A cyclone is a large rotating storm system that forms around a center of low atmospheric pressure. Winds spiral inward toward that low-pressure core, picking up energy from warm ocean water and producing heavy rain, powerful winds, and dangerous coastal flooding. The same type of storm goes by different names depending on where it forms: “hurricane” over the Atlantic and eastern Pacific, “typhoon” in the western Pacific, and simply “cyclone” in the Indian Ocean and near Australia.
Why Cyclones Spin
Earth’s rotation creates something called the Coriolis effect, which deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. When air rushes toward a low-pressure center, this deflection forces the air into a spinning pattern rather than a straight path. In the Northern Hemisphere, cyclones rotate counterclockwise. In the Southern Hemisphere, they rotate clockwise.
The Coriolis effect is weakest near the equator, where the planet’s surface isn’t moving sideways fast enough to deflect wind into a spin. That’s why tropical cyclones almost never form within about 300 miles (480 km) of the equator. You need enough rotational force to get the system spinning, and the equator simply can’t provide it.
How a Cyclone Forms
Tropical cyclones need a specific set of ingredients to come together. The ocean surface must be at least 80°F (27°C) down to a depth of about 150 feet. The atmosphere above needs to cool rapidly with altitude so that warm, moist air keeps rising. There also has to be relatively little wind shear, meaning winds at the surface and high up in the atmosphere are moving at roughly the same speed and direction. Strong wind shear tears a developing storm apart before it can organize.
Formation begins with a tropical disturbance, a broad area of low pressure where surface winds converge. If conditions stay favorable, the system tightens into a tropical depression with a recognizable circulation center. When sustained winds exceed 39 mph (63 km/h), it becomes a tropical storm and receives a name. Once winds reach 74 mph (119 km/h), the storm is officially classified as a hurricane, typhoon, or cyclone depending on its location.
Anatomy of a Cyclone
A mature cyclone has three distinct parts, each with its own weather conditions.
The eye is the calm center. It’s typically 20 to 40 miles (32 to 64 km) across, mostly cloud-free, with light winds usually below 15 mph. Sinking air in the eye suppresses cloud formation, which is why satellite images show that distinctive dark hole in the middle of the storm. An eye typically develops once maximum sustained winds surpass 74 mph.
The eyewall surrounds the eye and contains the most violent weather. It’s a ring of towering thunderstorms about 10 to 20 miles from the center where the heaviest rain falls and the strongest winds blow. If you could somehow stand in the path of a cyclone, the eyewall is the part you’d least want to experience.
Rainbands are the spiral arms of cloud and rain that curve outward from the eyewall. These bands produce bursts of heavy rain, strong winds, and sometimes tornadoes. Between the bands, there can be gaps with little rain and relatively light wind. Traveling from the outer edge of a cyclone to its center means passing through alternating periods of calm and increasingly intense rain and wind, each cycle stronger than the last.
How Cyclones Are Categorized
In the Atlantic and eastern Pacific, tropical cyclones are ranked on the Saffir-Simpson Hurricane Wind Scale, which runs from Category 1 to Category 5 based on sustained wind speed:
- Category 1 (74–95 mph): Damage to mobile homes, signs, and trees. Numerous power outages.
- Category 2 (96–110 mph): Roof, door, and window damage to buildings. Power outages lasting days.
- Category 3 (111–129 mph): Structural damage to small buildings. Mobile homes destroyed. Power out for days to weeks.
- Category 4 (130–156 mph): Wall failures and complete roof loss on many homes. Electricity unavailable for weeks.
- Category 5 (above 156 mph): Complete roof failure on many buildings. Some structures blown over entirely. Power outages lasting weeks to months.
Categories 3 through 5 are considered “major” cyclones. The scale measures wind damage potential only, not flooding or storm surge, which are often the deadlier threats.
The Biggest Danger: Storm Surge
Storm surge is currently the leading cause of fatalities from tropical cyclones. It happens when the cyclone’s strong onshore winds push ocean water toward the coast. In deep water, this effect is invisible because the water can circulate freely. But as the storm approaches shallow coastal areas, the water has nowhere to go but up and inland. The result is a rapid, often massive rise in sea level that can flood coastal communities in minutes. Hurricane Ian in 2022 killed 41 people through storm surge alone.
Not All Cyclones Are Tropical
The word “cyclone” technically applies to any low-pressure system with rotating winds, not just the powerful tropical storms most people picture. Extratropical cyclones form outside the tropics and draw their energy from temperature contrasts between warm and cold air masses rather than from warm ocean water. These are the large-scale storm systems that drive much of the winter weather across the middle latitudes. Subtropical cyclones fall somewhere in between, originating over warm water but with a less organized structure and their strongest winds located much farther from the center.
When a tropical cyclone moves over cooler water or land and loses its warm-core structure, meteorologists reclassify it as a post-tropical cyclone. It can still produce dangerous rain and wind, but its energy source has fundamentally changed.
How Many Cyclones Happen Each Year
Globally, about 85 to 90 named tropical storms form in a typical year. In 2024, 85 named storms developed worldwide, close to the 1991–2020 average of roughly 88. Of those, 42 reached full tropical cyclone strength (winds of 74 mph or higher), and 23 became major tropical cyclones with winds above 111 mph. Overall storm energy in 2024 was about 21% below the long-term average, a reminder that raw storm counts don’t tell the whole story. A season with fewer but more intense storms can be far more destructive than one with many weaker ones.
One complication in tracking long-term trends: the apparent increase in short-lived storms (lasting two days or less) since around 2000 is largely a result of better satellite coverage and analysis techniques, not necessarily a real increase in storm activity.