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How fast is 81 knots?

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It's about as fast as a Fastball (baseball)
(a.k.a. rising fastball, a.k.a. cross-seam fastball, a.k.a. heater, a.k.a. hummer, a.k.a. smoker; for four-seam grip) (major league average)
The average speed of major league fastball pitch is 81 knots. When up against the quickest professional fastball pitchers, a batter may have less than 0.4 seconds to react to a pitched ball.
It's about one-and-one-fifth times as fast as a Curveball (baseball)
In other words, 81 knots is 1.2 times the speed of a Curveball (baseball), and the speed of a Curveball (baseball) is 0.83 times that amount.
(a.k.a. hook, a.k.a. hammer, a.k.a. yakker) (major league average)
The average speed of major league curveball pitch is 65 knots. In the 1940's, debate over whether there really was a curve in the curveball pitch was settled with the conclusion that the ball does curve; however, an optical illusion caused by the spin of the ball and the batter's perception of motion exaggerates the extent of the curve.
It's about one-and-one-fourth times as fast as a Hurricane
In other words, 81 knots is 1.26 times the speed of a Hurricane, and the speed of a Hurricane is 0.794 times that amount.
(formally: Topical cyclone; a.k.a. typhoon)
A hurricane is defined by the US National Hurricane Center as a Northern Hemisphere tropical storm having one-minute average wind-speeds of at least 64.30 knots. Typhoons Tip (October, 1979) and Keith (October, 1997) and Hurricanes Camille (August, 1969) and Allen (August, 1980) jointly hold the record for highest tropical storm wind speeds at 167 knots.
It's about four-fifths as fast as a Skydiver (belly-to-earth)
In other words, 81 knots is 0.79 times the speed of a Skydiver (belly-to-earth), and the speed of a Skydiver (belly-to-earth) is 1.3 times that amount.
(Belly-to-Earth orientation, average conditions, terminal velocity)
A belly-to-Earth oriented skydiver's terminal velocity is about 100 knots. In a typical jump from 3,900 m (13,000 ft), a diver in this orientation will be in freefall for 60 seconds.
It's about three-fourths as fast as a Tornado
In other words, 81 knots is 0.7547622 times the speed of a Tornado, and the speed of a Tornado is 1.324921 times that amount.
(EF2) (wind speed range average)
According to the Enhanced Fujita scale implemented by the US National Oceanographic and Atmospheric Administration, a "significant" tornado has an Enhanced Funjita scale classification of EF2 and is characterized by wind gust speeds between 95.587390 knots and 119.0497 knots. The largest recorded tornado — an F4 event occurring in Nebraska in May, 2004 — was almost 4.02 km (2.5 mi) across.
It's about one-and-one-third times as fast as a Cheetah
In other words, 81 knots is 1.358 times the speed of a Cheetah, and the speed of a Cheetah is 0.7364 times that amount.
(Acinonyx jubatus)
The cheetah can reach speeds of up to 59.670 knots in short bursts. From a crouching position, the cheetah can attain these speeds in just 2.25 seconds.
It's about one-and-two-fifths times as fast as a Knuckleball (baseball)
In other words, 81 knots is 1.4 times the speed of a Knuckleball (baseball), and the speed of a Knuckleball (baseball) is 0.71 times that amount.
(a.k.a. knuckler, a.k.a. floater, a.k.a. dancer, a.k.a. butterfly ball) (major league average)
The average speed of major league knuckleball pitch is 59 knots. Eddie Cicotte, who was later implicated in the 1919 Black Sox scandal, is credited with developing the pitch ca. 1906.