As the Atlantic hurricane season gets underway, forecasters often use terms like tropical disturbance, tropical depression, tropical storm and hurricane.
While they may sound similar, each term describes a different stage in a tropical system’s development.
A tropical cyclone is the general term used for rotating storm systems that form over warm tropical or subtropical waters and have organized thunderstorms and a defined circulation. Depending on wind speeds and organization, these systems are classified into several categories.
Tropical disturbance
A tropical disturbance is the earliest stage of development. It is an area of disorganized showers and thunderstorms that forms in the tropics and may have some signs of circulation, but lacks a well-defined center. Many tropical disturbances never develop further.
Tropical depression
When a disturbance becomes more organized and develops a closed circulation with maximum sustained winds of 38 mph or less, it is classified as a tropical depression. These systems are numbered by the National Hurricane Center but are not given official names.
Tropical storm
Once sustained winds reach 39 mph, the system becomes a tropical storm and receives a name from the National Hurricane Center’s predetermined list. Tropical storms can produce heavy rainfall, flooding, strong winds and dangerous surf conditions. Sustained winds range from 39 mph to 73 mph.
Hurricane
A tropical storm becomes a hurricane when maximum sustained winds reach at least 74 mph. Hurricanes are categorized on the Saffir-Simpson Hurricane Wind Scale from Category 1 to Category 5 based on wind speed. Higher categories generally indicate a greater potential for wind damage, although flooding and storm surge can occur with any hurricane.
What about cyclones and typhoons?
Hurricanes, typhoons and cyclones are essentially the same type of storm. The difference is where they form.
Storms that develop in the Atlantic and eastern Pacific are called hurricanes. Similar storms in the western Pacific are known as typhoons, while those in the Indian Ocean and parts of the South Pacific are often called cyclones.
Why warm water matters
Tropical systems rely on warm ocean water as their fuel source. Most develop over waters that are at least 80 degrees Fahrenheit, along with favorable atmospheric conditions such as moist air and low wind shear.
With the Atlantic hurricane season running from June 1 through Nov. 30, forecasters encourage residents along the Gulf and Atlantic coasts to stay informed and have a hurricane preparedness plan in place before any tropical threat develops.