Thursday 30 May 2013

Two people killed as Hurricane Barbara makes landfall on Mexican south coast.

Hurricane Barbara made landfall on the southern coast of Oaxaca State in Mexico at around noon local time on Wednesday 29 May 2013, with sustained wind speeds of 120 km/h accompanied by a storm surge 1.5 m above normal tidal levels and up to 50 cm of rain. Two people are known to have died in the storm already, a 61 year-old American surfer and a local man in his twenties who was trying to cross a swollen river, and it is feared more lives will be lost. Mexico has already suffered a number of casualties due to flooding, landslides and weather-related traffic accidents in the past week.

The intensity and path of Barbara. The storm has been growing in strength as it moved northward, only being upgraded to a hurricane classification slightly before landfall, and is expected to lose strength as it passes over land, however it has already proven to be a dangerous storm with two known casualties. The thicker line represents its path to date, the thinner line its projected path. The circles are the areas that could potentially be affected by winds in 9, 21 and 33 hours; the circles get bigger as the storm is subsiding due to uncertainty about the path of the storm. Tropical Storm Risk.

Hurricanes, and tropical storms in general, form due to heating of air over the sea in tropical zones. As the air is heated the the air pressure drops and the air rises, causing new air to rush in from outside the forming storm zone. If this zone is sufficiently large, then it will be influenced by the Coriolis Effect, which loosely speaking means the winds closer to the equator will be faster than those further away, causing the storm to rotate, clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.

Whilst the high winds associated with hurricanes are extremely dangerous, the real danger from such storms is  the flooding. Each millibar drop in air pressure can lead to a 1 cm rise in sea level, and large hurricanes can be accompanied by storm surges several meters high. This tends to be accompanied by high levels of rainfall, caused by water picked up by the storm while still at sea, which can lead to flooding, swollen rivers and landslides; which occur when waterlogged soils on hill slopes lose their cohesion and slump downwards, over whatever happens to be in their path.

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