A tornado usually forms over land during a severe thunderstorm.
They begin to form when an updraft, a current of warm, moist air rising through a thunderstorm, meets a mass of cold dry air.
The rising warm air causes the cold air to be dragged down which creates instability in the atmosphere.
As the updraft rises higher it begins to rotate causing the winds to spiral around it and increase in speed. This create a horizontal spinning effect in the lower atmosphere.
Then the rising air within the updraft tilts the rotating air from horizontal to vertical. An area of rotation, 2-6 miles wide, now extends through much of the storm.
Most strong and violent tornadoes form within this area of strong rotation.