to a Category 3 Typhoon.

The path of Typhoon Bopha, with the strength of the storm indicated by colour and the potential future path of the storm indicated by concentric circles (the largest indicates possible positions for the centre of the storm 120 hours ahead of the time of writing: 8.20 am GMT on 12 December 2012). Tropical Storm Risk.

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.

Photograph of Typhoon Bopha taken from the International Space Station on 2 December 2012. NASA.

Tropical 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. They are categorised according to their wind speeds; using sustained wind strength (wind speeds maintained for a minute or more) rather than peak wind strength (the strongest gusts). A Super-Typhoon (or Super-Hurricane, or Super-Cyclone, depending on where these occur in the world), or Category 5 storm, has a sustained wind speed of 251 km/h or higher, a Category 3 Typhoon a sustained wind speed of 178-208 km/h.

The real danger of such storms is not, however, their wind speed, but the water they carry with them. The low pressure above the 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 waterlogging soils on slopes leading to landslides.

Evacuees at an emergency shelter in Cagayan de Oro City. US Environmental Protection Agency.

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