Hurricanes and Tornadoes

Tornadoes
With the amount of media coverage dedicated to American storm-chasers, you'd think the US had the monopoly on these twisters. It doesn't. It might come as a shock, but the United Kingdom is actually the world's most tornado-prone nation.



This fact was calculated by the late Dr Fujita of Chicago University. He devised the standard method of measuring tornado intensity. Fujita figured that since Britain has an average of 33 tornadoes every year in an area 38 times smaller than the USA, you're twice as likely to witness a tornado here.



How tornadoes happen
# Warm and cool airstreams collide
# A rotating area of low pressure storm clouds form
# Air within a low pressure front rises, creating a strong upward draught like a vacuum cleaner
# This draws in surrounding warm air from ground level, causing it to spin faster and faster
# These strong air currents can create a vortex - a spiralling funnel of wind - that can reach speeds of 300mph
# Where the funnel touches the ground, it creates a path of concentrated destruction, rarely more than 250m across




Heavy objects, like cars and cows, can be sucked up and flung around like confetti, and houses appear to explode. This is because air pressure within the vortex is extremely low. Inside the building the air pressure is normal, so when the tornado passes over, the air inside the building expands, creating an explosion.Wind speeds in tornadoes can vary from 72 to almost 300mph. Fortunately, only 2 percent of all tornadoes have winds greater than 200 mph.




Hurricanes



By definition, a hurricane is fierce rotating storm with an intense centre of low pressure that only happens in the tropics. In south-east Asia they're known as typhoons and in the Indian Ocean, cyclones.

They cause high winds, huge waves, and heavy flooding. In 1998, Hurricane Gilbert produced 160mph winds, killing 318 people, and devastating Jamaica. A tropical storm can only be classified as a hurricane if it sustains wind speeds above 73mph or force 12 on the Beaufort Scale. Each year about 50 tropical storms reach hurricane status.

One of the most powerful of all weather systems, hurricanes are powered by the heat energy released by the condensation of water vapour. However, the conditions have to be exact for a hurricane to form, with the sea's surface temperature being above 26.5°C.




How hurricanes happen
Air above warm tropical water rises quickly as it is heated by the sea. As the air rises it rotates or spins creating an area of low pressure, known as the eye of the storm. The eye can be clearly seen on satellite pictures, and is usually eerily calm.

The hurricane only moves slowly at speeds of 20-25mph bringing torrential rain and thunderstorms and very strong winds. However, they also cause flooding on low lying coastlines with a phenomenon known as a 'storm surge'.




Storm surge
This is caused by the intense low pressure at the eye of a hurricane, combining with the effect of strong winds. The sea rises 1cm for every millibar of pressure - if the pressure is 930 millibar, the sea surge will be about 80 cm. Hurricanes can raise the seas surface by as much as 4m.




The hurricane winds push the surge along in front of its path. When this surge hits low-lying coasts, the effects can be devastating. In addition to the sea surge, flooding can also result from torrential rain falling from the storm clouds.

Once it reaches the mainland, a hurricane may cause widespread damage for a few days, but with no warm water to supply heat, they quickly die out.