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
. While these storms are not given names, forecasters do issue warnings for them.
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