<P> 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). Most North Atlantic tropical storms and hurricanes form between June 1 and November 30 . 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 . </P> <P> 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 . 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). The climatological peak of activity is around September 11 each season . </P> <P> 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 35 kn (65 km / h; 40 mph). </P> <P> 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 influences hurricane tracks . </P>

What causes hurricanes to form in the atlantic ocean