How Do Hurricanes Form?

Hurricane, typhoon or tropical cyclone: what is the difference?

Hurricanes and typhoons are the same kind of storm, a tropical cyclone, and the only difference between them is where they happen.^[1] Meteorologists use "tropical cyclone" as the general term for an organized, spinning mass of clouds and thunderstorms that starts over tropical or subtropical seas and has a closed circulation near the surface.^[1] Its winds turn counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, and it is typically 200 to 500 km across but can reach 1,000 km.^[2]

Storms in the North Atlantic and the central and eastern North Pacific are called hurricanes, while those in the western North Pacific are called typhoons.^[1][2] The World Meteorological Organization (WMO) lists further regional names: "cyclone" in the Bay of Bengal and Arabian Sea, "severe tropical cyclone" in the western South Pacific and southeast Indian Ocean, and "tropical cyclone" in the southwest Indian Ocean.^[2]

What does a hurricane need to form?

The National Oceanic and Atmospheric Administration (NOAA) sums up the ingredients as a pre-existing weather disturbance, warm tropical oceans, moisture and relatively light winds.^[1] NOAA's JetStream pages give a more detailed list of conditions:^[3]

• ocean water of at least 80 °F (27 °C) down to a depth of about 150 feet (46 m);^[3]
• air that cools fast enough with height for storm clouds to build;^[3]
• fairly moist air at mid-levels, about 16,000 feet (4,900 m) up;^[3]
• generally, a distance of at least 300 miles (480 km) from the equator;^[3]
• low vertical wind shear, meaning the wind changes little with height: less than about 23 mph (37 km/h).^[3]

NASA gives the water temperature as typically 26.5 °C (80 °F) or greater.^[4] NOAA calls warm water the most important factor, since it is what powers the storm.^[3] Too much wind shear disrupts or weakens the thunderstorms, and dry air drawn in at mid-levels can kill them.^[7]

Distance from the equator matters because of the Coriolis force, which comes from Earth's rotation and makes the storm spin; it is zero at the equator.^[4][7] With only the rarest exceptions, tropical cyclones do not form within 5 degrees of latitude of the equator, and NASA says a storm generally needs to be at least 8 degrees away to start spinning.^[3][4]

Most tropical cyclones grow out of easterly waves, also called tropical waves: troughs of low pressure moving west in the tropical easterly winds.^[3] Even when all the conditions are met, many disturbances never develop.^[7]

How does a storm grow into a hurricane?

As warm, moist air rises, it cools and its water vapor condenses into clouds, releasing heat.^[3] The heat makes the air lighter so it keeps rising, and more air flows in near the surface to take its place, which is the wind.^[3] NASA describes a chain reaction: as pressure at the center drops, air flows in faster, the stronger winds draw more heat and moisture from the ocean, and more thunderstorms form.^[4]

NOAA's National Hurricane Center (NHC) classifies the storm by its maximum sustained wind, the highest one-minute average wind at a height of 10 meters.^[6] With winds of 38 mph (62 km/h) or less it is a tropical depression; from 39 to 73 mph (63 to 118 km/h) it is a tropical storm; and at 74 mph (119 km/h) or more it is a hurricane or typhoon.^[6]

As a depression organizes, its thunderstorms line up in spiral bands along the inflowing wind.^[7] Eventually the inner bands close off into an eyewall around a calm eye, usually around the time the winds reach hurricane force.^[7]

What are the eye, eyewall and rainbands?

The eye is the roughly circular area of comparatively light winds at the center of a strong tropical cyclone, and it has the lowest air pressure at the surface.^[6][7] It is mostly free of clouds because some of the air rising in the surrounding thunderstorms is forced toward the center, where it sinks and warms.^[4] NOAA's Hurricane Research Division says eyes range from 5 to 120 miles across, with most 20 to 40 miles, while NASA puts the usual size at 10 to 65 kilometers.^[7][4]

The eyewall is the ring of cumulonimbus, or thunderstorm, clouds around the eye, and it has the strongest surface winds in the storm.^[6][7] Farther out, rainbands (also called spiral bands) are long, narrow bands of rain and thunderstorms that spiral in toward the center.^[7]

How is hurricane strength measured?

The Saffir-Simpson Hurricane Wind Scale rates hurricanes from 1 to 5 based only on their maximum sustained wind speed, so it does not account for storm surge, rain flooding or tornadoes.^[5] Hurricanes of Category 3 and higher are called major hurricanes.^[5]

Why do hurricanes weaken over land or cold water?

A hurricane weakens when it moves over cooler water or over land, both of which cut off its supply of warm, moist air.^[4] Strong winds high in the atmosphere can also weaken it by carrying away its heat.^[4]

Two NOAA sources describe the role of land slightly differently.^[3][7] NOAA's JetStream says a storm weakens rapidly once its eye moves over land, not because of friction, but because it loses the moisture and heat the ocean provided and can no longer build thunderstorms near its center.^[3] NOAA's Hurricane Research Division lists increased friction over land as one factor, along with the loss of warm, moist ocean air and the drawing in of dry air from the continent.^[7] It adds that mountains can end a hurricane quickly, while over flat land the circulation may take two to three days to break down.^[7]