Saturday 29 August 2015

Tropical Storm Erika kills at least 25 in the Caribbean.

At least 36 people are known to have died and over 50 more are missing after Tropical Storm Erika swept across the islands of Dominica, Puerto Rico and Hispaniola this week. On Thursday 27 August the storm swept across the island of Dominica, bringing 38 cm of rain within 24 hours and causing widespread flooding and a number of 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. Twenty people have been confirmed dead on the island, and at least 31 more are known to be missing, with the death toll likely to rise as contact is made with remote parts of the mountainous country, where much of the transport infrastructure has been knocked out.

Flood damage in Roseau, Dominica on 28 August 2015. Robert Tonge/EPA.

After leaving Dominica the storm next passed over Puerto RIco, where power to around 200 000 households was cut, and crops with an estimated value of US$16 million, then the southern tip of Hispaniola, where it caused further flooding and at least five deaths. One person died in a landslide to the north of Port-au-Prince in Haiti, and four more were killed and another eleven injured after a truck carrying clairin (a form of raw liquor distilled from cane sugar) crashed into a bus shelter in heavy rain at Leogane and reportedly exploded.

The path to date of Tropical Storm Erika. Tropical Storm Risk.

Following its passage across southern Hispaniola the storm passed over the eastern part of Cuba, where it brought around two hours of heavy rain around the city of Santiago, prompting authorities to evacuate around 2000 people from low-lying areas.

Storm surge at Santo Domingo in the Dominican Republic on 28 August 2015. Erika Santelices/AFP.

Tropical storms are caused by the warming effect of the Sun over tropical seas. As the air warms it expands, causing a drop in air pressure, and rises, causing air from outside the area to rush in to replace it. If this happens over a sufficiently wide area then the inrushing winds will be affected by centrifugal forces caused by the Earth's rotation (the Coriolis effect). This means that winds will be deflected clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere, eventually creating a large, rotating Tropical Storm. They have different names in different parts of the world, with those in the northwest Atlantic being referred to as hurricanes.

Damage to a home in Port-au-Prince, Haiti, on 29 August 2015. Dieu Nalio Chery/AP.

Despite the obvious danger of winds of this speed, which can physically blow people, and other large objects, away as well as damaging buildings and uprooting trees, the real danger from these storms comes from the flooding they bring. Each drop millibar drop in air-pressure leads to an approximate 1 cm rise in sea level, with big tropical storms capable of causing a storm serge of several meters. This is always accompanied by heavy rainfall, since warm air over the ocean leads to evaporation of sea water, which is then carried with the storm. These combined often lead to catastrophic flooding in areas hit by tropical storms.

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