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Hurricane Katrina
　　A hurricane is a fiercely powerful, rotating form of tropical storm that can be 124 to 1,240 miles in diameter. The term hurricane is derived from Hurican, the name of a native American storm god. Hurricanes are typical of a calm central region of low pressure between 12 to 60 miles in diameter, known as the eye. They occur in tropical regions. Over its lifetime, one of these storms can release as much energy as 10,000 nuclear bombs.
　　The seed for hurricane formation5 is a cluster of thunderstorms over warm tropical waters. Hurricanes can only form and be fed when the sea-surface temperature exceeds 27℃ and the surrounding atmosphere is calm. These requirements are met between June and November in the northern hemisphere.
　　Under these conditions, large quantities of water evaporate and condense into clouds and rain - releasing heat in the process. It is this heat energy, combined with the rotation of the Earth, that drives a hurricane.
　　When the warm column of air from the sea surface first begins to rise, it causes an area of low pressure. This in turn creates wind as air is drawn into the area. This spinning wind drags up more moisture-laden air from the sea surface in a process that swells the storm. Cold air falls back to the ocean surface through the eye and on the outside of the storm.
　　Initially, when wind speeds reach 23 miles per hour, these mild, wet and grey weather systems are known as depressions. Hurricane Katrina formed in this way over the south-eastern Bahamas on 23 August 2005. Katrina has had a devastating impact on the Gulf Coast of the US, leaving a disaster zone of 90,000 square miles in its wake - almost the size of the UK. Thousands have been killed or injured and more than half a million people have been displaced in a humanitarian crisis of a scale not seen in the US since the great depression. The cost of the
　　damage may top $100 billion.

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