(Published December 22, 2017)
Hurricanes In General
A tropical cyclone, of which a hurricane is one type, is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. A tropical depression has maximum sustained winds of less than 39 mph. A tropical storm has maximum sustained winds of between 39 mph and 73 mph. A hurricane has maximum sustained winds of at least 74 mph. Traditionally, areas of tropical cyclone formation are divided into seven basins. These include the north Atlantic Ocean, the eastern and western parts of the northern Paci c Ocean, the southwestern Paci c Ocean, the southwestern and southeastern Indian Oceans, and the northern Indian Ocean (Arabian Sea and Bay of Bengal). While storms above 74 mph are called hurricanes in the Atlantic Ocean and the northern Pacific Ocean, they are called either “Typhoons” or “Cyclones” elsewhere. Since this book will focus almost exclusively on major storms formed in the Atlantic Ocean basin (including the Caribbean and the Gulf of Mexico) such storms will generally be spoken of as “Hurricanes.”
Whatever they are called, tropical cyclones all form the same way. Tropical cyclones are like giant engines that use warm, moist air as fuel. That is why they form only over warm ocean waters near the equator. They derive their energy through the evaporation of water from the ocean surface, which ultimately recondenses into clouds and rain when moist air rises and cools to saturation. The strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth’s rotation as air flows inwards toward the axis of rotation. As a result, they rarely form within 5° of the equator.
Tropical refers to the geographical origin of these systems, which form almost exclusively over tropical seas. Cyclone refers to their cyclonic nature, with wind blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis effect. Though a tropical cyclone typically moves from east to west in the tropics, its track may shift poleward and eastward as it moves west of the subtropical ridge axis or if it interacts with the mid-latitude flow, such as the jet stream or an extratropical cyclone. This motion, termed “recurvature,” commonly occurs near the western edge of the major ocean basins, where the jet stream typically has a poleward component and extratropical cyclones are common.
In addition to strong winds and rain, hurricanes are capable of generating high waves, damaging storm surge, and tornadoes. They typically weaken rapidly over land where they are cut off from the warm water, their primary energy source. For this reason, coastal regions, as compared to inland regions, are particularly vulnerable to damage from a hurricane. Heavy rains, however, can cause significant flooding inland, and storm surges can produce extensive coastal ooding up to 25 miles from the coastline. Continued …
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