What’s the Difference Between a Hurricane, Tropical Storm and Tropical Depression?

Hurricane Florence satellite image

Satellite image of Hurricane Florence moving closer to the coast of the Southeastern U.S. on September 12, 2018.
(Image Credit: NOAA)

Hurricanes are ferocious storms that cause widespread damage and devastation when they strike land. A hurricane doesn’t just emerge out of thin air. Instead, it goes through a process of development, with each stage having a different name. Some tropical disturbances never become hurricanes, but they still can cause a significant amount of damage in their own right.

What are these different terms used by meteorologists, and what do they mean? Here’s a list of the most commonly used terms you’ll need to know.

What Is a Tropical Disturbance?

A tropical disturbance (or tropical wave) is an initially disorganized area of low pressure. Often there are no clear signs of rotation. Although, when a disturbance gets better organized, it may begin to show signs of rotation as it starts to form into a tropical depression.

As we said earlier, not every disturbance in the tropics forms into a depression. In reality, only a small percentage do. Winds are not typically that powerful in a tropical disturbance. Normally sustained winds will be less than 30mph or so. However, rainfall may be extreme in some cases, leading to flooding in affected areas if it passes over land.

In special cases where a tropical disturbance may affect land, the National Hurricane Center has recently started to issue advisories to warn the public much like they already have done for tropical depressions, storms, and hurricanes. This gives those that may potentially be in harms way more time to prepare.

What Is a Tropical Depression?

When a tropical disturbance becomes organized enough and has clear rotation with maximum sustained winds below 39mph, it is classified as a tropical depression. Most tropical depressions have maximum sustained winds between 25 and 35 mph, although they may often gust to tropical storm force. Like tropical disturbances, depressions can produce extreme amounts of rain, and the strongest winds may be enough to uproot trees from the saturated ground.

What Is a Tropical Storm?

As a tropical depression becomes even more organized and its circulation tightens up, its winds will also begin to increase. Once it has sustained winds at or above 39mph and no higher than 73mph, it is upgraded to a tropical storm. Tropical storm status is when a storm is given a name. The major ocean basins around the world have their own list of names to be used each year.

With the stronger winds and heavier rainfall, tropical storms typically produce both wind and water damage. While there will be a good deal of flooding inland, the wind can also cause minor coastal flooding too as sea water is pushed onshore. The tropical storm’s tighter circulation also provides some degree of spin for the individual thunderstorms that make up these systems, increasing the chances for tornadic activity, causing even more severe damage.

What Is a Hurricane?

Tropical storms that continue to organize and develop will soon produce sustained winds at 74mph, and often far higher. When this occurs, a tropical storm is upgraded to a hurricane. On satellite, the hurricane’s appearance is distinct—the clouds clearly move in a circular fashion, and in stronger hurricanes take on a buzzsaw-like appearance with a circular area at the center with no clouds and calm winds called the “eye.”

The threat from a hurricane is threefold: wind, rain, and storm surge. Winds in hurricanes are severe enough in most cases to cause significant structural damage, especially to less robust structures like patios, awnings, roofs, and mobile homes. Tornadic activity becomes much more common in the strongest rain bands, which only exacerbates the damage. Rain from hurricanes can total upwards of a foot or more depending on the forward speed of the hurricane itself.

Perhaps most dangerous of all is the storm surge. The hurricane’s strong winds blow across the sea surface, building up massive waves that push water inland. The combination of the force of the water being pushed onshore combined with the ferocious winds will rip apart coastal structures, making these areas some of the most dangerous places to be in a hurricane.

Saffir-Simpson Hurricane Wind Scale

Hurricanes in the United States are classified by the speed of their sustained winds on a scale known as the Saffir-Simpson Hurricane Wind Scale. The classification aims to help the public understand more clearly the potential and the severity of destruction from a hurricane. Storms are rated from Category 1 to 5, with anything Category 3 or higher considered a “major hurricane.”

Saffir-Simpson Hurricane Wind Scale

While not a perfect match for every hurricane as it does not take rain or storm surge into consideration, the Saffir-Simpson Hurricane Wind Scale has still shown over time to be a reasonably accurate predictor of the type of damage to expect and it is straightforward and easy to understand.

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