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An Atlantic hurricane or tropical storm is a tropical cyclone that forms in the Atlantic Ocean, usually in the summer or fall. Tropical cyclones can be categorized by intensity. ''Tropical storms'' have one-minute maximum sustained winds of at least 39 mph (34 knots, 17 m/s, 63 km/h), while ''hurricanes'' have one-minute maximum sustained winds exceeding 74 mph (64 knots, 33 m/s, 119 km/h).〔National Hurricane Center. (Glossary of NHC/TPC Terms. ) Retrieved on 2006-10-28.〕 Most North Atlantic tropical storms and hurricanes form between June 1 and November 30.〔Chris Landsea. (Subject: E16- When did the earliest and latest hurricanes occur? ) Retrieved on 2008-06-10.〕 The United States National Hurricane Center monitors the basin and issues reports, watches, and warnings about tropical weather systems for the North Atlantic Basin as one of the Regional Specialized Meteorological Centers for tropical cyclones, as defined by the World Meteorological Organization. In recent times, tropical disturbances that reach tropical storm intensity are named from a predetermined list. Hurricanes that result in significant damage or casualties may have their names retired from the list at the request of the affected nations in order to prevent confusion should a subsequent storm be given the same name.〔NOAA (The Retirement of Hurricane Names ) Retrieved on 2008-06-10.〕 On average, in the North Atlantic basin (from 1966 to 2009) 11.3 named storms occur each season, with an average of 6.2 becoming hurricanes and 2.3 becoming major hurricanes (Category 3 or greater).〔http://www.nhc.noaa.gov/pastprofile.shtml〕 The climatological peak of activity is around September 11 each season.〔National Hurricane Center. (Tropical Cyclone Climatology. ) Retrieved on 2008-06-10.〕 In March 2004, Catarina was the first hurricane-intensity tropical cyclone ever recorded in the Southern Atlantic Ocean. Since 2011, the Brazilian Navy Hydrographic Center has started to use the same scale of the North Atlantic Ocean for tropical cyclones in the South Atlantic Ocean and assign names to those which reach . ==Steering factors== Tropical cyclones are steered by the surrounding flow throughout the depth of the troposphere (the atmosphere from the surface to about eight miles (12 km) high). Neil Frank, former director of the United States National Hurricane Center, used the analogies such as "a leaf carried along in a stream" or a "brick moving through a river of air" to describe the way atmospheric flow affects the path of a hurricane across the ocean. Specifically, air flow around high pressure systems and toward low pressure areas influence hurricane tracks. In the tropical latitudes, tropical storms and hurricanes generally move westward with a slight tend toward the north, under the influence of the ''subtropical ridge'', a high pressure system that usually extends east-west across the subtropics. South of the subtropical ridge, surface easterly winds (blowing from east to west) prevail. If the subtropical ridge is weakened by an upper trough, a tropical cyclone may turn poleward and then ''recurve'',〔U. S. Navy. (Section 2: Tropical Cyclone Motion Terminology. ) Retrieved on 2007-04-10.〕 or curve back toward the northeast into the main belt of the Westerlies. Poleward (north) of the subtropical ridge, westerly winds prevail and generally steer tropical cyclones that reach northern latitudes toward the east. The westerlies also steer extratropical cyclones with their cold and warm fronts from west to east.〔Hurricane Research Division. (Frequently Asked Questions: Subject G6 - What determines the movement of tropical cyclones? ) Retrieved on 2006-10-28.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Atlantic hurricane」の詳細全文を読む スポンサード リンク
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