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HOT TOPICS Published March : 28 : 2002
Anthrax bacteria
Tornado over Miami

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Print Version Natural Disasters
Key points
Tornadoes & hurricanes
Floods & storms
Tidal waves & droughts
Could natural disasters devastate Britain?
Quiz: what do you know about disasters?
Vote: do you feel at risk?
Timeline of events
Hurricanes and 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.

    The afternath of an Oklahoma tornado
    Tornado in Oklahoma

    Wind speeds in tornadoes can vary from 72 to almost 300 mph. Fortunately, only 2 percent of all tornadoes have winds greater than 200 mph.

    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 160 mph winds, killing 318 people, and devastating Jamaica. A tropical storm can only be classified as a hurricane if it sustains wind speeds above 73 mph 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 20-25 mph 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 1 cm 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.

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    back to the top
    Key points
    Tornadoes & hurricanes
    Floods & storms
    Tidal waves & droughts
    Could natural disasters devastate Britain?
    Quiz: what do you know about disasters?
    Vote: do you feel at risk?
    Timeline of events


    BBC Science - Horizon
    Volcano Hell

    BBC News
    Why volcanoes explode

    BBC Weather
    UK Flooding

    2002-2000 | 1999-1980 | 1979-1960 | 1959-1900 | 1899-70,000BC

    September 2002
    Earthquake hits UK

    May 2002
    'Earthquake risk' from dams

    March 2002
    An earthquake hits Northern Afghanistan killing two thousand people

    February 2002
    Over 50 people were killed by floods in La Paz, Bolivia

    January 2002
    Nyiragongo volcano erupts in the Republic of Congo, engulfing the city of Goma. 47 people died

    January 2002
    A tornado damages up to 100 homes in Cheshire as high winds sweep across England

    July 2001
    Typhoon Toraji slams central Taiwan and China, causing flash floods and mudslides, killing at least 72 people

    January 2001
    An earthquake hits India's Gujarat region killing over 25,000 people

    October 2000
    Weeks of rain bring widespread flooding to UK and Northern Europe

    November 2000
    An earthquake shakes Papua New Guinea, triggering at least one tidal wave


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    UK Flood Warnings
    UK Government Environment Agency

    Worldwide Earthquake Locator
    University of Edinburgh

    Hurricane Prediction Centre
    US National Hurricane Centre, Florida

    Tornadoes & Storms
    European Tornado and Storm Research Organisation, TORRO

    Volcano World
    University of Dakota

    University of Washington geophysics site

    About droughts
    US National Oceanic and Atmospheric Administration

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