Classification and Ranking

Intensity Classifications

There are three main groups of classification for tropical cyclones, based on their intensity: tropical depressions, tropical storms, and a third group of more intense storms, whose name depends on the region.

Tropical Depressions

A tropical depression is an organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of less than 17 m/s (33 knots, 38 mph, or 62 km/h). It has no eye, and does not typically have the organization or the spiral shape of more powerful storms. It is already a low-pressure system, however, hence the name "depression."

Tropical Storms

A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 and 32 m/s (34-63 knots, 39-73 mph, or 62-117 km/h). At this point, the distinctive cyclonic shape starts to develop, though an eye is usually not present. Government weather services assign first names to systems that reach this intensity (thus the term named storm).

Tropical Cyclones (Hurricanes, Typhoons)

A tropical cyclone (or hurricane or typhoon) is a system with sustained winds greater than 33 m/s (64 knots, 74 mph, or 118 km/h). A tropical cyclone tends to develop an eye, an area of relative calm (and lowest atmospheric pressure) at the centre of circulation. The eye is often visible in satellite images as a small, circular, cloud-free spot. Surrounding the eye is the eyewall, an area about 10-50 mi (16-80 km) wide in which the strongest thunderstorms and winds circulate around the storm's centre.

The circulation of clouds around a cyclone's centre imparts a distinct spiral shape to the system. Bands or arms may extend over great distances as clouds are drawn toward the cyclone. The direction of the cyclonic circulation depends on the hemisphere; it is counter clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Maximum sustained winds in the strongest tropical cyclones have been measured at more than 85 m/s (165 knots, 190 mph, 305 km/h). Intense, mature hurricanes can sometimes exhibit an inward curving of the eyewall top that resembles a football stadium: this phenomenon is thus sometimes referred to as the stadium effect.

Eyewall replacement cycles naturally occur in intense tropical cyclones. When cyclones reach peak intensity they usually - but not always - have an eyewall and radius of maximum winds that contract to a very small size, around 5 to 15 miles. At this point, some of the outer rainbands may organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and momentum. During this phase, the tropical cyclone is weakening (i.e. the maximum winds die off a bit and the central pressure goes up). Eventually the outer eyewall replaces the inner one completely and the storm can be the same intensity as it was previously or, in some cases, even stronger.

Categories and Ranking

Hurricanes are ranked according to their maximum winds using the Saffir-Simpson Hurricane Scale. A Category 1 storm has the lowest maximum winds (74-95 mph, 119-153 km/h), a Category 5 hurricane has the highest (> 155 mph, 249 km/h). The U.S. National Hurricane Center classifies hurricanes of Category 3 and above as major hurricanes.

The U.S. Joint Typhoon Warning Center classifies West Pacific typhoons as tropical cyclones with winds greater than 73 mph (118 km/h). Typhoons with wind speeds of at least 150 mph (67 m/s or 241 km/h, equivalent to a strong Category 4 hurricane) are dubbed Super Typhoons.

The Australian Bureau of Meteorology uses a 1-5 scale called tropical cyclone severity categories. Unlike the Saffir-Simpson Hurricane Scale, severity categories are based on estimated maximum wind gusts. A category 1 storm features gusts less than 126 km/h (78 mph), while gusts in a category 5 cyclone are at least 280 km/h (174 mph).

Meteorologists in the United States use maximum 1-minute average sustained winds 10 meters above the ground to determine tropical cyclone strength. Other countries use the maximum 10-minute average, as suggested by the World Meteorological Organization. Maximum wind speeds are typically about 12% lower with the 10-minute method than with the 1-minute method.

The rankings are not absolute in terms of damage and other effects. Lower-category storms can inflict greater damage than higher-category storms, depending on factors such as local terrain and total rainfall. For instance, a Category 2 hurricane that strikes a major urban area will likely do more damage than a large Category 5 hurricane that strikes a mostly rural region. In fact, tropical systems of minimal strength can produce significant damage and human casualties from flooding and landslides.

Regional Terminology

Terms used in weather reports for tropical cyclones that have surface winds over 64 knots (73.6 mph) or 32 m/s vary by region:

• Hurricane: Atlantic basin and North Pacific Ocean east of the International date line
• Typhoon: Northwest Pacific west of the dateline
• Severe tropical cyclone: Southwest Pacific west of 160°E and the southeast Indian Ocean east of 90°E
• Very severe cyclonic storm: North Indian Ocean
• Tropical cyclone: Southwest Indian Ocean and the South Pacific east of 160°E.
• Cyclone (unofficially): South Atlantic Ocean

There are many regional names for tropical cyclones, including bagyo in the Philippines and Taino in Haiti.