At this point in time, formation of tornadoes are not fully understood by scientists. They usually form in a giant rotating thunderstorm called a supercell. Supercells form when cold polar air meets warm tropical air. The result is a great instability caused by the rising warm air. A squall line, or narrow zone of cumulonimbus clouds forms, giving life to the tornadoes. Lightning flashes, and heavy rains and hail begin to fall. Soon after, the easiest recognizable part of the tornado, the funnel, seems to descend from the base of the cloud. In actuality, it does not, but rather the pressure within the cloud drops due to the increasing wind speeds. This is known as Bernoulli's principle. As the pressure drops, it causes moisture in the air to condense. This action continues down the spiral,