Tropical Terminology

By: Stu Ostro , 1:47 AM GMT on June 30, 2014

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With a system swirling offshore of the southeast U.S. and trying to become the first official numbered tropical depression or named tropical storm of the year in the Atlantic, even if it doesn't it reminds us that we're in hurricane season, and here is some basic, fundamental terminology related to tropical cyclones.

Rather than a comprehensive and/or technical glossary (there's one in that vein on the National Hurricane Center site), this represents the essence of the meaning & importance of some key, frequently used terms.


TROPICAL CYCLONE

Generic term for tropical depressions, tropical storms, and hurricanes. 

A tropical cyclone is characterized by lack of warm/cold fronts attached, a “warm core” (air is warmer in center of the cyclone than elsewhere), and persistent convection, attributes commonly referred to as a cyclone having “tropical characteristics."


HURRICANE

A tropical cyclone in which the maximum sustained surface wind speed is at least 74 mph. 

A “typhoon” is the same thing, just what it’s called in the western Pacific.

 
TROPICAL STORM  (T.S.)

A tropical cyclone in which the maximum sustained surface wind speed ranges from 39 to 73 mph.   Though less “intense” wind-wise, tropical storms and tropical depressions can produce destructive and deadly flooding from heavy rainfall.

Upon becoming a tropical storm, the National Hurricane Center (NHC) gives it a name.

 
TROPICAL DEPRESSION  (T.D.)

A tropical cyclone in which the maximum sustained surface wind speed is less than 39 mph. 

Upon being designated a tropical depression, NHC gives it a number.   When a named tropical storm weakens into a tropical depression, it retains the name.

This sequence of satellite pictures of Katrina illustrates typical levels of organization from depression to strong hurricane.


Images credit: NOAA

 
INVEST

When an area of disturbed weather becomes increasingly organized, the National Hurricane Center will designate "Invests" (short for investigative areas).  Each invest is assigned a number from 90-99, such as “Invest 90L,” with numbers repeating once 99 is reached. The letter identifier tells us the body of water, such as L for Atlantic (A is reserved for the Arabian Sea) and E for Eastern Pacific.

 
TROPICAL DISTURBANCE

Generic term for weather disturbances in the tropics and subtropics with persistent convection and sometimes low pressure.  Can be an early stage of tropical cyclone development.

 
TROPICAL WAVE

Rather than in the ocean, a tropical wave is in the atmosphere, and a la the graphic below it looks like a wave when its wind flow and pressure pattern are viewed from above. 

Also known as an “easterly wave” or “African easterly wave” (AEW) since they typically come from there and move east to west. 

There are many each year, a small percentage of which develop into tropical cyclones.


Image credit: The Weather Channel

CYCLONE

Low pressure system that has a “closed” (fully circular) wind circulation, as compared to a tropical wave, which does not.

 
TROPICAL LOW

A tropical low pressure system.   Typically the term is used for a weak one which is not yet, or is no longer, a tropical cyclone.

 
REMNANT LOW

A tropical low which used to be a tropical cyclone.

 
EXTRATROPICAL CYCLONE

A non-tropical cyclone.   A typical cyclone in winter over the United States is an extratropical cyclone.  

Tropical cyclones often become extratropical, undergoing “extratropical transition,” as they move out of the tropics; and once in a while an extratropical cyclone can transition into a tropical one.

 
POST-TROPICAL CYCLONE

As the prefix suggests, a former tropical cyclone -- which has either become extratropical or a remnant low.

 
SUBTROPICAL CYCLONE/DEPRESSION/STORM

A type of “hybrid” that has characteristics of both tropical and non-tropical cyclones.

 
SAFFIR-SIMPSON HURRICANE WIND SCALE

The scale which categorizes hurricane wind speeds on a scale from 1 to 5 (strongest). 



It refers exclusively to winds, not storm surge or other effects, which are also dependent upon factors such as the size of a hurricane; and per the NHC statement below from a few years ago when they were proposing to rename the scale, as a result of confusion about this the formal name is now specifically the Saffir-Simpson Hurricane Wind Scale. 



For example, Ike (2008) and Sandy (2012) were “only” Category 2 and 1, respectively, yet they caused catastrophic impacts from surge & waves.   Thus, although the official definition of a “major” hurricane is Category 3+, use of that term is problematic.

And as noted above with tropical storms and depressions, rainfall can be extreme and produce disastrous flooding independent of what the winds are.


STORM SURGE


As the term says: water surging onto a coastline, pushed by a storm.   The water rise above normal astronomical tide levels is primarily driven by winds; a very small percentage of it can be the result of lower atmospheric pressure.

As in this animation depicting storm surge, it can be accompanied by destructive waves.


Image credit: The COMET Program & National Hurricane Center; depiction of water rise in animation is sped up compared to actual storm surge

 
EYE

As tropical cyclones become stronger, “eyes” form, which are in the center of circulation and have relatively less wind and precipitation.

EYEWALL

Circular band of strong winds and heavy rain surrounding the eye.  The highest wind speeds are usually in the eyewall.


Image credit: NOAA


CONVECTION

In weather, refers to showers & thunderstorms, with air moving rapidly up (updrafts) and down (downdrafts).

It has a distinctive appearance on colorized satellite images such as this one as clouds boil up through the atmosphere.


Image credit: The Weather Channel


WIND SHEAR

Winds blowing at different speeds and/or directions at different levels of the atmosphere.    (This is referring to vertical wind shear; shear can also be horizontal or a combo.)

Strong upper-level winds blowing through a developing or existing tropical cyclone can discombobulate it, and inhibit intensification or result in weakening. 

Ironically, while wind shear can be hostile to hurricanes, it is conducive to severe thunderstorms and tornadoes.


Schematic image credit: The Weather Channel; pre-annotated satellite image credit: NOAA

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4. mfount
4:45 PM GMT on July 08, 2014
This is very helpful in clarifying things.
Member Since: February 6, 2003 Posts: 1 Comments: 17
3. slparker
10:40 AM GMT on July 08, 2014
This is not about this post, but I didn't know where else to comment. In a post about typhoons, you said:

"This jives well with observations at Kadena Air Base and Naha in Okinawa, both about 120 miles east of the typhoon, now reporting sustained winds of 55 mph with gusts to near 90 mph."

The correct verb is (to) jibe

so

"This _jibes_ well with observations at Kadena Air Base and Naha in Okinawa, both about 120 miles east of the typhoon, now reporting sustained winds of 55 mph with gusts to near 90 mph."

Hope this wasn't too troll-esque.
Member Since: July 8, 2014 Posts: 0 Comments: 0
2. vis0
6:36 PM GMT on July 01, 2014
Thank You Mr.Ostro.
Member Since: December 15, 2006 Posts: 243 Comments: 397
1. Astrometeor
2:09 AM GMT on June 30, 2014
Thanks Stu. Wish the NHC would use images more often.
Member Since: July 2, 2012 Posts: 96 Comments: 9875

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About stuostro

Senior meteorologist at The Weather Channel. Proud to be a weather-obsessed weather geek. Would be a DJ if not a meteorologist.

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