Hurricane (飓风) Katrina A hurricane is a fiercely powerful, rotating(旋转的)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 can release as much energy as 10,000 nuclear bombs. The seed for hurricane formation is a cluster(聚集) of thunderstorm 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 part of the world. Under these conditions, large quantities of water evaporate (蒸发) and condense (冷凝) 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 moist air from the sea surface in a process that strengthens 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, or low air pressure. Hurricane Katrina formed in this way over the southeastern 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(尾迹)-almost the size of the UK. Thousands have been killed or injured and more than half a million people have become homeless in a humanitarian(人道主义的) crisis of a scale not seen in the US since the Great Depression. The cost of the damage may top $100 billion. The eye of a hurricane refers to its ______.