How do tornadoes form?

I'm typing this while my mom makes a failing attempt to convince my brother, dad and myself that we're going to get killed by a giant tornado unless we all pile into the basement with her and our dog. I've lived on Long Island my whole life and we've never had a tornado warning before this that I can remember. I am personally not too concerned which is why I'm not in the basement with my mom. Anyway all this got me thinking, how do they form?

A tornado is a column of air that is rotating very quickly, this column extends from a cloud (usually a thunderstorm cloud) to the ground. Most tornados have winds under 100 mph but some can be over 250 mph.

It takes several conditions to form not only a tornado but the thunderstorm clouds they form from. A lot of low level moisture is required and a catalyst that pushes that moisture up into the air, usually this catalyst is something like cold winds. This moisture rising and getting saturated as it rises results in a very tall distinctly shaped cloud when the atmosphere is unstable.

In order for the tornado to form there needs to be a strong, low, wind that rotates horizontally to the ground. The rising air in the thunderstorm cloud will pick up the horizontal wind, rotating it from horizontal to vertical creating the column of air from the cloud to the ground known as a tornado.

Unstable Atmosphere - An unstable atmosphere is one where the temperature decreases rapidly with hight.