What Is a Tropical Depression?

Tropical cyclone sky and palm trees in wind

Sadik Demiroz / Getty Images

As the weakest tropical cyclone type, tropical depressions — a low pressure area surrounded by circulating thunderstorms and maximum sustained winds of 38 miles per hour or less — aren't talked about as much as tropical storms and hurricanes. Their formation, however, plays a key part in hurricane monitoring: Depressions are often one of the first indications that a hurricane is brewing over the tropical ocean. 

Because hurricanes also typically downgrade to tropical depressions at the end of their life cycles, the tropical depression stage can be a sign of storm dissipation, too. 

Dozens of tropical depressions form each hurricane season; the exact number varies depending on how active (or inactive) a season is. According to the National Oceanic and Atmospheric Administration, 14, on average, strengthen into tropical storms.

Tropical Depression vs. Tropical Storm

Tropical depressions exhibit most of the same weather conditions that tropical storms, including low pressure, humid air, showery precipitation, and moderate winds. The biggest difference is that these conditions are milder inside of tropical depressions. For example, tropical depression winds measure up to 38 mph, but range from 39 to 74 mph within tropical storms.

Another difference between the two is that depressions aren’t named. Upon forming, they receive a number only. For example, Tropical Depression Ten means the storm is the tenth tropical depression to form in a particular season. Tropical cyclones, on the other hand, are assigned the next available name on that season's names list. A tropical cyclone will keep its name — even as it downgrades to a depression and post-tropical cyclone — until it dissipates. (This is why some depressions appear to have names.)

Satellite image of a five tropical cyclones in the Atlantic
Satellite view of (from top right to bottom left) Tropical Storm Sally, Hurricane Paulette, Tropical Depression Renee (middle), Tropical Storm Teddy, and Tropical Depression 21 on Sept. 14, 2020.

NASA / Wikimedia Commons / Public Domain

When looking at weather radar and satellite imagery, depressions appear a bit lopsided, although they do show some rotation. Tropical storms tend to have a more symmetrical circular shape that's synonymous with tropical cyclones.

How Scientists Investigate Tropical Activity

Weather scientists know when tropical cyclones intensify from a depression to a tropical storm, for example, largely because they observe them via jet aircraft. NOAA scientists and Air Force pilots collectively known as “hurricane hunters” fly into the heart of tropical cyclones and collect pressure, temperature, humidity, and wind data using on-board weather instruments and dropsondes — instrument packages that parachute down through the cyclone to the ocean’s surface. These reconnaissance flights take place twice per day until the storm reaches land or begins dissipating. NOAA also uses vessels and ocean buoys to gather data about the storms at ground-level.

Potential Damage of a Tropical Depression

Tropical depressions may not wreak as much havoc as their tropical storm and hurricane peers, but they can still wallop locations with inches of rain, as was the case with 2019's Tropical Depression Imelda. From September 17-19, a slow-moving Imelda, which weakened from a tropical storm to a depression while over Southeast Texas, dumped up to 44 inches of rainfall across the region, triggering significant flooding. The flooding closed down a stretch of the I-10 Interstate and claimed at least five lives. According to NOAA's National Centers for Environmental Information, Imelda is one of the top ten wettest tropical systems to impact the United States.

Tropical depressions not only bring heavy downpours, but also gale-force winds which generate rough surf and life-threatening rip current conditions along coastal regions.

Rip Tide

Rip currents, or rip tides, are narrow jets of water that flow swiftly away from shore and out to sea. Among other causes, they can form when a tropical cyclone's wind-driven waves break along the shoreline.

Because tropical systems contain thunderstorms, wind shear, and instability, they have all the ingredients necessary to trigger tornadoes. Thankfully, tornadoes produced by tropical cyclones, especially fairly weak tropical depressions, tend to be relatively weak and short-lived.

What to Do During a Tropical Depression

When a tropical depression approaches, a tropical storm watch or warning will be issued for your area. Follow these tips to safely ride out the storm:

  • Before the storm arrives, tie down loose objects, such as outdoor furniture.
  • Be prepared for downed trees and possible power outages.
  • Be prepared for accumulating rainfall and localized flooding.
  • Don’t walk or drive through flooded roadways.
  • Avoid swimming at beaches, since a tropical depression's winds can churn up rip currents.