Saturday 23 November 2013

Cyclone Helen kills 10 in Andhra Pradesh.

Ten people are known to have died and several more are missing after Cyclone Helen made landfall in Andhra Pradesh State, India, at about 1.30 pm local time (8.00 am GMT) on Friday 22 November 2013, bringing with it high winds of over 100 km per hour, torrential rains (exceeding 250 mm in a day in places), and a 1.5 m storm surge. The storm also caused extensive damage to homes, infrastructure, agricultural land and seagoing vessels. A number of fishing boats are reported to be missing after the storm, and many areas are without electricity after power lines were brought down. Higher casualties were almost certainly avoided by the evacuation of around 17 000 people from low-lying coastal areas in the state.

Floodwaters washing over a road in Vizag, Andhra Pradesh. PTI.

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.

Cleanup operations beginning in Andhra Pradesh in the wake of Cyclone Helen. AP.

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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