Hurricanes are powerful, whirling tropical storms. They are also called willywillies, cyclones or typhoons.
Hurricanes develop in late summer as clusters of thunderstorms build up over warm seas (at least 27°C).
As hurricanes grow, they tighten into a spiral with a calm ring of low pressure called the ‘eye’ at the centre.
Hurricanes move westwards at about 20 km/h. They strike east coasts, bringing torrential rain and winds gusting up to 360 km/h.
Officially a hurricane is a storm with winds exceeding 119 km/h.
Hurricanes last, on average, 3-14 days. They die out as they move towards the Poles into cooler air.
Each hurricane is given a name in alphabetical order each year, from a list issued by the World Meteorological Organization. The first storm of the year might be, for instance, Hurricane Andrew.
The whirling winds of a hurricane can cause widespread destruction. The storm measures between 320 and 480 km in diameter.
The most fatal cyclone ever was the one that struck Bangladesh in 1970. It killed 266,000 with the flood from the storm surge – the rapid rise in sea level created as winds drive ocean waters ashore.
A hurricane generates the same energy every second as a small hydrogen bomb.
Each year 35 tropical storms reach hurricane status in the Atlantic Ocean, and 85 around the world.
A satellite view of a hurricane approaching Florida, USA. Notice the yellow eye in the center of the storm.
A weather front is where a big mass of warm air meets a big mass of cold air.
At a warm front, the mass of warm air is moving faster than the cold air. The warm air slowly rises over the cold air in a wedge. It slopes gently up to 1.5 km over 300 km.
At a cold front, the mass of cold air is moving faster. It undercuts the warm air, forcing it to rise sharply and creating a steeply sloping front. The front climbs to 1.5 km over about 100 km.
In the mid-latitudes, fronts are linked to vast spiraling weather systems called depressions, or lows. These are centered on a region of low pressure where warm, moist air rises. Winds spiral into the low anticlockwise in the Northern Hemisphere, clockwise in the Southern.
Lows start along the polar front, which stretches round the world. Cold air spreading out from the Poles meets warm, moist air from the subtropics.
Lows develop as a kink in the polar front. They then grow bigger as strong winds in the upper air drag them eastwards, bringing rain, snow and blustery winds. A wedge of warm air intrudes into the heart of the low, and the worst weather occurs along the edges of the wedge. One edge is a warm front; the other is a cold front.
The warm front arrives first, heralded by feathery cirrus clouds of ice high in the sky. As the front moves over, the sky fills with slate-grey nimbostratus clouds that bring steady rain. As the warm front passes away, the weather becomes milder and skies may briefly clear.
Feathery cirrus clouds high up in the sky are a clear warning that a warm front is on its way, bringing steady rain. When there is a warm front, a cold front is likely to follow; bringing heavy rain, strong winds and perhaps even a thunderstorm.
After a few hours, a build-up of thunderclouds and gusty winds warn that the cold front is on its way. When it arrives, the clouds unleash short, heavy showers, and sometimes thunderstorms or even tornadoes.
After the cold front passes, the air grows colder and the sky clears, leaving just a few fluffy cumulus clouds.
Meteorologists think that depressions are linked to strong winds, called jet streams, which circle the Earth above the polar front. The depression may begin with Rossby waves, giant kinks in the jet stream up to 2000 km long.