Tornadoes usually form during a super cell thunderstorm. During these storms, the warm and moist air near the base of the storm begins to rise quickly, this causes a major change in wind. While this is going on, near the top of the storm, cold dry air is dropping down and creating a wind current. When these two forces meet, the two wind currents mix and a funnel cloud is formed. As the hot air rises, the currents shift from being horizontal, to vertical, forming the beginnings of a tornado. Eventually, these currents touch the ground and a tornado is formed.