Every spring the southern states of
the USA get ready to be battered by tornadoes. These
destructive towers of air, dust and debris are created by winds
blowing at hundreds of miles per hour.
Let's take some time to
learn more about this fearsome phenomenon...
To see a tornado in action, take a
look at this video clip:
The key to a tornado is temperature
- when air warms up it becomes less dense. Warmer air weighs less
than the cold air. Just like a cork floats in water, the warm air
floats on the heavier, colder air.
This causes a convection current to
form, often called a thermal. Glider pilots and birds use
thermals to stay in the air. Birds are really good at taking
advantage of thermals. They glide through them with outstretched
wings, saving a lot of energy as they don't have to flap their
wings as much.
We'd all be in trouble if tornadoes
formed every time the temperature changed. Luckily, tornadoes only
tend to happen when there are dramatic changes in temperature.
Unfortunately for people
living in USA's Tornado Alley, they live
somewhere where this happens pretty often...
When this happens, columns of air
begin to spin as they rise. It's the spinning motion that reduces
the density inside the column and this causes it to spin out of
control!
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All shapes and
sizes...
Tornadoes aren't the only kind of
twister. Dust devils, waterspouts and fire whirls are all types of
tornado. Read on to find out more about each one.
Dust devils
You can find these in deserts, and
they are nowhere near as dangerous as twister tornados. As they
spin they pick up dust particles. They are formed in just the same
way as twister tornados, but they generally form during good
weather: blue skies with no clouds in sight. As they
spin, dust is picked up into a vortex.
To see a dust devil in action, take
a look at the video clip:
Dust devils have even been recorded
on Mars. But the temperature change which causes them ranges
between -60oC and -100oC! To see one in
action, look at the video.
Waterspout
These occur over water, but they
don't pick up water as they spin. The water droplets inside
them have been formed from condensation. They normally occur
in the tropics and also tend to be much weaker than land
tornadoes.
To see a waterspout in action, take
a look at the video clip:
Fire Whirl
These are very rare, but if they do occur it's usually during a
wild fire. When there has been a drought and it is dry, it is
easier for fires to spread. But they don't normally last for
very long. Watch this video to see how fire whirls are
created, but don't try it at home!
Or go directly to youtube.