Although tornadoes can occur at any time of the year, springtime brings the most favorable tornado conditions, tornadoes are formed in the clouds of thunderstorms. The main conditions required for thunderstorms to form are moisture in the air at the lower to mid levels of the atmosphere. Unstable air that will continue rising from near the ground, and a lifting force, the most common lifting force is heating of air near the ground, as hot air rises.
When all the conditions are present, humid air will rise and cool and condense into clouds, forming thunderstorms. This air rising into a thunderstorm is called an updraft which is where the tornado itself is formed.
The strongest tornadoes are often near the edge of the updraft, not far from where air is descending in a downdraft caused by the thunderstorms with falling rain or hail. This is why a burst of heavy rain or hail often precedes the tornado itself. Tornadoes are common in an area stretching from Texas to Iowa, the area known as Tornado Alley also covers Colorado, Nebraska, Illinois, Indiana, Missouri and Oklahoma.
Tornadoes have at one point or another occurred in all 50 U.S. states and are actually more common in Florida than they are in Oklahoma, although the tornadoes in Florida are generally weaker than those in Tornado Alley. Weak tornadoes generally last 10 minutes or less, and only cover a short distance. Tornadoes to hit Oklahoma are some of the most violent on record. In 1999 a tornado with winds of nearly 320mph struck and ravaged the city and its southern suburbs.
Tornadoes are ranked by the damage they cause using the Fujita Scale. F0 and F1 tornadoes on the scale are considered "weak" and cause minimal to moderate damage with winds from 40-112 mph. F2 and F3 tornadoes are considered strong, with winds of 113-206 mph that can cause major damage. Violent tornadoes are those classified F4 and F5 with winds exceeding 206 mph these tornadoes can leave catastrophic remains in their wake.