Doppler radar observations have shown that the rotation begins in the middle of the thunderstorms and gradually works downward. As air rushes into toward the low pressure of the mesocyclone, the rotational wind speeds increase and vertical stretching occurs due to angular momentum principles. Eventually, the tornado cyclone forms when the spinning vortex inside the mesocyclone appears near the midlevel of the cloud and gradually extends downward to the cloud base. As the air beneath the funnel is drawn into the core, it cools rapidly and condenses, and the funnel cloud descends toward to the surface. Tornadoes may also begin as debris clouds on the ground in the absence of a condensation funnel.
Some tornadoes actually begin at the ground and build upward.