Let’s twist again like we did last summer

In a talk I was giving last week on the physics of thunderstorms, to a joint meeting of the Institute of Physics and the Institute of Engineering and Technology, I could not resist mentioning tornadoes.  Always spectacular and at times devastating, I have been fascinated by them for years.  We tend to hear more about tornadoes in the US, and they do have around 75% of the world’s tornado occurrences, but there are many other locations across the globe that have the right conditions for tornadoes to form.  Canada comes second to the US in the number of tornadoes and then in Europe it’s the UK and a swath of countries in Europe’s tornado alley that runs from France, through Germany and Poland to Russia.  South America, Asia, Australia and the southern part of Africa also regularly see tornadoes.  They are of course so wide spread because the conditions to form tornadoes occur in so many different places.  So what are they?

At a very simply level you need cold, dry air and warm, moist air coming together.  That’s why we see this well-known tornado alley in the US formed along the long diagonal line spreading up in to Canada, where the cold air from the Rocky Mountains meets the warm, moist air travelling north from the Gulf of Mexico.  At these cold-warm air boundaries, where you have a strong temperature gradient, lots of moisture, and so lots of convectively available potential energy, then very intense thunderstorms can form and these are the birthplace of tornadoes.  These intense thunderstorms draw the warm air up and in to the cloud and the cold air flows down spreading out from the bottom of the cloud, forming a strong convective circulation of warm air in and up, cold air down and out.

I’m sure you have felt this yourselves.  Have you ever been out and about on that warm sunny summer afternoon and felt the breeze pick up a little, with a slight chill, then 30 minutes later the storm shower follows.  Well that cool breeze is the nose of cold air pushing out as a gust front ahead of the storm.  If you know what you’re looking for then it’s a great predictor of pending showers, and has rescued several BBQs and washing drying out on the line.  Going back to the thunderstorm, what can happen is that as the cold air spreads out it cuts off the warm air supply, which is like turning off the gas and the storm will just dissipate away.  But if the winds change direction with height, what we call windshear, then this can help to separate the up and down flows of air and keep the thunderstorm going.  With enough energy the big brother of all thunderstorms, called supercells, are borne and these in turn give birth to tornadoes.

There is a simple demonstration you can do at home to show why in particular we see the characteristic funnel that defines all tornadoes.  So go-on, take two plastic fizzy drink bottles.  Fill one with water and place the other upside down on top of the first so they are joined at the two screw tops.  Then tape them together, so you have what looks like a plastic bottle, water-based egg timer.  If you tip it up the water tries to fall down to the lower bottle as the air tries to pass up in to the top bottle and it slowly glugs through.

Now as you tip it over to try again, hold the bottles in the vertical and move them round in a circle to get the water in the bottles to spiral round and you’ll see something that looks like a tornado forming in the water.

What you will also notice is that the water and the air pass each other much quicker.  That’s what is happening in the tornado.  The air wants to equalise and the fastest way to do that is through a spiral and it really is moving fast, reaching speeds of up to 300 mph.  That causes the air pressure inside the funnel to drop significantly, so the air cools and condenses forming the funnel cloud.  Physicists know this process well as the Bernoulli effect.

The speed of the winds is what also helps to lift objects in to the tornado.  As the air passes rapidly over the top of the object it creates lift, like in an aeroplane wing, because the pressure is lower on top of the object than underneath it.  The forces are huge, because it increases with the cube of the wind speed, so double the speed and this exerts eight times the lift on the object.  That’s why tornadoes are so destructive.  Ted Fujita developed a scale that defines the damage caused by tornadoes, which became known as the Fujita scale.  The scale has been adapted over more recent times but the original scale was defined as F0 up to F16, but only F0 (light damage) to F5 (incredible damage) were used.

It has become very popular over recent years to go on storm chasing holidays to follow tornadoes, but you don’t have to go much further than the south coast in the UK during the summer months to sight a tornado out at sea – these are called water spouts, and are commonly spotted in the Dover Straits.  Also, if you’ve never seen pictures of a fire tornado then you should definitely Google that (other search engines are also available).

by Prof Paul Hardaker, Chief Executive of The Institute of Physics