Why do tornadoes form in the shape of a funnel?

Answer 1

I like Jason's answer but I think he missed a few points. The circulation for a tornado may actually begin as a horizontal tube of cyclonic circulation caused by vertical wind shear which when drawn up into the base of a cumulonimbus cloud becomes visible with the debris it picks up from the surface. This funnel then shrinks as it spins up due to a conservation of angular momentum much as a skater does when he draws his arms into his side. The wind shear is caused by warm, moist southerly air near the surface and cold, dry, westerly air aloft.

Answer 2

It's short and straight to the point