A Tropical Cyclone can be (in order of magnitude) a Tropical DISTURBANCE, Tropical DEPRESSION, Tropical STORM, or a HURRICANE.
TROPICAL WAVE – (easterly wave) An area of disturbed weather with low pressure, but lacking organization, and moving from west to east in the Inter Tropical Convergence Zone. May or may not develop into a tropical cyclone.
TROPICAL DISTURBANCE – An area of disturbed weather with some organized convection that maintains its identity for at least 24 hours.
TROPICAL DEPRESSION – Also an area of disturbed weather, but with more organized convection, but not to the point of having an eye feature, and having winds of less than 39 mph.
TROPICAL STORM – This storm will usually have a closed surface center of circulation, around which stronger convection is loosely organized and having sustained winds of 39 to 73 mph. They may have the rough beginnings of a cyclonic shape, but there is usually not a developed eye.
HURRICANE – This storm will have sustained winds of at least 74 mph and a defined eye, or center, surrounded by an eye wall, in which the strongest winds and storms circulate around the center. The eye is a clear area of calm and low pressure. Generally, the stronger the hurricane, the better defined the eye, and the lower the barometric pressure will be. Hurricanes are classified on the Saffir Simpson Scale according to their maximum sustained winds.