A tornado is a column of air that turns around at extremely high speed, usually with a funnel of cloud sucked down inside it from above.

A tornado usually happens due to a combination of several weather conditions. First of all, there would normally be a very violent thunderstorm. Then there must be unstable conditions, which in this case means very humid, warm air in the atmosphere nearer the ground, and very often cooler air higher up. The unstable air currents cause updrafts and downdrafts (vertical movements of the air). For a tornado to happen, these movements should then come into contact with wind shear - the wind getting faster, and changing direction, as it gains height. All these events together cause the rapid air rotation that makes a tornado.

There are other ways for a tornado to form - for example, they can sometimes form early in a storm rather than later, and occasionally they appear without the cloud funnel.