Tropical cyclone intensity scales

Tropical cyclones are big storms with strong winds that form over warm ocean waters. They are ranked on different scales depending on how strong their winds are and where they occur in the world. Only a few classifications are used officially, but scientists also use other ways to measure their strength.

In the Northern Hemisphere, storms in the North Atlantic Ocean or the North-eastern Pacific Ocean are called tropical depressions or tropical storms. If they get stronger, they are called hurricanes and are ranked using the Saffir–Simpson hurricane wind scale based on their wind speeds. In the Western Pacific, a different system is used with four classifications.

The India Meteorological Department uses a scale with seven classifications for storms in the North Indian Ocean. In the Southern Hemisphere, storms are ranked using two different scales, one for the Australian or South Pacific region and another for the South-West Indian Ocean, which is used by Météo-France in places like New Caledonia and French Polynesia.

Different places measure wind speeds in slightly different ways, which makes it tricky to compare storms from one area to another. All around the world, tropical cyclones get names when their winds reach at least 35 knots.

Background

Tropical cyclones are big storms that form over warm ocean water. They have strong winds that spin around a center. Scientists measure how strong these winds are to rank the storms. Different places use different ranking systems, like the Saffir–Simpson hurricane wind scale in some areas and special scales in Australia. These rankings help us understand how powerful each storm is based on its wind speed.

Atlantic, Eastern and Central Pacific

Main article: Saffir–Simpson scale

Tropical cyclones in the Northern Hemisphere to the east of the anti-meridian are watched by the National Hurricane Center or the Central Pacific Hurricane Center. These storms are warm, swirling systems of air that form over tropical or subtropical waters. When they develop, they are first called tropical or subtropical depressions if their winds are less than 34 knots.

As they grow stronger, these storms may become tropical or subtropical storms and get names. If they become even stronger, with winds over 64 knots, they are called hurricanes. The Saffir–Simpson hurricane wind scale rates these hurricanes from Category 1 to Category 5 based on wind speed. Category 1 has winds of 64–82 knots, and Category 5 has winds of at least 137 knots. Even weaker storms can cause big damage, depending on where they hit and how much rain they bring.

Western Pacific

Tropical cyclones in the Western Pacific are watched by the Japan Meteorological Agency. These storms start as tropical depressions with winds slower than 34 knots. When they grow stronger, they become tropical storms with winds between 34 and 47 knots. Even stronger storms are called severe tropical storms, with winds between 48 and 63 knots. The strongest storms are called typhoons, with winds faster than 64 knots.

Different groups, like the China Meteorological Administration and Hong Kong Observatory, have their own ways to describe typhoons. Some call very strong typhoons "super typhoons" if their winds are very powerful. The Joint Typhoon Warning Center in the United States also tracks these storms and gives them numbers and names.

North Indian Ocean

Tropical cyclones that form in the North Indian Ocean between 100°E and 45°E are watched by the India Meteorological Department (IMD, RSMC New Delhi). In this area, a tropical cyclone is a storm over warm water with strong winds and heavy rain. The weakest storms are called depressions, with winds from 17 to 27 knots (about 20 to 31 miles per hour). When these storms get stronger, they are called deep depressions, then cyclonic storms, and finally severe cyclonic storms. The strongest storms are called super cyclonic storms, with very powerful winds.

The Joint Typhoon Warning Center in the United States also tracks these storms and gives them numbers, using a different way to measure wind speed. Storms in the Arabian Sea get an "A" after their number, and storms in the Bay of Bengal get a "B".

South-West Indian Ocean

Any tropical cyclone that forms in the Southern Hemisphere between Africa and 90°E is watched by Météo-France’s La Réunion tropical cyclone centre (MFR, RSMC La Réunion).

A tropical disturbance is MFR’s name for a non-frontal area of low pressure that has organized convection and definite cyclonic surface wind circulation. The system should be estimated to have wind speeds of less than 28 knots (52 km/h; 32 mph).

A system becomes a tropical depression or a subtropical depression when it reaches 10 min sustained wind speeds above 28 knots (52 km/h; 32 mph). If a tropical depression reaches wind speeds of 35 knots (65 km/h; 40 mph) then it is called a moderate tropical storm and given a name by either the Sub Regional Center in Mauritius or Madagascar. Since the 2024–25 cyclone season, a subtropical system is called a subtropical storm if it reaches wind speeds of 35 knots (65 km/h; 40 mph).

If the named tropical system gets stronger and reaches wind speeds of 48 knots (89 km/h; 55 mph), it is called a severe tropical storm. A severe tropical storm becomes a tropical cyclone when it reaches wind speeds of 64 knots (119 km/h; 74 mph). If a tropical cyclone gets even stronger and reaches wind speeds of 90 knots (170 km/h; 100 mph), it is called an intense tropical cyclone. A very intense tropical cyclone is the highest category on the South-West Indian Ocean Tropical Cyclone scale, and has winds of over 115 knots (213 km/h; 132 mph).

At the tenth RA I tropical cyclone committee held during 1991, it was recommended that the intensity classifications be changed ahead of the 1993–94 tropical cyclone season. Specifically it was decided that the classifications: Weak Tropical Depression, Moderate Tropical Depression and Severe Tropical Depression would be changed to Tropical Depression, Moderate Tropical Storm and Severe Tropical Storm. This change was implemented ahead of the 1993–94 tropical cyclone season.

The United States Joint Typhoon Warning Center also monitors the basin, and issues warnings on significant tropical cyclones on behalf of the United States Government; these systems are unofficially assigned TC numbers with suffix "S" (which spans the whole South Indian Ocean, including both BMKG and BoM areas of responsibility west of 135°E). These warnings use a 1-minute sustained wind speed and can be compared to the Saffir–Simpson hurricane wind scale, however, regardless of intensity in this basin the JTWC labels all systems as tropical cyclones with TC numbers (plus any parenthesized names or placeholders, like typhoons and North Indian Ocean cyclones above).

Australia and Fiji

Tropical cyclones in the Southern Hemisphere east of 90°E are watched by several weather services, including the Fiji Meteorological Service, New Zealand's MetService, Indonesia's Badan Meteorologi, Klimatologi, dan Geofisika, Papua New Guinea's National Weather Service, and Australia's Bureau of Meteorology. These storms are defined as systems with strong winds and low pressure over warm water.

The Australian tropical cyclone intensity scale rates these storms from Category 1 to Category 5 based on wind speed measured over 10 minutes. A Category 1 storm has winds of about 39–54 mph, while a Category 5 storm has winds of at least 124 mph. Other terms like Tropical Disturbance and Tropical Depression are used for weaker systems that may develop into full tropical cyclones.

Alternative scales

There are other ways to measure the strength of tropical cyclones that are not officially used by weather centers, but some groups, like the National Oceanic and Atmospheric Administration, use them. One example is the Integrated Kinetic Energy index, which looks at how much a storm could damage the coast, ranging from one to six, with six being the most powerful.

Accumulated cyclone energy (ACE) is a tool used to show how active a tropical cyclone or a whole season of storms is. It is worked out by measuring the wind speeds of storms every six hours and doing some math with those numbers. The result is usually made smaller by dividing by 10,000.

The Hurricane Severity Index (HSI) is another way to measure tropical cyclones. It looks at both how strong the storm is and how big its windy area is, giving scores out of 50. This scale was created by weather experts at ImpactWeather, now known as StormGeo, but it doesn’t always get things right because it doesn’t consider how much rain a storm might bring.