Explaining tornadoes

Tornadoes, sometimes called twisters or whirlwinds, are intense vortices stretching from the Earth’s surface to the base of stormy, cumulonimbus clouds – these clouds extend deeply in the vertical and often have an anvil-shaped top.

Typically, tornadoes measure around 100 metres in diameter, travelling on average around five kilometres before they dissipate. Tornadoes form from mesocyclones, large-scale regions of rapidly rotating air that flow within the cumulonimbus clouds. Cool, dry air can wrap around these mesocyclones, creating a strong temperature contrast which intensifies the spinning, drops the central pressure and extends the vortex to the ground, thus the tornado is born.

As tornadoes are made up of air, they are invisible to the naked eye unless they are transporting some other material, such as condensed water in the form of water vapour, dust, or larger debris, so more often than not take on a dark appearance. Given the chaotic nature of tornadoes, they are a nightmare to predict, so often it is the original mesocyclone that will start to alert the forecasters on where to look. Weather agencies around the world include different phenomena in their classification of tornadoes. So for instance, intense waterspouts, which form over water from a mesocyclone, often only officially count as a tornado if they make landfall. An extremely rare subset of these are known as snowspouts – essentially the same type of circulation but formed in regions of snow.

There are also a number of other tornado-like vortices, for instance, dust- or fire-devils. The former being a small vortex consisting of dust that can form from updraughts without the need for storm clouds. The latter being a small vortex funnelling fire, often formed in large wildfires. While these phenomena typically look much like tornadoes, they are generally not considered part of the grouping, because their formation is so different, and the vortex itself does not usually extend from the Earth’s surface to a cloud base.

Tornadoes can occur at any time in the year if the conditions are favourable but are far more common during the stormy season. For most mid-latitude countries this is often spring or autumn, because tropical and polar frontal systems meet in these transitional seasons, giving rise to the stormy weather. Tornadoes occur where the atmospheric and land conditions for tornado-generation are most common, and nowhere are these ingredients more prominent than the Great Plains of the US, which gives rise to ‘Tornado Alley’. Tornado Alley is generally accepted to extend from Texas to South Dakota and contains more violent tornadoes than anywhere else in the world. Canada, for instance, comes in at a distant second with an order of magnitude fewer tornadoes.

While the most intense tornado activity is found on that continent, tornadoes also exist in a number of other places, namely in the mid-latitudes which have favourable conditions for the storm systems. Argentina and Brazil give rise to their own tornado alley around the Pampas Plains, but also parts of Europe, South Africa, Australia, New Zealand, and South and East Asia. Indeed, the country with the highest density of tornadoes for its area is actually the UK, where we see around 30 tornadoes each year.

Science Focus