Severity of Tropical Storms

Watching the Weather Channel, I’ve heard them use the following terms relating to situations developing in the Atlantic Ocean:
[li]Tropical Wave[/li][li]Tropical Disturbance[/li][li]Tropical Depression[/li][li]Tropical Storm[/li][li]Hurricane[/li][/ul]

Obviously, Tropical Storm and Hurrican are the worst, but how do the other three stack up, in terms of severity. Are there any other terms?

A map of the Atlantic showing the intensities of storms as of today.

This is a rather cool site with a description of hurricane formation and degrees of intensity, including a chart of wind speed associated with each level.

To add to the links above, a tropical wave is an area of showers and thunderstorms, usually associated with a large-scale area of low pressure. These are extremely common, especially this time of year in the northern hemisphere. For the most part, tropical wave and tropical disturbance are interchangeable, however there is a stage of development after the tropical wave and before a tropical depression (which has its definition in the link), and it is characterized by having 12 hours of persistent convection (thunderstorms) and a definitive low level circulation center. When these two criteria are met, the storm is officially a disturbance. A depression is a disturbance that has at least one closed isobar, and usually has maximum sustained winds of a least 20 mph.

As for other terms…I think the link covered it well for the most part, though their “track” graphic is a bit inaccurate, especially in the Indian Ocean.


In other words, a “tropical disturbance” is a tropical wave that has taken on the features of a cyclone (in the technical meteorological sense, not the Australian storm sense) but has not yet ascended to “tropical depression” status. Do I have that right?

I believe so, as I’m not sure I could pull up a cite stating that if asked. I did worldwide tropical cyclone satellite analysis for several years, and that’s what I personally would believe.


A tropical wave is a disorganized area of convection. If any organization can be found (i.e., circulation around a low pressure area), it becomes a tropical depression or can immediately jump to a TS, as Hermine has just done. As Viscera said, usually a tropical wave is also called a tropical disturbance. The intermediate stage between a tropical wave and a tropical depression being called a tropical disturbance is a new nuance to me.

Before Gaston was labeled a tropical depression, it was sitting off the coast of SC spinning bands of rains over the coastal areas for a few days. It had all the features (as I could see) of a tropical depression, yet it wasn’t labeled a tropical depression until it developed significantly Friday afternoon. The local meteorologists were saying there was a low pressure area out there which was stationary. I suppose that it was not a tropical phenomenon then because it did not have the tropical characteristics, particularly development through latent heat (diabatic energy). Perhaps ** Viscera** can fill me in on Gaston?