Why do large explosions form into mushroom clouds?

A mushroom cloud is a distinctive pyrocumulus mushroom-shaped cloud of condensed water vapor or debris resulting from a very large explosion. They are most commonly associated with nuclear explosions, but any sufficiently large blast will produce the same sort of effect. Mushroom clouds are formed by many sorts of large explosions under earth gravity, though they are best known for their appearance after nuclear detonations.

Inside a mushroom cloud: cooler air is drawn into the rising toroidal fireball, which itself cools into the familiar cloud appearance.