An elementary model of a tornado is presented that is based on the concept of downward vertical velocity everywhere within the tornado. A tornado forms because some air, which is denser than the surrounding air, falls from the cloud to the ground. The funnel shape is then accounted for qualitatively by conservation of mass, and conservation of angular momentum explains the high wind speeds at the narrow end of the funnel. The outward centrigual force on the rotating winds is balanced by an inward pressure gradient. Because of the dynamic effect of the falling air, compared to the static effect of the environment, the pressure inside the funnel is always lower than that outside the funnel, and the pressure difference increases with decreasing altitude. ¿ American Geophysical Union 1989