Tropical Wave: A "bump" or disruption of normal tropical easterly flow. Associated turning of wind causes low-level convergence of air, which helps with falling pressure and enhanced showers.
This can evolve into a Tropical Depression, which is a closed circulation of air in the low levels. This in turn increases convergence and pressure falls, and wind speeds increase in a Catch-22 effect (i.e. the stronger the wind blows the greater the convergence, the quicker the pressure falls... so the stronger the wind, etc.).
Once sustained winds reach 39 mph in the closed circulation a Tropical Storm is named. Usually there are at least 2 closed isobars (lines of equal pressure) of 4 mb increments around the center. If atmospheric conditions remain correct the system will evolve into a...
Hurricane. There is usually a difference in pressure of at least 0.60 inches of mercury between the center and surrounding pressure field, with the greatest change near the center (eyewall). It is this great difference in pressure, which sometimes can be as great as 2.95 inches of mercury, that causes the wind to be so strong.
A mature hurricane is a well-oiled meteorological machine, but disruption of the processes that drive the storm (i.e. interaction with land or colder air feeding in) will begin to destroy the storm, and the disintegration of a hurricane can often be quick and dramatic.
The Saffir-Simpson Wind Scale is a categorical classification of hurricanes based on their wind speed, used by the U.S. government's National Hurricane Center.
To qualify as a Category 5 hurricane on the Saffir-Simpson hurricane intensity scale, maximum sustained winds must exceed 155 mph (135 kt).
Hurricanes that have a severe impact on lives or the economy are remembered by generations after the devastation they caused, and some go into weather history.
AccuWeather.com has created a number of specialty maps designed for tracking the progress of tropical storms and hurricanes. Use these maps in conjunction with our Hurricane Position graphic, as well as statements issued by the NHC with storm positions.
Hurricanes (by whatever name) are by far most common in the Pacific Ocean, with the western Pacific being most active. In some years, the Philippines are struck by more than 20 tropical storms and typhoons.
Low pressure in the hurricane can act as a plunger, slightly pulling up the water level. However, the components that contribute to the greatest storm surge affect are the winds blowing to the left side of the storm and the topography of the land as the storm makes land fall.