1.A large thunderstorm occurs in a cumulonimbus cloud
2.A change in wind direction and wind speed at high altitudes causes the air to swirl horizontally
3.Rising air from the ground pushes up on the swirling air and tips it over
4.The funnel of swirling air begins to suck up more warm air from the ground 5.The funnel grows longer and stretches toward the ground
6.When the funnel touches the ground it becomes a tornado
Hope this can help you! >-<