A "tropical storm" usually refers to a tropical low pressure area whose winds
have increased to at least 35 knots (about 40 mph) but less than 65 knots (about 75 mph, which then
becomes "hurricane" strength).
A tropical storm goes through a "tropical depression" stage first,
which has winds less than 35 knots, and a closed cyclonic circulation. Some tropical waves can
be very strong, with peak winds of at least 35 knots, but if they do not have a closed cyclonic
(circular) circulation, they are classified as a wave rather than a depression.
Even weaker
systems are sometimes called a "tropical disturbance". These may be no more than a persistent
area of showers and thunderstorms.
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