For a tornado to form, a severe thunderstorm must be present. Severe thunderstorms have large hail, damaging winds, and heavy rain with them. Two specific types of severe thunderstorms are associated with tornado formation: super-cell thunderstorms and frontal thunderstorms. Super-cell thunderstorms are ones that form about 100-150 miles ahead of a cold front, while frontal thunderstorms form along the boundary of a cold front.

Tornadoes form by the following process:

1. Warm moist air rises, causing upward vertical motion of air into the thunderstorm.
2. The air diverges, or spreads out, above the storm.
3. Cold air from about 20,000 feet sinks into the thunderstorm, giving the storm downward vertical motion
4. The cold air and the warm air collide in the thunderstorm, and this creates a vortex, or a spinning, in the thunderstorm
5. This spinning leads a sinking cloud to form what is called a wall cloud.
6. The spinning and sinking intensify, allowing the cloud to get closer to the ground. This is known as the funnel cloud.
7. Now the spinning and sinking leads to extreme downward motion. The cloud touches the ground, and when it does it is known as a tornado

The diagram below shows all these steps in motion.