Maximum sustained wind

Saffir–Simpson scale
CategoryWind speeds
(for 1-minute maximum sustained winds)
m/sknots (kn)mphkm/h
Five ≥ 70 m/s   ≥ 137 kn   ≥ 157 mph   ≥ 252 km/h  
Four  58–70 m/s    113–136 kn    130–156 mph    209–251 km/h  
Three  50–58 m/s    96–112 kn    111–129 mph    178–208 km/h  
Two  43–49 m/s    83–95 kn    96–110 mph    154–177 km/h  
One  33–42 m/s    64–82 kn    74–95 mph    119–153 km/h  
Related classifications
(for 1-minute maximum sustained winds)
Tropical storm  18–32 m/s    34–63 kn    39–73 mph    63–118 km/h  
Tropical depression  ≤ 17 m/s    ≤ 33 kn    ≤ 38 mph    ≤ 62 km/h  

The maximum sustained wind associated with a tropical cyclone is a common indicator of the intensity of the storm. Within a mature tropical cyclone, it is found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unlike gusts, the value of these winds are determined via their sampling and averaging the sampled results over a period of time. Wind measuring has been standardized globally to reflect the winds at 10 metres (33 ft) above the Earth's surface, and the maximum sustained wind represents the highest average wind over either a one-minute (US) or ten-minute time span (see the definition, below), anywhere within the tropical cyclone. Surface winds are highly variable due to friction between the atmosphere and the Earth's surface, as well as near hills and mountains over land.

Over the ocean, satellite imagery determines the value of the maximum sustained winds within a tropical cyclone. Land, ship, aircraft reconnaissance observations, and radar imagery can also estimate this quantity, when available. This value helps determine damage expected from a tropical cyclone, through use of such scales as the Saffir–Simpson scale.

Definition

The maximum sustained wind normally occurs at a distance from the center known as the radius of maximum wind, within a mature tropical cyclone's eyewall, before winds decrease at farther distances away from a tropical cyclone's center.[1] Most weather agencies use the definition for sustained winds recommended by the World Meteorological Organization (WMO), which specifies measuring winds at a height of 10 metres (33 ft) for 10 minutes, and then taking the average. However, the United States National Weather Service defines sustained winds within tropical cyclones by averaging winds over a period of one minute, measured at the same 10 metres (33 ft) height.[2] This is an important distinctions, as the value of the highest one-minute sustained wind is about 14% greater than a ten-minute sustained wind over the same period.[3]