Tornadoes and Waterspouts
- What is tornado?
- What causes tornado?
- Measuring tornado's strength
- Waterspouts

What is a Tornado?

A tornado is a violent, spinning column of air. From a distance, it looks like a cloud in the shape of an ice- cream cone. Tornadoes are most numerous and devastating in central, eastern and north- eastern USA, where an average of five per day are reported every May. They also occur in Australia (where there are around fifteen per year) and in UK, Italy, Japan and Central Asia.
>> A tornado in the Midwest of the USA creates a swirling cloud of dust and debris.

What causes tornadoes?

Tornadoes usually form over land rather than over tropical oceans, when there is warm, moist air near the ground and cold, dry air above it. These conditions occur frequently in late spring and early summer over the Great Plains of the USA. As the sun heats the ground, the warm, moist air rises. As it does so, it cools and from large cumulonimbus clouds. The strength of the updraft produced by the rising air affects how much of the surrounding air is sucked into the bottom of the tornado. >> Dramatic shots like this one, of a tornado in the US Midwest, are taken by "tornado chasers", who enjoy the thrill of getting close to the violent storm.

Two things help the tornado to spin violently: Coriolis force and the jet stream. As the jet stream passes over the top of the storm, it adds an extra twist to the tornado.

Amazing but True
On 4 September 1981, a tornado hit Ancona in Italy. It lifted a baby asleep in his pram 15m into the air and set it down safely 100m away. The baby was still sleeping soundly!

Measuring Tornado Strength

Measuring tornado strength Two types of measurement are used to describe the strength of a tornado. The Fujita (F) Scale classifies the strength of a tornado according to the speed at which it rotates. This is important as it indicates how destructive the storm will be. Tornadoes classified as F1 are usually (but not always) harmless. F4 and F5 tornadoes are truly scary. However, on average there are only two F5 tornadoes per year in the USA, and they usually reach level 5 for just a few seconds. An F6 tornado is possible but no one has ever observed one.

Fujita Scale

The second type of measurement is called the Pearson Scale. This scale is used to measure the length and width of a tornado's path. Using these two scales, the potential damage and the area that is likely to be affected can be predicted.

Waterspouts

Over the sea, waterspouts may form with wind speeds of up to 150 km/h, although 80 km/h is more normal. Waterspouts can cause damage if they cross land, but they never cause as much damage as tornadoes. >> This is an example of a waterspout.

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