Breaking down tropical terminology: What is an ‘invest’ anyway?

Meteorologist Rachael Penton explains what commonly used tropical terms mean
This GOES-16 GeoColor satellite image taken Friday, June 18, 2021, at 11 a.m. EDT, and provided...
This GOES-16 GeoColor satellite image taken Friday, June 18, 2021, at 11 a.m. EDT, and provided by NOAA, shows a tropical weather system in the Gulf of Mexico.(AP)
Published: Jun. 30, 2021 at 4:59 PM CDT
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ALEXANDRIA, La. (KALB) - This is the time of the year you’ll hear meteorologists discuss things like tropical waves, disturbances, invests and tropical cyclones. All of these terms can be confusing, but we’re here to break each of them down.


When it comes to tropical activity, it all begins with something we call a tropical wave or an easterly wave. They’re the same thing. A tropical wave is a type of atmospheric trough, or an elongated area of low pressure. Tropical waves move from east to west across the Tropics, causing areas of cloudiness or thunderstorm activity. In the Atlantic basin, these emerge off the coast of Africa. Sometimes tropical waves are also called “tropical seeds” because they can start as small waves and grow into something as powerful as a hurricane. Tropical waves are extremely common during hurricane season.

Not all tropical waves have thunderstorm activity, but if they do and the activity grows strong enough, we start calling them a tropical disturbance. That means the area is nothing official, but it’s something worth watching.

If a tropical disturbance continues growing, it will become an “Investigation” or “Investigative” area. This means the National Hurricane Center will usually begin collecting data on the storm to determine if the storm has the potential for further development. On TV you’ll hear us call these “Invests”, along with a number and the letter L. Invests are numbered from 90 to 99 and assigned an “L” to the name when the disturbance has developed in the Atlantic Ocean or Gulf of Mexico. For example, the area we’ll be watching over the next week has been given the name “Invest 97-L”.

Some invests never develop into anything, but some continue strengthening. If the National Hurricane Center determines that an invest has organized deep convection (storms) and a closed wind circulation around a well-defined center (what we sometimes call “the eye”) we’ll begin calling it a tropical cyclone.

A tropical cyclone with maximum sustained winds of 38 mph or less is known as a “Tropical Depression”. Depressions do not receive names, but are assigned numbers based on the order that they have formed within the hurricane season, such as “Tropical Depression Two”. A tropical cyclone with maximum sustained winds of 39-73 mph receives the designation of “Tropical Storm”. It is at this point that the storm receives a name. If winds strengthen to 74 mph or more, the storm becomes a hurricane. Tropical depressions, tropical storms, and hurricanes are all tropical cyclones of varying strengths.

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