<human>:  I'd like to understand the physics behind a tornado. Can you explain?
<bot>: A tornado is a rapidly rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is a swirl of air that has very low pressure at its center. The wind speed in a tornado can reach up to 300 miles (480 kilometers) per hour or more. Tornadoes form when warm and cold air combine and create an invisible horizontal spinning effect in the lower atmosphere. This spinning effect, combined with changes in wind speed and direction, can cause an area of low pressure to develop which can result in a tornado.