How Hurricanes Form and Why They Are So Powerful

Hurricanes, typhoons and cyclones are the same kind of storm known by different names in different regions. They are among the most powerful weather systems on Earth, and they all draw their energy from warm ocean water.

Key takeaways

• Tropical cyclones need warm sea-surface water, moist air and light upper-level winds.
• They are called hurricanes in the Atlantic and eastern Pacific, typhoons in the northwest Pacific and cyclones in the Indian Ocean and South Pacific.
• Earth's rotation makes the storm spin, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
• The calm eye is surrounded by the most intense winds in the eyewall.
• Storm surge and flooding rain often cause more harm than wind.

The ingredients

A tropical cyclone starts over tropical ocean water that is generally around 26 degrees Celsius or warmer. Warm water evaporates, loading the air above it with moisture. This air rises, and as the vapour condenses into clouds and rain it releases heat, which fuels more rising air. The process pulls in more warm, moist air at the surface, so the system can strengthen, provided winds higher in the atmosphere do not tear it apart.

Why the storm spins

Air flowing toward the low-pressure centre does not travel in a straight line. Because Earth rotates, moving air is deflected, a phenomenon called the Coriolis effect. The deflection is weak near the equator, which is why these storms rarely form within a few degrees of it. Away from the equator, the inflowing air curls into a rotating system with a clearly organised circulation.

Eye and eyewall

In a mature storm, a relatively calm and often clear area called the eye forms at the centre. Around it lies the eyewall, a ring of towering thunderstorms where the strongest winds and heaviest rain occur. Beyond the eyewall, spiral rainbands stretch outward for many kilometres. When the eye passes over a location, conditions can briefly calm before the opposite side of the eyewall brings the wind back.

Measuring strength

Forecasters classify storms by sustained wind speed. In the Atlantic, the Saffir-Simpson scale rates hurricanes from category 1 to category 5. The scale describes wind speed only, so it does not capture the full danger of a storm. Rainfall totals, storm size and the speed at which a storm moves also matter.

The main hazards

Storm surge, a rise in sea level pushed onto land by strong winds, is often the deadliest hazard along coasts. Heavy rain can cause flooding far inland, and the storm can also spawn tornadoes. Warnings, evacuation orders and building standards all play a role in reducing harm, which is why following official advice matters.

How storms are tracked

Satellites, aircraft flown into storms, ocean buoys and radar all feed computer forecast models. Track forecasts have improved considerably over recent decades, while forecasting sudden changes in intensity remains harder.

Names and categories

Tropical storms are named to make warnings clearer. Lists of names for each region are maintained by the World Meteorological Organization, and in the Atlantic the lists rotate every six years. When a storm is especially deadly or costly, its name is retired and replaced. The Saffir-Simpson scale rates hurricanes by sustained wind speed: category 1 begins at about 119 kilometres per hour, or 74 miles per hour, and category 5 starts at about 252 kilometres per hour, or 157 miles per hour. Because the scale measures only wind, a category 1 hurricane can still cause devastating flooding.

Forecasting and the cone of uncertainty

Forecasters publish a cone around the predicted track of a storm. The cone shows the probable path of the centre of the storm, based on past forecast errors, and it does not show the size of the storm or the area that will be affected. Dangerous winds, rain and surge can occur far outside the cone. Track forecasts have become much more accurate over recent decades thanks to better satellites, aircraft observations and computer models, but forecasting sudden changes in intensity remains difficult.

Storm surge and rain

Water, not wind, causes most hurricane deaths. Storm surge, a dome of seawater pushed inland, destroyed large parts of the Gulf coast during Hurricane Katrina in 2005. Heavy rain can also cause deadly floods far from the coast, as happened with Hurricane Harvey in 2017, which dropped more than a metre of rain on parts of Texas. Hurricanes can also spawn tornadoes, especially in the right-front part of the storm after landfall.

Hurricanes and climate change

Scientists are studying how warming affects hurricanes. Warmer oceans provide more energy, and a warmer atmosphere holds more moisture, so heavy rainfall from storms is expected to rise. Studies suggest that the proportion of storms reaching the most intense categories may increase, and that some storms are strengthening more quickly, a process called rapid intensification. Rising sea level increases the reach of storm surge. It is less clear whether the total number of storms will change, and research continues.

Preparing

Preparation saves lives. Know whether you live in an evacuation zone and follow official instructions, since leaving early can be safer than staying. Prepare a kit with water, food, medicines, torches, batteries, a charged phone and important documents. Secure your home as advised, check insurance cover for flood, and sign up for local alerts. Never drive through floodwater, because even shallow moving water can sweep a vehicle away.

Frequently asked questions

Do hurricanes form near the equator?

Rarely. The Coriolis effect is too weak very close to the equator to start the necessary rotation.

What happens when a hurricane moves over land?

It loses its warm-water fuel and friction slows the winds, so it weakens, though it can still bring heavy rain and flooding.

Why are storms given names?

Names make it easier to communicate clearly about several storms at the same time. Lists are maintained by international weather organisations.

What is the difference between a hurricane watch and a warning?

A watch means hurricane conditions are possible in the area, typically within about 48 hours, and it is the time to prepare. A warning means they are expected, typically within about 36 hours, and it is the time to complete preparations and follow evacuation orders.

What is rapid intensification?

It is defined as an increase in a storm's maximum sustained winds of at least about 55 kilometres per hour, or 35 miles per hour, within 24 hours. It is hard to forecast and makes some storms much more dangerous than predicted.

Why do hurricanes weaken over land?

They lose their supply of warm, moist air from the ocean, and friction with the land slows the winds. They can still cause serious flooding from rain after they have weakened.

Sources and further reading

• Tropical cyclone (Wikipedia)
• National Hurricane Center