What is a hurricane?
Have you ever wondered how a hurricane develops? How forecasters predict the strength and direction of a hurricane? Read more about how these giant storms grow, change, and how weather experts judge whether it will affect you.
When a hurricane develops
Disturbance
Depression
Tropical storm
Hurricanes striking the eastern United States typically form in equatorial waters from Africa to the Caribbean. Hurricanes in the northern hemisphere spin counterclockwise as they cross the Atlantic toward the U.S.
Warm, moist air rises from the ocean surface to cold air above. The water condenses, forming thunderstorms.
Wind increases in speed and rotates around a center. The National Hurricane Center begins tracking and forecasting at this stage.
When winds reach 39 mph, the storm is named to identify it. At 74 mph, the storm becomes a hurricane.
Read this year's list of hurricane names here.
Inside a hurricane (back to top)
Critical data are obtained from flying into the storm. Flights through a hurricane's eye measures minimum barometric pressure in the eye wall, maximum windspeed, and through the outer rainbands determine the hurricane's extent.
Click here to watch a 3D rendering of how hurricanes form.
Tracking by range (back to top)
LONG RANGE
Scientists rely on weather satellites for surveillance throughout a hurricane's lifetime. Satellites, however, generally can't see below clouds. For that, forecasters rely on other weather-observing equipment.
Polar Orbiting Satellites (POES)
Orbits the earth daily to help forecasters see the cloud cover and a storm's location.
Geostationary Satellites (GOES)
Stationed the same spot 23,000 miles above the equator, they transmit continuous storm images.
MIDRANGE
Weather balloons
Launched from sites in the U.S. and the Caribbean to measure wind speed, air pressure, humidity, temperature and the air steering currents that can alter the course of a storm.
Weather buoys
Moored weather stations that broadcast conditions at sea, such as ocean temperature, needed to predict a storm's strength.
SHORT RANGE
Doppler radar
Observes a storm when it is about 250 miles from land. Radars help forecasters measure wind speed and precipitation and can detect tornadoes spawned by hurricanes.
ASOS
Automated Surface Observing Systems at airports measure wind, temperature, humidity and pressure when the storm makes landfall.
Reconnaissance aircraft (back to top)
Forecasters rely on data collected by three different aircraft to monitor a hurricane's intensity and motion. Two are highly specialized planes operated by the National Oceanic and Atmospheric Administration; the third is operated by the U.S. Air Force Reserve.
WP-3D Orion
The most sophisticated plane, it carries advanced instruments to measure surface wind speed. The NOAA aircraft flies into the eye of a hurricane at 10,000 feet, releasing dropwindsondes.
Dropwindsondes
Up to 50 per mission are released, and they transmit data by radio.
The top is a parachute to stabilize decent, and the bottom is a humidity sensor. They are about 16 in. long.
Gulfstream IV-SP
This unique NOAA aircraft flies at high altitudes in the environment around a hurricane. The jet uses dropwindsondes to measure the steering currents that determine a storm's direction.
C-130 Hercules
Operated by the Air Force Reserve, it penetrates the storm's eye wall like NOAA's Orion, following an "alpha" flight path. By releasing dropwindsondes, it records a hurricane's most violent winds near the ocean surface.
Saffir-Simpson scale (back to top)
| Amount of damage | Barometric pressure | Winds | Storm surge | |
|---|---|---|---|---|
| Category 1 | MINIMAL Damage to shrubs, trees, mobile homes |
28.94 inches | 74 to 95 mph | 4 to 5 ft. |
| Category 2 | MODERATE Some damage to roofs, doors, windows; some downed trees |
28.50 to 28.93 inches | 96 to 110 mph | 6 to 8 ft. |
| Category 3 | EXTENSIVE Minor damage to small buildings, homes; large trees blown down |
27.91 to 28.49 inches | 111 to 130 mph | 9 to 12 ft. |
| Category 4 | EXTREME Major damage to buildings; mobile homes destroyed |
27.17 to 27.90 inches | 131 to 155 mph | 13 to 18 ft. |
| Category 5 | CATASTROPHIC Buildings, roofs destroyed; all trees, shrubs, signs downed |
Less than 27.17 inches | More than 155 mph | Higher than 18 ft. |
The danger zone (back to top)
The danger zone is usually in the forward right quadrant, (the front of the storm to the right of its track). Here the storm generates the greatest wind speeds, builds the highest storm surge and creates the most spin off tornadoes.
Storm surges (back to top)
The storm surge occurs as low pressure elevates the sea level underneath the hurricane and winds push a wall of water ahead of the storm.
Surge height can be 10-15 feet above normal sea level.
The surge-driven waves can rise to 30 feet or more above sea level.
Spin-off tornadoes (back to top)
HOW TORNADOES DEVELOP
Sometimes the center of a "supercell" thunderstorm begins to rotate, spawning a dangerous tornado.
1. Supercell thunderstorm
Tall clouds with flat "anvil" tops develop; column of rising air forms in the center of the strong thunderstorm.
2. Pre-tornado phase
Updrafts: Rising air rotates
Wind shear: Winds blow in different directions at bottom of storm, increase in speed at high altitude.
3. Funnel cloud appears
Wind in center of the storm speeds up as the funnel cloud is stretched thinner. If the funnel cloud reaches the ground, the storm is considered a tornado.
Typical tornadoes:
- Diameter: 66-1,300 ft.
- Wind speed: 62-125, up to 370 mph.
- Moving speed: 30 mph