Scientists explain a hurricane as an engine run by heat. Water vapor that cools and condenses back into liquid water releases latent heat, and that heat powers the storm. Warm ocean water feeds a hurricane: water evaporates into vapor and rises, and the warmed air is pulled into the column of clouds. The warmer the water under a hurricane, the more powerful the storm is likely to grow. Hurricanes need very warm ocean water, about 80 degrees Fahrenheit (27 degrees Celsius) or more, and fade if the water is cooler than about 79.7 degrees Fahrenheit (26.5 degrees Celsius). Cold water and land do not give off much evaporated water, so they cannot feed a hurricane the way warm ocean can. A tropical cyclone is a rotating low-pressure weather system that has organized thunderstorms but no fronts.

Earth's turning explains why a hurricane spins. The Coriolis effect comes from different parts of the Earth moving at different speeds. A hurricane is a low-pressure system, so it pulls air toward its center, and the Coriolis effect bends that moving air. This bending of the air, the Coriolis deflection, is what makes tropical cyclones spin. Hurricanes spin counterclockwise north of the Equator and clockwise south of it. Tornadoes can be spawned by hurricanes, and they most often form in the right-front part of the storm where wind shear is greatest.

A tornado needs a different ingredient. Wind shear usually weakens a hurricane, but wind shear is always needed to make a tornado. Directional wind shear is the change in wind direction with height. Speed shear, a change in wind speed with height, is believed to be a key part of how rotating thunderstorm updrafts called mesocyclones form. Rising air in a thunderstorm tilts rotating air from horizontal to vertical, and once the updraft rotates and is fed by warm, moist air at ground level, a tornado can form. Tornado formation is believed to depend mainly on things that happen on the scale of the storm, in and around the mesocyclone. The tornado itself is a very small extension of the much larger rotation inside a supercell.

Doppler radar shows wind motion: red is a target moving away from the radar and green is a target moving toward it. A pattern found in NSSL Doppler radar data was named the Tornado Vortex Signature. It shows up several kilometers above the ground before a tornado touches the ground. This discovery led NOAA to build a nationwide network of Doppler radars. The Tornado Intercept Project of 1975 began the National Severe Storms Laboratory's tradition of organized field experiments. The VORTEX project began in 1994 with the aim of explaining how tornadoes form. In VORTEX2, in 2009 and 2010, researchers deployed 10 mobile radars and 40 other instrumented vehicles. VORTEX2 scientists sampled 36 supercells and 11 tornadoes to learn about tornado strength, lifetime and behavior.

Aircraft carry the same idea into hurricanes. The history of hurricane hunting starts in 1943, when Colonel Joe Duckworth of the Army Air Forces flew a single-engine trainer plane through a hurricane for the first time and showed it could be done. After that flight, interest in flying into storms grew: the Army Air Force became the Air Force, and the Air Force took over the military side of hurricane flying. Hurricane hunter missions fly to locate the center of the storm and measure its central pressure and surface winds. Dropsondes dropped from the aircraft continuously send back pressure, humidity, temperature and wind direction and speed. A P-3 mission typically lasts 8 to 10 hours.

Balloons and satellites watch from the sky. A radiosonde is a small, expendable instrument package that weighs 60 to 80 grams and hangs below a weather balloon. It reports pressure, temperature, humidity and GPS position every second, and it can rise higher than 35 km (about 115,000 feet). On April 1, 1960, NASA launched TIROS-1, the first successful weather satellite. In 1975 the SMS series of satellites became the first operational Geostationary Operational Environmental Satellites, called GOES. GOES satellites orbit 22,236 miles above Earth's equator at the speed of Earth's rotation, so each keeps watching the same area. As of November 2026, a GOES imager can take pictures of weather patterns, hurricanes and severe storms as often as every 30 seconds. Polar-orbiting satellites fly over a storm about twice a day at a lower altitude than the satellites that stay in view.

Ratings turn damage and wind into numbers. The original Fujita scale's precise wind speeds were guesses that were not verified by science or engineering tests. In 1971, T. Theodore Fujita of the University of Chicago developed it to sort tornadoes by their intensity. The Enhanced Fujita scale began operating in the United States in February 2007. It starts from 28 damage indicators and estimates a three-second wind gust. The National Weather Service is the only federal agency with authority to give official tornado EF scale ratings. Ratings EF0 and EF1 are called weak, EF2 and EF3 strong, and EF4 and EF5 violent.

Words to know

• latent: hidden
• radiosonde: balloon instrument
• mesocyclone: rotating updraft
• Coriolis: spin effect
• geostationary: fixed in place
• deflection: a bend