Stages of Development
from disturbance to hurricane
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 and a tropical depression has at least one closed isobar that accompanies a drop in pressure in the center of the storm.

Terms for using data resources. CD-ROM available.
Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.