Atlantic hurricaneFrom Wikipedia, the free encyclopedia

Tracks of North Atlantic tropical cyclones(18512012)AnAtlantic hurricaneortropical stormis atropical cyclonethat forms in theAtlantic Ocean, usually in the summer or fall. Tropical cyclones can be categorized by intensity.Tropical stormshave one-minute maximum sustained winds of at least 39mph (34knots, 17m/s, 63km/h), whilehurricaneshave one-minute maximum sustained windsexceeding 74mph(64knots, 33m/s, 119km/h).[1]Most North Atlantictropical stormsand hurricanes form between June 1 and November 30.[2]TheUnited StatesNational Hurricane Centermonitors the basin and issues reports, watches, and warnings abouttropicalweather systems for the North Atlantic Basin as one of the Regional Specialized Meteorological Centers fortropical cyclones, as defined by theWorld Meteorological Organization.[3]In recent times, tropical disturbances that reach tropical storm intensity are named from apredetermined list. Hurricanes that result in significant damage or casualties mayhave their names retired from the listat the request of the affected nations in order to prevent confusion should a subsequent storm be given the same name.[4]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 3or greater).[5]Theclimatologicalpeak of activity is around September 11 each season.[6]In March 2004,Catarinawas the first hurricane-intensity tropical cyclone ever recorded in the Southern Atlantic Ocean. Since 2011, theBrazilian Navy Hydrographic Centerhas started to use the same scale of the North Atlantic Ocean for tropical cyclones in the South Atlantic Ocean and assign names to them which reach 35kn (65km/h; 40mph).[7]Contents 1Steering factors 2Climatology 2.1June 2.2July 2.3August 2.4September 2.5October 2.6November 2.7December to May 3Extremes 4Trends 5See also 6References 7External linksSteering factors

The subtropical ridge (in the Pacific) shows up as a large area of black (dryness) on this water vapor satellite image from September 2000Tropical cyclonesare steered by the surrounding flow throughout the depth of thetroposphere(the atmosphere from the surface to about eight miles (12km) high). Neil Frank, former director of theUnited StatesNational 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 aroundhigh pressuresystems and towardlow pressure areasinfluence hurricane tracks.In thetropical latitudes, tropical storms and hurricanes generally move westward with a slight tend toward the north, under the influence of thesubtropical ridge, a high pressure system that usually extends east-west across the subtropics.[8]South of the subtropical ridge, surface easterly winds (blowing from east to west) prevail. If the subtropical ridge is weakened by an uppertrough, a tropical cyclone may turn poleward and thenrecurve,[9]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. Thewesterliesalso steerextratropical cycloneswith their cold and warm fronts from west to east.[10]ClimatologyTotal and Average Number ofTropical Storms by Month (18512011)

MonthTotalAverage

January April5