How Hurricanes Form and How They Are Rated, Explained
What a hurricane needs to form, how storms are named and rated on the Saffir-Simpson scale, why storm surge is so dangerous and how to prepare.
Hurricanes are among the most powerful weather systems on Earth. Understanding how they form, and how they are rated, makes forecasts and warnings easier to follow.
What a hurricane needs
A tropical cyclone usually forms when several conditions come together:
- Warm ocean water, typically at least about 26.5°C (80°F), to supply heat and moisture.
- Moist, unstable air that can rise easily and form towering storm clouds.
- Low wind shear, meaning little change in wind speed or direction with height, so the storm can stay organised.
- A spin from the Earth’s rotation, called the Coriolis effect, which is why they rarely form very close to the equator.
As warm, moist air rises over the ocean, it condenses and releases heat, which powers the storm. Air rushes in at the surface to replace the rising air, and begins to rotate around a low-pressure centre.
Stages of development
- Tropical disturbance: a cluster of thunderstorms with a weak circulation.
- Tropical depression: a defined circulation with winds up to 38 mph (62 km/h).
- Tropical storm: winds of 39 to 73 mph (63 to 118 km/h). At this stage it is given a name.
- Hurricane: winds of 74 mph (119 km/h) or higher.
The Saffir-Simpson Hurricane Wind Scale
Hurricanes are rated from Category 1 to 5 based on their maximum sustained wind speed:
| Category | Sustained winds |
|---|---|
| 1 | 74 to 95 mph |
| 2 | 96 to 110 mph |
| 3 | 111 to 129 mph |
| 4 | 130 to 156 mph |
| 5 | 157 mph or higher |
Categories 3, 4 and 5 are considered major hurricanes. The scale measures wind only. It does not capture the danger from rain, flooding or storm surge, which can be severe even in a lower-category storm.
Why storm surge is so dangerous
Storm surge is a rise in sea level pushed ashore by the storm’s winds, and it can flood coastal areas quickly. Heavy rainfall inland can cause flash floods and river flooding too. For many people, water rather than wind is the greatest threat.
Names around the world
The same type of storm has different names depending on where it forms: hurricane in the Atlantic and eastern Pacific, typhoon in the northwestern Pacific, and cyclone in the Indian Ocean and South Pacific. Storms are named from rotating lists agreed by regional weather organisations.
How to prepare
- Know your local evacuation zone and follow instructions from local officials.
- Prepare a kit with water, food, medicines, chargers and important documents.
- Secure your home, and move vehicles and loose items.
- Follow official forecasts from your national weather agency, not social media rumours.
Frequently asked questions
What is the difference between a hurricane, a typhoon and a cyclone?
They are the same kind of storm. It is called a hurricane in the Atlantic and eastern Pacific, a typhoon in the northwestern Pacific, and a cyclone in the Indian Ocean and South Pacific.
When is hurricane season?
The Atlantic season runs from 1 June to 30 November, and the eastern Pacific season from 15 May to 30 November, though storms can occasionally form outside these dates.
What is storm surge?
Storm surge is a rise in sea level pushed onto land by a storm's winds. It is often the most dangerous part of a hurricane, and the cause of many deaths.
