Tropical cyclone intensity scales

In the Western Pacific, the ESCAP/WMO Typhoon Committee uses four separate classifications for tropical cyclones that exist within the basin, which are based on the estimated maximum sustained winds over a 10-minute period.

The definition of sustained winds recommended by the World Meteorological Organization (WMO) and used by most weather agencies is that of a 10-minute average at a height of 10 m (33 ft) above the sea surface.

All of the scales rank tropical cyclones using their maximum sustained winds, which are either observed, measured or estimated using various techniques, over a period between one and ten minutes.

[1] Once either of these classifications are met, then advisories are initiated and the warning centers will classify the system as either a tropical or subtropical depression, if the one-minute sustained winds estimated or measured as less than 34 kn (39 mph; 63 km/h).

Should the system intensify further or already have one-minute sustained winds of 34–63 kn (39–72 mph; 63–117 km/h), then it will be called either a tropical or subtropical storm and assigned a name[1] (which replaces the spelled-out TC number; the two-digit number is still kept for purposes like the Automated Tropical Cyclone Forecasting System, as in 2018's TS 12 (KIRK)).

[1][3] A post tropical cyclone is a system that has weakened, into a remnant low or has dissipated and formal advisories are usually discontinued at this stage.

[1] They may also continue if the remnants of the system have a chance of regeneration and producing tropical storm or hurricane-force winds over land within 48 hours.

[7] Tropical cyclones that occur within the Northern Hemisphere between the anti-meridian and 100°E are officially monitored by the Japan Meteorological Agency (JMA, RSMC Tokyo).

[8] Should the system continue to intensify further then it will be classified as a severe tropical storm, which has winds speeds between 48–63 kn (25–32 m/s; 55–72 mph; 89–117 km/h).

[16] In addition to the national meteorological services of each nation, the United States' Joint Typhoon Warning Center (JTWC) monitors the basin, and issues warnings on significant tropical cyclones for the United States Government,[17] assigning them two-digit TC numbers (with suffix "W").

TD 20W, became Tropical Storm Bebinca, but was referred to as TS 20W (BEBINCA) in JTWC advisories); however, in cases when the JTWC upgrades a depression to tropical storm without the JMA following suit (due to the differences between JTWC and JMA wind-speed scales), the spelled-out number (without the suffix) is parenthesized and appended to the TC number as placeholder name, as in TS 16W (SIXTEEN), until JMA upgrades and names it, on which case the name replaces the placeholder.

[20] Any tropical cyclone that develops within the North Indian Ocean between 100°E and 45°E is monitored by the India Meteorological Department (IMD, RSMC New Delhi).

[21] The lowest official classification used in the North Indian Ocean is a depression, which has 3-minute sustained wind speeds of between 17 and 27 kn (20 and 31 mph; 31 and 50 km/h).

[21] If the system intensifies further, it will be classified as a cyclonic storm and be assigned a name by the IMD, if it should develop gale-force wind speeds of between 34 and 47 kn (39 and 54 mph; 63 and 87 km/h).

[21] The highest classification used in the North Indian Ocean is a super cyclonic storm, which has hurricane-force winds of at least 120 kn (140 mph; 220 km/h).

[22] During 2015, another modification to the scale took place, with the IMD calling a system with 3-minute maximum sustained wind speeds between 90 and 119 kn (104 and 137 mph; 167 and 220 km/h): an extremely severe cyclonic storm.

[24] A tropical disturbance is MFR's generic term for a non-frontal area of low pressure that has organized convection and definite cyclonic surface wind circulation.

[27] 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;[17] 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).

[28] The FMS numbers these systems when they have a potential to develop into a tropical cyclone or persist to cause significant impact to life and property, within its area of responsibility and have been analysed for the previous 24 hours.

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;[17] these systems are unofficially assigned TC numbers with either suffix "S" (if originating west of 135°E; spans the whole South Indian Ocean, including MFR's area of responsibility) or suffix "P" (if east of 135°E; spans the whole South Pacific Ocean, merging BoM, PNG-NWS, FMS, and MSNZ AORs together).

[32] It is calculated by taking the squares of the estimated maximum sustained velocity of every active tropical storm (wind speed 35 knots or higher) at six-hour intervals.

[32] As well as being squared for ACE, wind speed can also be cubed, which is referred to as the Power Dissipation Index (PDI).

[35] HSI was developed by a private company program in competition with the National Weather Service's accumulated cyclone energy index.

[citation needed] HSI was developed by ImpactWeather (now StormGeo) meteorologists as a proprietary method of hurricane severity classification.