What Is a Hurricane? How It Forms and How It Differs from a Cyclone or Typhoon

What is a hurricane?

A hurricane is a tropical cyclone with sustained winds of at least 74 mph (119 km/h), the name used for these storms in the North Atlantic and the northeast Pacific. The National Hurricane Center (NHC) glossary defines a cyclone as a closed circulation of air that turns counterclockwise in the Northern Hemisphere and clockwise in the Southern. The Treccani encyclopedia ties it to an area of low pressure: on a weather forecasting chart it sits inside closed isobars, with values dropping toward the center.

Wind comes from that pressure difference, which pushes air from high toward low. Earth’s rotation deflects the moving air, to the right in the Northern Hemisphere and to the left in the Southern, so it stops heading straight for the center and starts circling it. The deflection carries the name of Coriolis, the French scientist who described it mathematically in 1835.

An anticyclone is the reverse pattern: pressure rises toward the center, and in the Northern Hemisphere the wind turns clockwise around it. The sky changes too. In a low, converging air is forced upward and cools, which is why the weather near cyclones is often poor; under an anticyclone the air sinks, warms and usually brings fair weather.

Cyclones come in two families, tropical and extratropical. The NHC tells them apart by their energy source.

How a hurricane forms

NOAA, the US agency for the oceans and atmosphere, lists the starting conditions. The sea is at least 80 °F (27 °C) through a layer about 150 feet (46 m) deep. Air cools fast enough with height to be unstable, and it is still fairly moist at around 4,900 meters. Between the surface and the upper troposphere the wind changes little: the difference, called vertical wind shear, stays below roughly 23 mph. The birthplace also generally lies at least 300 miles (480 km) from the equator. Warm water needs a disturbance to start from, and most of the time that is a tropical easterly wave.

NOAA calls warm water the engine of the system. Vapor rising from the sea cools and condenses into cloud, the same step found in the water cycle, and condensing releases heat. The warmed air rises, more air flows in at the surface to take its place, and that inflow is the storm’s strong wind. The NHC describes the overall exchange: the cyclone takes heat from the warm ocean and gives it up high, in the cold upper troposphere. Air spirals in toward the center and leaves from the top turning the opposite way, and once aloft that heat is carried toward the poles by the winds. According to NOAA, proposals to weaken a hurricane with chemicals, or even nuclear weapons, badly underestimate the energy involved.

Close to the equator the Coriolis force is too weak to set the system spinning, and with very rare exceptions no tropical cyclones form within 5 degrees of latitude. Typhoon Vamei, which formed near Singapore on December 27, 2001, is one of those exceptions.

According to Treccani, a tropical cyclone drifts toward the mid-latitudes once formed, on a roughly parabolic track at an average speed of 10 to 40 km/h. The World Meteorological Organization (WMO) says it can die out within a day or stay active for up to a month. When its center moves over land it weakens fast, and NOAA puts this down to the loss of ocean moisture and heat; friction with the ground is not the cause.

Practical example: in the North Atlantic the official hurricane season runs from June 1 to November 30, peaking between early and mid-September, while in the eastern Pacific it opens earlier, on May 15. In 2005 the Atlantic had 27 named storms, and 15 of them became hurricanes.

Hurricane, typhoon, cyclone or tropical storm: what’s the difference?

The WMO treats hurricane and typhoon as regional names for the tropical cyclone. NOAA counts seven basins worldwide where they are likely to form, and the US National Weather Service lists these names, among others, once sustained winds pass 74 mph (119 km/h):

Basin	Name of a tropical cyclone above 74 mph
North Atlantic	hurricane
Northeast Pacific, east of the International Date Line	hurricane
South Pacific, east of 160°E	hurricane
Northwest Pacific	typhoon (super typhoon from 150 mph)
Southwest Pacific west of 160°E, southeast Indian Ocean east of 90°E	severe tropical cyclone

Below that threshold the name depends on wind speed, measured as maximum sustained wind. For the NHC this is the highest one-minute average, taken 10 meters above the ground at an unobstructed site.

Stage	Maximum sustained wind (NHC)
Tropical depression	38 mph or less (62 km/h, 33 knots)
Tropical storm	39-73 mph (63-118 km/h, 34-63 knots)
Hurricane or typhoon	74 mph or more (119 km/h, 64 knots)

A tropical storm, then, is a defined stage in the life of a tropical cyclone: at 39 mph the system moves up a class and usually gets a name of its own. Large mid-latitude weather systems are called storms too. The WMO used “Storm Daniel” for the system that hit Greece and Libya in September 2023, which changed character along the way.

The word hurricane comes from the Caribbean. Spanish adapted it from Hurican, the storm god of the Taíno, the Indigenous people of those islands.

How a hurricane works: eye, eyewall and rainbands

A mature tropical cyclone has three main parts. At the center is the eye, a roughly circular area usually 20 to 40 miles (32-64 km) across, where winds generally stay under 15 mph. It tends to appear once sustained winds pass 74 mph, and NOAA admits that why it forms is still not fully understood. Inside it the air sinks and keeps clouds from forming.

Around the eye runs the eyewall, an organized ring of cumulonimbus, the clouds that build thunderstorms. It brings heavy rain and usually the most violent winds, concentrated on its inner edge. Farther out spread the rainbands, curved belts of cloud spiraling away from the center that can bring downpours, gusts and even tornadoes. Because the strongest winds sit a little away from the exact center, a coast can feel them even if the eye never makes landfall.

A hurricane is typically about 300 miles (483 km) wide, though sizes vary a great deal, and NOAA warns that size is no guide to strength. The largest on record is Typhoon Tip in 1979, with gale-force winds reaching 675 miles (1,087 km) from its center.

Practical example: someone in the eye at night often sees stars overhead and may think the storm has passed. It is only halfway through: soon the far side of the eyewall arrives, and the wind starts up again from the opposite direction.

Measuring strength: the Saffir-Simpson scale

The Saffir-Simpson scale, used by the NHC, rates hurricanes from 1 to 5 on maximum sustained wind alone.

Category	Sustained wind	km/h	Knots
1	74-95 mph	119-153	64-82
2	96-110 mph	154-177	83-95
3	111-129 mph	178-208	96-112
4	130-156 mph	209-251	113-136
5	157 mph or more	252 or more	137 or more

From Category 3 up a storm counts as a major hurricane, and for Categories 4 and 5 the NHC expects much of the affected area to be uninhabitable for weeks or months. A storm can also strengthen fast: rapid intensification means the maximum wind rises by at least 30 knots in 24 hours.

Wind is only one of the hazards. The scale leaves out storm surge, rainfall flooding and tornadoes, which can kill even in a low-category hurricane, and the WMO adds lightning to the list. Storm surge is an abnormal rise of the sea, driven ashore mainly by strong winds, and it can push up rivers and canals and reach far inland. When it coincides with high tide the water can in some cases rise 20 feet or more, and with other conditions equal a larger storm drives it higher. For Hurricane Ian in 2022, the NHC attributes 41 deaths to surge.

Among the records, NOAA lists Wilma (2005) as the Atlantic basin’s most intense hurricane, with a central pressure of 882 millibars, and Patricia (2015) as the strongest in the Western Hemisphere, with estimated sustained winds of 213 mph.

Mid-latitude cyclones and medicanes

In NOAA’s account, extratropical cyclones, the ones that form in the mid-latitudes, shape the weather across much of the world from autumn to spring. The NHC defines them as cyclones of any intensity that run mainly on the temperature gap between warm and cold air masses. Treccani places their birth outside the tropics, usually over the oceans and mostly in winter, and links them to polar air reaching the temperate zones, while noting that scientists do not fully agree on the causes.

Norwegian meteorologists were the first to describe their life cycle, in the 1910s and 1920s. At the start, a front divides two air masses, the warmer one lying to the south and the colder one to the north. As the air masses move, the front splits into a cold front and a warm front; the faster cold front catches up with the warm one and forms an occluded front. Once the warm air is cut off from the center, the low slowly winds down.

One family can turn into the other. A tropical cyclone moving poleward can become extratropical, switching energy source while keeping hurricane-force winds, and even after losing its tropical features it can still bring heavy rain and strong winds.

Over the Mediterranean something close to the reverse happens. A 2023 study in Scientific Reports explains that this sea cannot produce true tropical cyclones, but some mid-latitude cyclones crossing it occasionally turn into similar systems, called medicanes or Mediterranean hurricanes. They are smaller and symmetrical, with cloud bands wrapped around a warm, calm low-pressure core that is ringed by strong winds. The WMO describes them as hybrids, historically most frequent between September and January, so across the whole of autumn, and notes that a sea warmer than 26 °C can strengthen them after they form.

Italy has been hit as well. In October 2021 Medicane Apollo crossed the Strait of Sicily and the central-western Ionian Sea, with intense rain, extensive coastal flooding, seven deaths and severe damage in Sicily and Calabria; the same study also lists Qendresa (2014), Zorbas (2018) and Ianos (2020). In September 2023 Storm Daniel, which formed over Greece and was named by the Greek weather service, took on medicane features as it headed for Libya, over a sea that topped 27.5 °C near the coast. On September 5 and 6 a station in Zagora, Greece, recorded 750 mm of rain in 24 hours. In Derna the failure of two dams flooded the city. The WMO reported that thousands of deaths were feared and that the Libyan Red Crescent put the number of missing at up to 10,000, figures that had not been confirmed when the report came out.

As the Italian research institute CNR-ISAC pointed out in February 2026, scientists had used the term medicane inconsistently. A shared definition, coordinated by the institute, has since appeared in the Bulletin of the American Meteorological Society and entered the society’s glossary.

Why hurricanes get names

For the WMO, a name is the fastest way to spread warnings, and it avoids confusion among forecasters, the media, emergency services and the public. Before names, storms were identified by latitude and longitude. Female names came into use in the mid-twentieth century, and since 1979 they have alternated with male ones.

Five WMO regional bodies draw up the lists in advance, on proposals from national weather services, and no storm is named after a real person. In the Atlantic, where names have come from lists started at the NHC since 1953, six lists rotate and each one returns after six years: the 2026 list will be used again in 2032. If a storm was so deadly or costly that reusing its name would be inappropriate, the WMO committee retires it and chooses a replacement. When a season runs past 21 named storms, the next ones take names from a supplemental list.