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 ハリケーンはどのようにして生まれるか
To understand why the U.S. coastline is so vulnerable to a hurricane strike, you have to travel over 4,000 miles east...to the place where hurricanes are born. Most hurricanes in the Atlantic begin as very small atmospheric disturbances in the jet stream that flows from east to west of the sub-Saharan Africa. Scientists call this disturbance an ‘easterly wave’. What triggers the development of an easterly wave could be anything – could be a little girl playing in the sand...it’d trigger off a little dust devil, for example, that perturbs the atmosphere downstream in such a way that you get one of these waves. The easterly wave creates a system of turbulent eddies which go on to develop into a cluster of thunderstorms. These travel west across the African continent, and then encounter the warm tropical waters of the Atlantic. One begins to see on satellite pictures – these thunderstorms appear to become better organized and begin to rotate around each other. The rotation rate increases, the storms become more vigorous, and winds continue to increase. The storm rapidly gathers strength and develops into a hurricane, feeding off the warm, moist air coming from the ocean. The heat is coming out of the ocean and the hurricane heat engine converts that heat energy into the mechanical energy of the winds...and that’s what’s powering the storm. When the winds reach typically around 70 miles per hour, the storm may develop an eye. It’s surrounded by an eye wall, which is a region of very intense heat and the strongest winds. And you’re off and running. These are the hurricanes that make landfall in the U.S. every year, causing enormous damage.