Tuesday 27 August 2013

Thirteen dead after Tropical Storm Ferdinand hits Veracruz State in southeast Mexico.

Tropical Storm Ferdinand blew into Veracruz State in southeast Mexico on Sunday 25 August 2013, brining with it torrential rains that triggered a string of landslides across the state, killing at least 13 people. Nine people, including two children, died when a landslide swept away four homes in the village of Roca de Oro on the outskirts of Yecuatla. Three more were killed in the port of Tuxpan when another mudslide hit their homes in the early hours of Monaday 26 August and one was killed by a landslide in the town of Atzalan. Landslides 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.

Rescue workers searching for survivors the in the aftermath of the Tuxpan landslide in Veracruz State on Monday 26 August 2013. Koral Carballo/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 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.

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