WHIRLWINDS OF TERROR

Powerful tornadoes killed at least 35 people in the United States in April 2014. They flipped cars, ripped up homes and uprooted trees across south-central United States.

How do tornadoes form, and do they always cause so much destruction?

Tornadoes are among the most violent and destructive storms on earth. They form when hot humid air meets dry, cold air. As the hot air rises, the denser, cold air gathers underneath. The condensation and moisture produced cause thunder clouds to form.

The two air masses rotate around each other without mixing. Not long after that, a cone or funnel can be seen. This is a warning sign a tornado is on the way. Once the spiral touches the ground it is called a tornado.

The amount of damage a tornado causes is related to how fast it spins. The Enhanced Fujita Scale is used to measure intensity and damage potential.

Most tornadoes are classified as Level Zero, with wind speeds of between 105 and 137 kilometres per hour. Damage is usually slight, for example tiles might be blown off roofs. Level Five tornadoes spin at 320 to 500 kilometres per hour. They can uproot whole houses and throw cars more than 100 metres.

Tornadoes can form all across the world but they hit the US most frequently. In the Mid-West area of the US, there is a place called Tornado Alley where tornadoes form most easily.