Hurricanes
A hurricane is a giant rotating storm that forms over warm tropical oceans. They are by far the largest storms on Earth, sometimes over 1,000 km wide, with sustained winds over 180 km/h (and gusts much higher). Hurricanes draw their power from warm ocean water, which is why they only form in tropical and sub-tropical regions. In different parts of the world they have different names: hurricane in the Atlantic and eastern Pacific, typhoon in the western Pacific, cyclone in the Indian Ocean and southern Pacific. All three are the same kind of storm.
- Other namesTyphoon, cycloneSame storm, different ocean
- Sustained wind speed118+ km/hTo officially count as a hurricane
- Largest knownTyphoon Tip, 19792,220 km wide
- Strongest knownPatricia, 2015345 km/h sustained winds
- Atlantic hurricane seasonJune 1 to November 30Peak in September
- Hurricanes named since1953By the US National Hurricane Center
How hurricanes form
Hurricanes need very specific conditions to form.
- Warm ocean water: at least 26.5 degrees C, to a depth of at least 50 metres.
- Tropical latitude: 5 to 30 degrees from the equator. Closer to the equator, there is not enough Coriolis force to spin the storm.
- High humidity in the lower atmosphere.
- Low wind shear: the wind needs to be blowing roughly the same direction at all altitudes, otherwise the storm gets ripped apart.
- A trigger: an existing tropical disturbance or low-pressure area.
When all five come together, warm ocean water evaporates quickly into the atmosphere. The wet air rises, cools, and releases huge amounts of heat as the water vapour condenses. This heat warms the surrounding air, which rises even faster, sucking in more warm wet air from below. The whole system spirals upwards and outwards, growing into a hurricane over several days.
Parts of a hurricane
- Eye: the calm centre, usually 30 to 65 km wide. Clear skies and light winds.
- Eye wall: a ring of intense thunderstorms around the eye. The strongest winds and heaviest rain.
- Rainbands: spiralling bands of cloud and rain that wrap around the storm, sometimes for hundreds of kilometres.
- Outflow: high-altitude winds blowing outwards from the top, helping to ventilate the storm.
The Saffir-Simpson scale
Hurricanes are rated for strength on the Saffir-Simpson scale, from category 1 to category 5, based on sustained wind speed.
- Category 1: 119 to 153 km/h. Some damage to homes and trees.
- Category 2: 154 to 177 km/h. Extensive damage, power outages.
- Category 3: 178 to 208 km/h. Devastating damage, some buildings destroyed.
- Category 4: 209 to 251 km/h. Catastrophic damage, most areas uninhabitable for weeks.
- Category 5: 252 km/h or higher. Total destruction over large areas.
The strongest hurricane ever measured, Hurricane Patricia in October 2015, had sustained winds of 345 km/h, well above category 5. It thankfully weakened before making landfall in Mexico.
Why hurricanes have names
Hurricanes have been given personal names since the early 1950s, originally just female names but male names were added in 1979. Each ocean basin has its own rotating list of names, used in alphabetical order each season. If a hurricane is particularly destructive (like Katrina or Andrew), its name is "retired" and never used again. The system makes it much easier for the public to keep track of which storm is which when several are happening at the same time.
Famous hurricanes
- Galveston Hurricane, 1900: deadliest natural disaster in US history. 8,000 to 12,000 deaths in Texas.
- Hurricane Andrew, 1992: a Category 5 hurricane that devastated southern Florida.
- Hurricane Katrina, 2005: flooded New Orleans, killing 1,833 and causing $125 billion in damage.
- Typhoon Haiyan, 2013: hit the Philippines with the highest-ever sustained winds at landfall, killing over 6,300 people.
- Hurricane Maria, 2017: devastated Puerto Rico, killing approximately 3,000 people and leaving much of the island without power for months.
- Hurricane Ian, 2022: caused approximately $113 billion in damage in Florida.
Hurricanes and climate change
Warmer oceans mean more evaporation and more energy for hurricanes. Most climate scientists agree that climate change is making the strongest hurricanes more intense, even if the total number does not change much. Hurricanes are also producing more rainfall than in the past (warmer air holds more water), causing more flooding. And rising sea levels make the storm surge from each hurricane more dangerous.
Deeper dive: hurricane hunters and the science of forecasting
One of the most extraordinary jobs in meteorology is the hurricane hunter: pilots and scientists who deliberately fly aircraft into active hurricanes to gather data. The most famous are the US Air Force's 53rd Weather Reconnaissance Squadron, which has been flying into hurricanes since 1944.
Modern hurricane hunter flights use heavily modified C-130 turboprops loaded with weather instruments. They fly through the storm at approximately 3 km altitude, sometimes passing through the eye several times in a single mission. Inside the eye, the air is calm and clear, often with blue sky overhead and a visible ring of towering thunderclouds around them. Then they fly back out through the eye wall (the most violent part of the storm) and continue measuring.
The data they gather (including dropping disposable instruments called dropsondes through the storm) is fed directly into hurricane forecasting models. The improved data has made modern hurricane track forecasts dramatically more accurate. A 3-day forecast today is as accurate as a 1-day forecast was in the 1980s. This has saved thousands of lives by giving people more time to evacuate.
The flights are dangerous but surprisingly few aircraft have been lost. The biggest threat is not the wind itself (the aircraft are designed for severe turbulence) but the possibility of severe icing or a sudden hailstorm damaging engines. Modern crews say flying through a hurricane is bumpy but generally not as terrifying as it sounds, until you remember what is happening on the ocean below.
For more, see wind, thunderstorms and evaporation.