Tornadoes are basically violent and rotating column of air that comes in contact with the surface of earth and a cumulonimbus cloud. They come in the form of a visible condensation funnel. The narrow end touches the earth and the broader end is upwards. It is encircled by a cloud of debris.
Tornadoes can have a speed of 40 mph (64 km/h) to 110 mph (177 km/h). It is not exactly clear to scientists How tornadoes are formed. However, the are formed because of a giant rotating thunderstorm which is called supercell. This happens when the cold polar air meets the warm tropical air. This makes way for great instability as warm air arises.
Because of this, a narrow zone of cumulonimbus cloud is formed which in turn forms a tornado.