How do we get from a “tropical depression” to a “hurricane” and back? We think mostly in terms of watches and warnings and wind speeds, but the process is more involved. It all begins with a tropical storm, which may or may not develop into a tropical cyclone, and then into a full-blown hurricane.
Tropical waves occur when a kink or bend occurs in the normally straight flow of air over water in tropical areas. A low pressure trough, or pressure boundary, forms. A tropical wave, once well established, can involve thunderstorms. If the thunderstorms become organized, a tropical storm can develop.
Tropical storms begin as organized areas of thunderstorms that form in tropical water. Winds are below 30 mph, low pressure may or may not form at the surface, and the activity persists for more than 24 hours. When winds increase to 33-39 mph, and are sustained near the surface, a tropical depression/cyclone is declared and a number is assigned.
Tropical cyclones continue to develop around a center axis, which is defined as the area of minimum wind and pressure. The rotation is counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere.
When the central core of a tropical cyclone becomes warmer than the surrounding air, there is no front and the winds are stronger at the water’s surface, a tropical depression is confirmed. If the air at the core is colder than the surrounding air AND the storm is formed outside of the tropics, the storm is declared to be an Extra-tropical Cyclone.
The Fujiwhara Effect: occurs when two tropical cyclones are near each other and begin to rotate around each other.
A hurricane is a tropical cyclone that has sustained winds in excess of 74-79 mph, with a well-developed eye and an eye wall, and a definite circular rotation. The eye and is ringed by thunderstorms and is calm at the center. The eye wall is composed of thunderstorms.
The term hurricane is used for Northern Hemisphere tropical cyclones east of the International Dateline to the Greenwich Meridian. The term typhoon is used for Pacific tropical cyclones north of the Equator west of the International Dateline.
Storms in the Atlantic basin, the Pacific Ocean, and East of the International dateline are the most common to be designated as hurricanes. After the hurricane is assigned a name, the hurricane goes through watch levels, based on wind speed and air pressure. When the hurricane loses force, it returns to the tropical cyclone, then tropical depression states until it dissipates.
National Weather Service “Glossary of NHC Terms”