英语六级阅读备考：快速阅读练习(5)_大学英语阅读

英语六级阅读备考：快速阅读练习(5)

　　Forming a Storm

　　The seed for hurricane formation is a cluster of thunderstorms over warm tropical waters. Hurricanes can only form and grow when sea-surface temperatures exceed 27C and the surrounding atmosphere is calm. These requirements are met between June and November in the northern hemisphere. In Australia and the southern hemisphere, the cyclone season runs from October to May.

　　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 wind drags up more moisture-laden air from the sea surface in a positive feedback process that swells the storm. Cold air falls back to the ocean surface on the outside of the storm.

　　When wind speeds hit 37 kilometers per hour, these mild, wet and gray weather systems are called as tropical depressions. Hurricane Katrina formed in this way over the south-eastern Bahamas on 23 August 2005 and was labeled Tropical Depression No.12 of that year.

　　If this process happens far enough from the equatortypically at latitudes of more than 10then the Coriolis force associated with the rotation of the Earth sets the massive weather system spinning, with winds spiraling in towards the center.