Most hurricanes follow a similar cycle of development, called a hurricane life cycle. These life cycles may run their course in as little as a day or last as long as a month. The longest-lasting tropical cyclone ever observed was Hurricane/Typhoon John, which existed for 31 days as it traveled a 13,000 km (8,100 mi) path from the eastern Pacific to the western Pacific and back to the central Pacific. There have also been many tropical cyclones that remained at hurricane intensity for 12 hours or less, including the Atlantic hurricane, Ernesto, in 2006.
All hurricanes begin as an area of low pressure in the atmosphere, where surface winds are converging toward each other. This low-pressure area is called a tropical disturbance. If suitable conditions exist (see Hurricane Genesis: Birth of a Hurricane), the circulation may become more organized and wind speeds may increase. Once the system obtains a clearly identifiable circulation center, the system is upgraded to a tropical depression. If winds continue to intensify to greater than 63 km/hr (39 mph), the system will be classified as a tropical storm, and once winds are sustained above 119 km/hr (74mph), the system is officially upgraded to a hurricane (In the Atlantic, Central Pacific, and Eastern Pacific regions. Other terms apply elsewhere.)
Tropical Disturbance (Tropical Wave)
The majority of tropical storms and hurricanes start out as tropical disturbances. These weather systems are unorganized masses of thunderstorms with very little, if any, organized wind circulation. During the hurricane season in the Atlantic Ocean and Gulf of Mexico, tropical disturbances often grow from a pattern of stormy weather, called an African easterly wave. These waves typically emerge every three or four days off the west coast of Africa and then drift west within the trade winds into the Atlantic Ocean. If weather and ocean conditions continue to be favorable, the system may then strengthen. For more details, see Hurricane Genesis: Birth of a Hurricane.
As a system continues to become organized and winds begin to circulate, it may become a tropical depression, the weakest form of tropical cyclone. It is called a “depression” because it has low, or depressed, air pressure at its center. As the system develops, winds converge towards the center and the pressure near the center drops. Tropical depressions exhibit tropical transition. During this transition, the disturbance begins to obtain its energy from the ocean instead of from horizontal temperature gradients in the atmosphere and the environmental wind field (see Hurricane Development: From Birth to Maturity). Sometimes, tropical transition occurs when the maximum surface wind speed is less than or equal to 61 km/hr (38 mph). Maximum surface wind speed is defined as the maximum 10-minute averaged (1-minute averaged in the U.S. only) wind speed found anywhere in the tropical cyclone at 10 m (33 ft) height.
As bands of thunderstorms continue to develop, the depression may intensify into a tropical storm with maximum sustained wind speeds of 63-117.5 km/hr (39-73mph). A tropical storm usually forms in this manner (i.e. from an intensifying tropical depression), but sometimes it can form directly from the tropical transition of an extratropical cyclone or subtropical storm (see: Hurricane Genesis: Birth of a Hurricane). Once a system is classified as a tropical storm, it is given a name (for more detail, please see Hurricane Forecasting). Approximately 100 tropical cyclones form globally each calendar year. Many of them die out before they can grow stronger, with only approximately half of them (50) eventually strengthening into a mature hurricane (or typhoon).
If a tropical cyclone obtains a maximum sustained wind speed greater than or equal to 119 km/hr (74 mph), it is reclassified as a hurricane if it is located in the Atlantic, Central Pacific, or Eastern Pacific regions. In other ocean regions, the terms typhoon, severe tropical cyclone, severe cyclonic storm, or simply tropical cyclone are used instead. At this point, the recognizable, cloud-free eye of a hurricane typically forms (see Hurricane Structure and Primary Circulation).
In the Atlantic, Central Pacific, or Eastern Pacific regions, hurricane intensity is often classified based on maximum surface wind speed using the five categories of the Saffir-Simpson Hurricane Wind Scale, where categories 3-5 are considered major hurricanes:
In general, damage from landfalling hurricanes increases roughly 4 times for each category they intensify. However, lower category storms (and tropical storms) can cause substantial damage depending on their size, the other weather features with which they interact, where they strike, and the speed with which they propagate. In addition to wind, damage from hurricanes also depends on storm surge (which can cause coastal flooding) and rainfall-induced inland flooding. Storm surge is as much a function of storm size (hurricane force wind radii) as it is the peak winds of a hurricane. Hurricane Katrina had an extreme storm surge normally associated with category 5 hurricanes (storm surge flooding of 7.6 to 8.5 meters (25 to 28 ft) above normal tide level occurred along portions of the Mississippi coast, with storm surge flooding of 3 to 6 meters (10 to 20 ft) above normal tide levels along the southeastern Louisiana coast) even though it was only a Category 3 hurricane at landfall. Tropical Storm Charley (1998) caused heavy rainfall between 127 - 254 millimeters (5-10 in) in some Texas cities. Included in these was the city of Del Rio, TX, where more than 432 millimeters (17 in) of rain fell in just one day. Twenty people died from drowning during Tropical Storm Charley and total damage was estimate at $50 million (1998 USD). Tropical storm Allison (2001) was even worse, dumping over 762 millimeters (30 in) of rain in some areas near Houston, TX . Allison was responsible for 41 deaths and at least $5 billion (2001 USD) in damage in the United States (most of which occurred in TX), making it the deadliest and costliest U.S. tropical storm on record. For more information on hurricane risks and impacts please see Hurricane Hazards and Impacts.
Most hurricanes do not reach Category 5 intensity. Category 5 systems develop only about once every three years on average in the Atlantic region. Only the 1960, 1961, 2005, and 2007 Atlantic hurricane seasons have seen multiple Category 5 hurricanes form, and only in 2007 did more than one hurricane make landfall at Category 5 strength.