Hurricanes are formed from low-pressure systems that come with rain and thunderstorms. These storms are known by different names depending on where they occur: they’re called tropical cyclones in the Caribbean, typhoons in the Indian Ocean and Sea of Japan, baguios in the Philippines, and willy-willies in Australia.
They’re also classified by wind speed:
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Tropical Depression: Winds of up to 62 km/h (38 mph) at sea level.
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Tropical Storm: A warm-core tropical cyclone with maximum winds between 63 and 117 km/h (39–73 mph).
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Hurricane: A warm-core tropical cyclone with sustained winds of 118 km/h (74 mph) or more at sea level. The intensity depends on the hurricane category.
In warm tropical waters, low-pressure systems and tropical waves often develop. There are five main factors needed for a hurricane to form:
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A pre-existing weather disturbance – usually a cluster of thunderstorms that can trigger development.
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Warm ocean water, at least 26°C (79°F) from the surface down to about 15 meters deep. At this temperature, seawater evaporates rapidly, fueling the storm. The evaporation followed by condensation into clouds releases heat, giving the storm energy to grow.
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Winds: Light winds high in the atmosphere that don’t shift much in speed or direction. Warm surface winds help fuel the system with moisture. As the warm, moist air rises, it creates a low-pressure area that pulls in more air, starting a spiral motion. Strong winds aloft can disrupt this structure, so upper-level winds must stay light to allow the cycle to continue.
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Rotation (Coriolis effect): Earth’s rotation gives the system its spinning motion—counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. This spin turns the storm into a massive rotating vortex.
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Humidity: Hurricanes feed on moist air. The more humidity over the ocean, the more energy the storm gathers. Over land, this fuel source disappears, which is why hurricanes weaken once they make landfall.