Thursday 5 November 2015

Cyclone Chapala makes landdall in Yemen.

Cyclone Chapala made landfall in the close to the city of Al Mukalla in the Hadhramaut region of Yemen early on Tuesday 3 November 2015.This is the first tropical cyclone to have hit the coast of Yemen since modern meteorological recording began, and it is thought likely that the local population, which typically only receives tens of millimeters of per year may have been largely unprepared for the flooding that accompanied the storm. Al Mukalla is currently held by Al-Qaeda in the Arabian Peninsula, one of several competing groups currently fighting in a civil war in Yemen, and an organization particularly opposed to outside interference, making it hard to assess the level of damage caused by the storm and unlikely that outside help will be able to reach the area.

Flooding in Al Mukalla following the landfall of Cyclone Chapala. Saeed Al-Batati/Twitter.

Cyclone Chapala was a Category 1 storm, defined as a storm having sustained winds of between 119 and 153 kilometers per hour (a sustained wind is a windspeed recorded for at least 1 minute), but is was a Category 3 storm, defined as a storm having sustained winds of between 178 and 208 kilometers per hour, when it swept over the Yemeni island of Socotra on Monday 2 November. Here it is known to have killed at least three people, destroyed around 450 homes and displaced around 40 000 people (out of a total population of about 50 000), though most should be able to return to their homes promptly. While Socotra is no more used to tropical storms than the Yemeni mainland, it has been little affected by the country's civil war, and aid efforts have been dispensed from several countries in the region to help with recovery after the storm.

A man calls for help after his vehicle is swept away in a flash flood caused by Cyclone Chapala on the island of Socotra. Reuters.

Tropical cyclones 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 passage of Cyclone Chapala as of 3 November 2015. Tropical Storm Risk.

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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