nasa hurricane isabel

A Guide to Hurricane Terminology

September 12, 2018

Hurricane and typhoon season are in full swing. As more than a million Americans were evacuated from their homes during recent Hurricane Florence, people around the world are closely following weather updates.

To help you understand some of the terms being used by meteorologists right now, we’ve compiled this alphabetical list of tropical storm terminology.

ACE is an acronym for Accumulated Cyclone Energy. This is a measure used to express the combined duration and strength of tropical cyclones. It is also a good metric for scientists to compare storms to each other, as well as across different seasons. The ACE of a season is the sum of the ACE for each storm — taking into account the number, strength, and duration of all the tropical storms in that season.


Atlantic basin cyclone intensity by Accumulated Cyclone Energy, 1850-2014.

cone of uncertainty is a cone shape that extends out from the storm on forecasting maps to warn the public of the projected path of a tropical system. It is known as the cone of “uncertainty” because its shape is made by graphing the accuracy of initial estimates against the actual stage of a hurricane. Because estimates made at the beginning will be limited in their accuracy, meteorologist use the cone shape to reduce uncertainty so that estimates can become more accurate as the hurricane develops.

Convection is the movement of moisture and heat from the lower layers of the atmosphere to upper layers through updrafts of air. This movement sometimes occurs as a thunderstorm inside a hurricane.

A cyclone is a tropical storm that occurs around the Indian Ocean and the Southeast Pacific. They tend to appear in countries like Japan, Indonesia, Malaysia Madagascar, India and parts of Australia. Cyclones rotate counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

nasa hurricane isabel2

An image of Hurricane Isabel as seen from the International Space Station showing a well-defined eye at the center of the storm.

The eye is the center of a hurricane, where winds are calm and skies virtually rain free. The eye develops as a hurricane grows and intensifies, occurring in the middle of huge swirling clouds, which are known as the eyewall. This is the most destructive part of a hurricane, containing very strong winds and thunderstorms.

Feeder bands are low-level clouds that move into the thunderstorms of hurricanes.

A hurricane is a severe tropical storm with maximum sustained winds over 74 mph at the surface. Hurricanes occur in the Caribbean and parts of the Southeastern U.S. Most hurricanes emerge thousands of miles away from these areas — in the Atlantic Ocean and close to Northwest Africa. Hurricanes receive their energy from warm seas and can only develop when the sea temperature is at least 26 degrees Celsius (78.8 F), developing into major hurricanes only when the sea temperature is at least 28 degrees Celsius (82.4 F).

Hurricane season is the part of the year in which there is a relatively high incidence of hurricanes. The hurricane season in the Atlantic, Caribbean and Gulf of Mexico runs from June 1 to November 30. In the Eastern Pacific basin it runs from May 15 to November 30. Hurricane season in the Central Pacific basin runs from June 1 to November 30. Hurricanes can and do occur outside of these windows, however.

hurricane season

This map shows the tracks of all tropical cyclones in the 2017 Atlantic hurricane season. The points show the location of each storm at 6-hour intervals. The color represents the storm's maximum sustained wind speeds as classified in the Saffir-Simpson Hurricane Scale, and the shape of the data points represent the type of the storm.

is when the eye of the hurricane crosses the shoreline. This is the moment when meteorologists regard the hurricane as being at a certain point in its lifecycle. It does not mean that the hurricane has “hit land,” arrived or started to inflict damage, however. The outer bands that carry flooding rain and severe winds arrive many hours or even days beforehand, meaning coastal areas can have already been destroyed before landfall.

Madden-Julian Oscillation (MJO) is one of many variables used to forecast tropical development. When small pockets of active weather move from west to east across the equator, this is called the enhanced phase of MJO. It is important for forecasting because it indicates an increased chance of tropical development.

The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 rating system based on a hurricane’s sustained wind speed. The scale estimates potential property damage in five categories:

Category 1: 74- 95 mph. Damage potential: minimal
Category 2: 96-110 mph. Damage potential: moderate
Category 3: 110-130 mph. Damage potential: extensive
Category 4: 131-155 mph. Damage potential: extreme
Category 5: more than 156 mph. Damage potential: catastrophic

Spiral bands are clouds and thunderstorms that spiral inward toward the center of a hurricane.

A synoptic track is a weather reconnaissance mission flown to provide vital meteorological information in data-sparse ocean areas to supplement existing surface, radar and satellite data. Synoptic flights define the upper atmosphere and aid in the prediction of tropical cyclone development and movement.

storm surge2

Total destruction of the Bolivar Peninsula (Texas) by Hurricane Ike's storm surge in 2008.

A storm surge is an abnormal rise in sea level accompanying a hurricane or other intense storm. Sea water is pushed inland across coastal locations by a broad wind field circulating around the storm’s center. The severity of the storm surge is affected by the storm’s intensity and wind field, plus the shape of the coastline and the slope of the sea floor. Most injuries and deaths associated with tropical cyclones are related to storm surge. Note: Because each location that might experience storm surge is different, storm surge can be very difficult to forecast with precision. Therefore, it is critical to heed the directions for preparation and/or evacuation from local officials.

A tropical depression is a tropical cyclone with maximum sustained winds less than 39 mph at the surface.

Tropical storms are one of the most destructive weather systems on Earth. Essentially, a tropical storm is a system of thunderstorms, characterized by surface circulation and accompanied by sustained winds with speeds of 39-73 miles per hour. Cyclones, typhoons and hurricanes are all tropical storms, categorized according to where in the world they occur.

A tropical wave is a cluster of thunderstorms in the tropics that has the potential to become a tropical depression. 

Tropical Wave Dorian

Tropical Wave Dorian on July 29, 2013, two days after losing tropical cyclone status.

Typhoons are tropical storms that occur in the Northwestern Pacific basin and in the proximity of the Philippines, China and the Marshall islands.