Monday 16 September 2013

At least eighteen deaths as Tropical Storm Manuel hits Guerro State in southern Mexico.

At least eighteen people have died as Tropical Storm Manuel hit the Pacific coast of Guerro State in Mexico on Sunday 15 September. In Acapulco a family of six were wiped out when their house was his by a landslide, three people were swept away by floods and a tenth person was crushed by a collapsing wall. Elsewhere in Guerro State a further six people died when the vehicle they were traveling skidded in wet conditions and two more people were killed in separate landslides. Landslides 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. 

An overturned car on top of landslide debris in Acapulco, Mexico, on Sunday 15 September 2013, following Tropical Strom Manuel. AP.

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. Tropical Storm Manuel has brought 25-40 cm of rain within 24 hours across much of Guerro State.

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