A cyclone that eventually reaches hurricane intensity first
passes through two intermediate stages known as tropical
depression and tropical storm. Hurricanes start over the oceans
as a collection of storms in the tropics. The deepening low-
pressure center takes in moist air and thermal energy from the
ocean surface, convection lifts the air, and high pressure
higher in the atmosphere pushes it outward. Rotation of the wind
currents tends to spin the clouds into a tight curl; as the
winds reach gale force, the depression becomes a tropical storm.
The mature hurricane is nearly circularly symmetrical, and its
influence often extends over an area 500 mi (805 km) in
diameter. As a result of the extremely low central pressure
(often around 28.35 in./960 millibars but sometimes considerably
lower, with a record 25.69 in./870 millibars registered in a
1979 NW Pacific typhoon) surface air spirals inward cyclonically
(counterclockwise in the Northern Hemisphere and clockwise in
the Southern Hemisphere), converging on