Stages of development - tropical
depression
If sustained winds
increase to at least 20 knots, a disturbance is
upgraded to a tropical depression.
Surface wind
speeds vary between 20 and 34 knots
A tropical
depression has at least one closed isobar that
accompanies a drop in pressure in the center of
the storm.
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![](graphics/stages2.gif) image adopted from WW2010 Online Guides
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