subtropical cyclone
They occur in regions of weak to moderate horizontal
temperature gradient and extract the associated
available potential energy, as do
baroclinic cyclones, but they also receive some or most of their
energy from convective redistribution of
heat acquired from the sea, as do tropical cyclones. These storms usually have a radius of maximum
winds that is larger than what is observed in purely tropical systems, and their maximum sustained winds have not been observed to exceed about 32 m s
−1 (64 knots). Subtropical cyclones sometimes become true tropical cyclones, and likewise, tropical cyclones occasionally become subtropical storms. Subtropical cyclones in the Atlantic
basin are classified by their maximum sustained surface winds: Subtropical depressions have surface winds less than 18 m s
−1 (35 knots), while subtropical storms have surface winds greater than or equal to 18 m s
−1.
Term edited 3 December 2021.
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