Thursday 2 January 2014

Fifteen injured as Cyclone Bejisa batters Réunion.

Fifteen people have been injured, two seriously, as Cyclone Bejisa moved past the island of Réunion (an overseas department of France in the southwest Indian Ocean) on Thursday 2 January 2013, brinig with it winds of up to 150 km per hour and eight meter high waves. The storm also left 200 000 people (roughly a quarter of the island's population) without water supplies, and 175 000 without electricity. The island's main airport, Roland Garros, was closed, and postal services suspended.

A deserted street in La Possession, on the northeast coat of Réunion, on 2 January 2014. Richard Bouhet/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.

Waves battering a harbour wall at Saint-Gilles on the west coast of Réunion, on 2 January 2014. Richard Bouhet/AFP.

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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