Why a mushroom cloud?

Why do nuclear (or nuculer if you prefer) bomb explosions for a mushroom shape cloud?

Any large enough explosion will produce a mushroom cloud – but A-bomb and H-bomb explosions nearly always will owing to the size.

Essentially, the shock wave and blast effect from the explosion expands in a semicircle, the above-ground area around the explosion site, but much of the force is carried upwards. When it hits the tropopause, it begins to expand outward, producing the mushroom effect.

http://en.wikipedia.org/wiki/Mushroom_cloud

The explosion doesn’t even have to be large. On a calm day even a small explosion that takes place in the air produces a mushroom shaped cloud that lasts for a while.

Even better than Wikipedia is Cecil’s column on the subject.