Another day, another explainable weather event. Subtropical Storm Fay is on the cusp of developing in the Atlantic Ocean, and it's likely going to brush Bermuda in a few days. What is a "subtropical storm" and why is it different from a regular ol' tropical storm?

Tropical systems (depressions, storms, and hurricanes) have to meet certain criteria in order to be considered tropical. The United States experiences three types of cyclones, or low pressure systems—extratropical, tropical, and subtropical. Understanding subtropical systems requires a brief summary of the other two types of cyclones.

Extratropical Cyclones

Extratropical cyclones are by far the most common in the world. Whenever you hear your local television meteorologist talking about a low over the Midwest or even a Nor'Easter, it's an extratropical low. These cyclones are marked by temperature advection—they produce cold fronts and warm fronts, and they feed their energy off of the jet stream. The extratropical cyclone pictured above was the strongest ever recorded over the United States, dubbed the "Chiclone."

Tropical Cyclones

Tropical cyclones, as the name suggests, have to consist entirely of a tropical airmass—from side to side, top to bottom, the atmosphere through the system has to be warm and humid. They don't produce fronts since there's no collision of different airmasses; tropical systems are tropical through and through. Tropical cyclones also gain their strength through intense, well-organized thunderstorm activity around the eye.

Pictured above is an infrared satellite image of Super Typhoon Vongfong near its peak strength of 180 MPH.

Subtropical Cyclones

This brings us to subtropical cyclones. Subtropical cyclones are like the step-brother of tropical cyclones. Like tropical systems, subtropical systems have a closed low pressure center at the surface. Unlike traditional tropical cyclones where the strongest winds are concentrated in thunderstorm activity around the center, subtropical cyclones often feature their winds and thunderstorm activity displaced a good distance from the center of circulation.

In the system that's likely to develop into Subtropical Storm Fay, the National Hurricane Center says that the feature's maximum winds extend over a 100 nautical mile radius. That's a huge wind field for such a small, weak storm.

Subtropical cyclones are also cooler than tropical systems. While tropical cyclones have a warm core (warm air from surface to the top of the atmosphere), subtropical cyclones usually aren't completely warm through their entire cores. Since they're not quite tropical, they're...subtropical.

Bermuda is under a tropical storm watch in anticipation of future Subtropical Storm Fay's brush with the island. There are a few more areas of interest out in the Atlantic worth watching. It's been a quiet year, but coastal residents shouldn't let their guards down yet—hurricane season doesn't end until November 30.

[All images via NASA, annotations on last image by the author]

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