Tornadoes are usually a by-product of strong thunderstorms, but not all thunderstorms create tornadoes.
For a tornado to form the atmosphere has to be out of balance. What does that mean? When winds from the upper atmosphere are going faster than the winds on the surface of the Earth horizontal spinning clouds are created. Thunderstorms will turn these columns vertically in their updraft. As up and downward motion continues within a thunderstorm the rotating column of air will drop below the cloud base and eventually create a tornado on the surface.
In pictures courtesy of the BBC website:
As the ground temperature increases, moist air heats and starts to rise.
When the warm, moist air meets cold dry air, it explodes upwards, puncturing the layer above. A thunder cloud may begin to build.
A storm quickly develops - there may be rain, thunder and lightning.
Upward movement of air can become very rapid. Winds from different directions cause it to rotate.
A visible cone or funnel drops out of the cloud towards the ground.
The vortex of winds varies in size and shape, and can be hundreds of metres wide. A tornado can last from several seconds to more than an hour and may travel dozens of miles.
A tornado is a wind funnel that is connected to the Earth’s surface and the cloud above.