Tropical terminology
With the “official” Atlantic hurricane season starting on June 1, and already prior to that two named storms including a hurricane in January and a landfall of a tropical depression in the U.S. in May, here is a primer on tropical terminology.
Rather than a comprehensive and/or technical glossary (there’s one more in that vein on the National Hurricane Center site), this is a selection of some basic terms that meteorologists commonly use, and their key points.
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 deep convection wrapped close to the center, 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. The U.S. Joint Typhoon Warning Center uses the term super typhoon to apply to typhoons with sustained winds of at least 150 mph.
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 than hurricanes, 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 the system a name. Other ocean basins have other lists of names. Note that the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA-DOST) uses their own set of typhoon and tropical storm names, e.g. Haiyan was known as Yolanda there.
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.
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 after 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 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.
CYCLONE
Generic term for a low pressure system that has a closed circulation (fully circular), as compared to a open 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, officially a tropical cyclone. (Logically any tropical low pressure system would be a tropical cyclone, with depressions, storms and hurricanes having specific criteria therein, but for some reason that’s not the convention used.)
POST-TROPICAL CYCLONE
As the prefix suggests, a former tropical cyclone — which has either become extratropical or a remnant low.
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 or subtropics; and once in a while an extratropical cyclone can transition into a tropical one.
REMNANT LOW
A tropical low which used to be a tropical cyclone.
SUBTROPICAL CYCLONE/DEPRESSION/STORM
A type of “hybrid” whose nature is different than an extratropical cyclone, but not fully tropical.
Extratropical cyclones get their energy primarily from the imbalance between air masses of different temperatures, tropical cyclones get theirs primarily from deep convection and warm ocean water, and subtropical cyclones are a combo. Subtropical cyclones, although non-frontal like tropical cyclones, are typically associated with cold upper troughs rather than warm upper ridges, their warm cores are shallower than those of tropical cyclones, and their strongest winds are not as closely wrapped around the center.
SAFFIR-SIMPSON HURRICANE WIND SCALE
The scale which categorizes hurricane wind speeds on a scale from 1 to 5.
It refers exclusively to winds, not storm surge or other effects, and per the NHC statement 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” of Category 2 and 1 strength, respectively, at the time they hit the U.S., yet they caused catastrophic impacts from surge & waves — they were very large in size, which matters with tropical cyclones. 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 coast, 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.
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.
CONVECTION
In weather, refers to showers & thunderstorms, with air moving rapidly up (updrafts) and down (downdrafts).
A fundamental element of tropical cyclones is deep convection, extending high into the atmosphere.
It typically has a vivid appearance on colorized satellite images.
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 both.)
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.
