Friday, 14 December 2012

Cyclone Evan kills at least two in Samoa.

Cyclone Evan hit the island of Samoa on Thursday 13 December 2012, leading to widespread flooding and causing the Vaisigano River to burst its banks in the capitol, Apia. At least three people are believed to have been killed, and the Tanugamanono Power Plant is said to have been completely destroyed, leaving the island nation without electricity.

Flooding in Apia after Cyclone Evan. AAP.

Cyclones (and other tropical storms) form due to solar heating of the air over the sea in the tropics. As the air warms up it rises, causing more air to rush into the area from outside (wind). If the area over which this is happening is large enough, then the winds closest to the equator will be veering eastward relative to the winds furthest away, due to the Earth's rotation (the Coriolis Effect), causing the system to rotate; clockwise in the northern hemisphere and anticlockwise in the south. Such storms are graded according to their sustained wind speed (the highest speed that winds can maintain for periods of at least a minute); Evan was a category 1 Cyclone when it hit Samoa, with sustained wind speeds of 119-153 km/h, and gusts of up to 175 km/h. This is a long way from being as big as these storms get, but is considerably larger than Samoa is used to, resulting in widespread damage.

However it is not the high winds that cause the majority of the damage with cyclones, but the flooding that accompanies them. The low pressure at the center of a storm system causes the sea to rise roughly 1 cm for every millibar drop in pressure. This creates a storm surge that may be several meters in height and which can overwhelm sea defenses and flood low-lying areas. In addition the heat and low pressure associated with cyclones leads to high levels of water evaporation within the systems, and therefore high levels of rain around the edges (i.e. where the winds are strongest). This adds to the flooding associated with cyclones and can cause additional problems of landslides in upland areas.

After leaving Samoa Cyclone Evan veered to the northwest, and is now drifting towards the Wallis and Futuna Islands; Fiji is also in a state of high alert.

The path of Cyclone Evan to date, and the predicted future path of the storm. Tropical Storm Risk.

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