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d ~ 277'862'400 km: day 108

For most people in the UK, the raw power of a tornado is hard to imagine.
But tornado season has started in the US, and reports suggest that 62 tornadoes have ripped through North Carolina alone in the last few days.

During the next few weeks, thousands of scientists and storm-chasers will head to "Tornado Alley" in the hope of studying and witnessing a giant twister. Where do these massive whirlpools in the sky come from?'

First of all, you need an energy source. In the case of a tornado the energy comes from water vapour, which is a bit like an aqueous battery. When liquid water gets hot enough to evaporate into a gas, it doesn't just need to be the right temperature, it also needs a little extra kick of energy (known as latent heat) to convert from a liquid to a gas. When the water molecules are floating around as vapour, they carry this extra energy with them and when they turn back into a liquid they give that energy back to their surroundings. When water from the ocean evaporates and blows away, it carries this stored energy with it. Harmless though water vapour sounds, it's fantastically effective storm fuel.

The storm starts when warm moist air starts to rise, and since this air is full of water vapour it's carrying lots of energy. As it rises, it releases that energy and starts an updraft, an upside-down waterfall in the sky. This stream of air rushes upwards and new air is sucked into the gap that is left behind. Above, condensing water forms a huge cloud.

Tornadoes form in "supercell" storms, which have one really strong updraft that sucks in air from the surroundings at an enormous rate. In your bathtub, water starts to rotate before it's sucked downwards, because it can't all fall down the hole at once. In the same way, air being sucked towards the updraft rotates, but instead of falling downwards in the centre, it rushes upwards. These upward winds can reach speeds of 100 mph. Just like a skater pulling their arms in, the air coming in from the sides rotates faster as it gets closer to the centre. These upside-down waterfalls officially become tornadoes when they cause surface winds faster than 40 mph.

So the warm moist air has fueled a huge and destructive rotating column of air, a tornado. But air is usually invisible. Why can we see twisters?
What we're seeing are water droplets. There is really low pressure in the tornado centre, up to 10% lower than the surrounding air. As air is sucked into this core, the low pressure allows the water in the air to condense much closer to ground level than normal and it's these droplets that we can see. So when you see a funnel cloud from a tornado, you're seeing the shape of the region of low pressure. All the surrounding air is being sucked into that "hole" in the atmosphere.

If you want to see a tornado, you'll need patience and luck. Less than 20% of all supercell storms form tornadoes and scientists are still not sure what makes some storms generate twisters while others don't.

You don't have to be in the US to see tornadoes though. According to the , an average of 33 tornadoes per year are reported in the UK. They're small ones, but they still count.

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