[cubicle]

The prompt X-rays from the physics package heat up a lot of air very quickly. This expands, producing the supersonic shockwave front.

Then the fireball, being very hot and much less dense than the air around it, rises. Air is sucked towards ground zero, producing the backwash, and the characteristic mushroom cloud of any very large explosion.

[klodolph]

And the fireball created by the X-rays is, for a moment, hidden behind the shock wave. This makes a "double flash" which can be detected from orbit without even the need for a camera, just point a photodetector at the Earth's surface.

[arethuza]

Bhangmeters are one kind of specialised photo detector used to detect nuclear explosions:

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

I'm pretty sure these were used in bunkers (at least in the UK) as well as satellites.