Hurricane（飓风）Katrina

A hurricane is afiercely 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 nativeAmerican storm god.Hurricanes are typical of a calm central region of lowpressure between 12 to 60 miles in diameter,known as the eye.They occur in

tropicalregions.Over its lifetime,one of these can release as much energy as 10,000nuclear bombs.

The seed forhurricane formation is a cluster（聚集）of thunderstorm over warm tropical waters. Hurricanes can only formand be fed when the sea一surface temperature

exceeds 27℃and the surrounding atmosphere iscalm. These requirements are met between June and November in the northern partof the world.

Under theseconditions, large quantities of water evaporate (蒸发）and condense（冷凝）into clouds andrain一releasing heat in the process.It is this heatenergy,combined

with the rotation ofthe Earth,that drives a hurricane.

When the warm columnof air from the sea surface first begins to rise,it causes an area of lowpressure.This in turn creates wind as air is drawn into the area. This spinning

wind drags up moremoist air from the sea surface in a process that strengthens the storm. Coldair falls back to the ocean surface through the eye and on the outside of the

storm.

Initially,when windspeeds reach 23 miles per hour,these mild,wet and grey weather systems areknown as depressions,or low air pressure.Hurricane Katrina formed in this

way over thesouth-eastern Bahamas on 23August 2005.Katrina has had a devastating impact on the GulfCoast of the U.S.,leaving adisaster zone of 90,000 square miles in

its wake（尾迹）—almost the size of theU.K.Thousands have been killed or injured and more than half a million peoplehave become homeless in a humanitarian（人道主

义的）crisis of ascale not seen in the U. S. since the Great Depression.The cost of the damagemay top$100 billion.The eye of ahurricane refers to its_______.

A.lower part

B.rotating part

C.calm central part

D.most violent part