How Do Hurricanes Form? From Warm Ocean to Monster Storm

Seen from space, a hurricane is one of the most beautiful sights on Earth: a vast, perfect spiral of white cloud hundreds of kilometres wide, with a dark eye at its centre. On the ground it is one of the most destructive. Winds can flatten buildings, the sea can surge inland and rain can flood entire cities. How does such a giant form?

The ingredients

Hurricanes, also called tropical cyclones, need a special combination of conditions:

• Warm ocean water, at least about 26.5 °C, down to a depth of around 50 metres. This is the storm’s fuel.
• Moist air high into the atmosphere.
• Low wind shear: winds that do not change much in speed or direction with height. Strong shear tears developing storms apart.
• An existing disturbance, such as a cluster of thunderstorms, often a tropical wave drifting off Africa in the Atlantic.
• Enough distance from the equator, at least about 5 degrees of latitude, so that Earth’s rotation can make it spin.

How a hurricane is born

1. Evaporation. Warm sea water evaporates, loading the air above with heat and water vapour.
2. Rising and condensing. Moist air rises, cools and condenses into towering clouds. Condensation releases latent heat, warming the air and making it rise even faster.
3. Falling pressure. As air rises, the pressure at the surface drops. Surrounding air rushes in to fill the gap, picking up more heat and moisture from the warm sea.
4. Spin. Because Earth rotates, air moving towards the low-pressure centre is deflected, an effect called the Coriolis effect. In the Northern Hemisphere storms spin anticlockwise; in the Southern Hemisphere, clockwise.
5. Organisation. As the system strengthens, it becomes a tropical depression, then a tropical storm, which receives a name. When sustained winds reach 119 km/h, it becomes a hurricane, and a calm eye often forms at its centre.

Inside a hurricane

At the centre is the eye, usually 30 to 65 kilometres across, where air gently sinks and skies can be almost clear. Around it is the eyewall, a ring of towering thunderstorms with the strongest winds and heaviest rain. Beyond it, spiral rainbands stretch outwards for hundreds of kilometres. People caught in the eye may think the storm has passed, only to be hit by the other side of the eyewall minutes or hours later.

Measuring hurricanes

In the Atlantic and northeast Pacific, hurricanes are rated on the Saffir–Simpson scale according to their sustained winds:

Water, not wind, is the biggest killer

The scale is based on wind, but most hurricane deaths are caused by water:

• Storm surge: powerful winds and low pressure pile up seawater, which floods coasts. In 2005, Hurricane Katrina’s surge breached levees in New Orleans and flooded most of the city.
• Heavy rain: slow-moving storms can drop more than a metre of rain, causing flash floods far from the coast.

Same storm, different names

A hurricane in the Atlantic is a typhoon in the northwest Pacific and a cyclone in the Indian Ocean and South Pacific. They are the same type of storm. The Mediterranean occasionally produces smaller hurricane-like storms nicknamed medicanes. In September 2023, Storm Daniel, a medicane, caused catastrophic flooding in Libya, where two dams collapsed near the city of Derna.

Hurricanes and climate change

Scientists are studying how a warming climate affects these storms. In 2005, Kerry Emanuel reported that the destructive potential of tropical cyclones had increased markedly over the previous 30 years, linked to warmer oceans. A major assessment led by Thomas Knutson, published in 2019 and 2020, concluded that with further warming:

• the proportion of the most intense storms is likely to increase;
• storms are likely to produce more rainfall;
• storm surges will be made worse by rising sea levels.

The total number of storms may not increase, and could even decrease, but those that form are expected to be more dangerous on average.

Staying safe

Hurricanes can be tracked for days before landfall, thanks to satellites, aircraft that fly into the storms, and computer models. If you live in a hurricane area, know your evacuation route, follow official warnings, and remember that storm surge and flooding are often more dangerous than the wind.

In short

• Hurricanes form over warm oceans of at least about 26.5 °C.
• Rising moist air releases heat, lowering pressure and drawing in more air.
• The Coriolis effect makes the storm spin; hurricanes rarely form near the equator.
• They are rated on the Saffir–Simpson scale, from Category 1 to 5.
• Storm surge and flooding cause most hurricane deaths.
• A warmer climate is expected to make the strongest storms more common and rainier.