Most tornadoes occur where cold, dry air meets warm, moist air. The moist air is lifted quickly and if the air is blowing in the same direction at all levels of the atmosphere then a tornado is formed. The destructive power of the tornado is in this rotating mass.
A tornado moves forward as a whirling funnel extending down to the ground from a mass of dark clouds. The tornado turns brown or grey because of the dirt and other material sucked up from the ground. A waterspout is a tornado over water.
A tornado can advance at a speed from 40 to 80 km an hour and often passes through an area in a minute or so, running its entire course in less than an hour.
Tornadoes are rare in the United Kingdom but occur frequently in the southern and central areas of the USA.