How are tornadoes formed?

Tornadoes are rotating columns of air that is in contact with the ground. The catalyst that lead to tornadoes are temperature and humidity instability. Warm moist air near the ground, with cooler dry air at higher altitudes combined with wind shear, a change in wind speed and/or direction with height. This unstable airmass promotes the development of strong updrafts, while wind shear will further increase the strength of the updraft, and promotes the  rotation from which tornadoes are produced. Tornado intensities are measured using the Enhanced Fujita (EF) scale