Tropics Terminology: What You Need To Know

Hurricane season can get pretty confusing when it comes to terminology - we get it. There are a lot of terms that sound similar, but their distinctions are crucial throughout the life cycle of a hurricane.

From a cluster of thunderstorms, to a major hurricane, let's dive into what you need to know.

Life Cycle Of A Hurricane

“Tropical cyclone” is a generic term for warm-core low pressure systems with a defined surface wind circulation, born over tropical, or sometimes subtropical, waters, accompanied by persistent thunderstorms. Tropical depressions, tropical storms, hurricanes, and typhoons are all tropical cyclones.

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“Hurricane” classifies the powerful tropical cyclones that develop in the North Atlantic Ocean, the Gulf of Mexico, the Caribbean Sea, and the eastern North Pacific Ocean.

A hurricane’s life cycle passes through four distinct stages: tropical disturbance, tropical depression, tropical storm, and finally, a hurricane.

1. Tropical Disturbance: A tropical disturbance is a system of clouds, showers, and thunderstorms that originate in the tropics. Tropical waves, a type of tropical disturbance, are troughs of low pressure without a closed surface wind circulation that move generally from east to west within the tropical easterlies. Tropical waves are the most common way for tropical systems to form, but it’s not the only way.
2. Tropical Depression: When a tropical disturbance develops a closed circulation (counterclockwise winds blowing around a center of low pressure in the Northern Hemisphere), it is designated a tropical depression. Tropical depressions contain maximum sustained winds of 38 mph or less.
3. Tropical Storm: A tropical cyclone is given a name by the National Hurricane Center (NHC) once it reaches tropical storm status. Tropical storms have maximum sustained winds of 39-73 mph.
4. Hurricane: If a tropical cyclone has sustained winds of at least 74 mph, it becomes reclassified as a hurricane. Winds in most hurricanes can become much stronger, and hurricanes are categorized on a scale of 1 to 5 based on their wind speed, known as the Saffir-Simpson Scale.

Other Terms You May Hear

Potential tropical cyclones (PTC) and invests are other terms you may hear thrown around when referencing tropical systems. Although they aren’t technically a new step in the hurricane life cycle, they are designated terms used for some tropical disturbances.

A PTC is a disturbance that has not yet developed into a tropical cyclone but poses the threat of bringing tropical storm or hurricane conditions to land areas within 72 hours. Designating a disturbance as a PTC allows the NHC to issue watches and/or warnings with more lead time.

A tropical disturbance is labeled an invest when the NHC is interested in further monitoring the system for potential development into a tropical depression or storm. Once designated as an invest, specialized data sets can be collected and computer models can be run on the area, such as spaghetti model tracks.

Miriam Guthrie graduated from the Georgia Institute of Technology with an undergraduate degree in Atmospheric and Oceanic Sciences and is now a meteorology intern with weather.com before returning to school for her masters.