Tropical systems are ranked on one of several tropical cyclone scales according to what oceanic basin they are located in and their maximum sustained winds. Five scales of classifications are officially used by the meteorological agencies monitoring tropical cyclones in those specific regions designated by the World Meteorological Organization as a Regional Specialized Meteorological Centre (RSMC).

Global Distribution of Tropical Storms:
1985 - 2005
Source: Wikipedia

A tropical cyclone formed in the North Atlantic Ocean or the Northeastern Pacific Ocean is classified using the Saffir-Simpson Hurricane Scale by the US National Hurricane Center. In the Western Pacific, tropical cyclones are ranked using the Japan Meteorological Agency's scale. The U.S. Joint Typhoon Warning Center (JTWC), the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), the China Meteorological Agency (CMA) and the Hong Kong Observatory (HKO) also monitor tropical cyclones in this basin and may use different ratings scales. The RSMC run by the Indian Meteorological Department in New Delhi, India uses its scale to assess tropical cyclones in the Northern Indian Ocean, particularly the Bay of Bengal and the Arabian Sea. Other warning centres that watch this region include RSMC La Reunion, the Bangladesh Meteorological Department and the United States Joint Typhoon Warning Center.The Mιtιo-France forecast center on La Reunion covers the Southwest Indian Ocean. Both the Australian Bureau of Meteorology and the RSMC in Nadi, Fiji use the Australian tropical cyclone intensity scale for storms in their jurisdictions in the South Indian Ocean and South Pacific Ocean.

The definition of sustained winds recommended by the World Meteorological Organization (WMO), used by most weather agencies, is a 10-minute average measured at a height of 10 m (33 ft). There are exceptions. The Saffir-Simpson Hurricane Scale is based on wind speed measurements averaged over a 1-minute period, at 10 m (33 ft) above the surface. The scale used by RSMC New Delhi applies a 3-minute averaging period, and the Australian scale is based on both 3-second wind gusts and maximum sustained winds averaged over a 10-minute interval.

Atlantic Ocean and East Pacific Ocean Basins

Saffir–Simpson Hurricane Wind Scale
US National Hurricane Center

Category

Wind Speeds

Western Pacific Ocean Basin

Tropical Cyclone Intensity Scale
Japan Meteorological Agency

North Indian Ocean Basin

Tropical Cyclone Intensity Scale
India Meteorological Department

Southwest Indian Ocean Basin

Tropical Cyclone Intensity Scale
Mιtιo-France
Regional Specialized Meteorological Center
in La Reunion

Australia and Fiji Basin

Australian Tropical Cyclone Intensity Scale
Australian Bureau of Meteorology