Following on from this question, here is a high-speed photograph of a nuclear explosion, taken about 1 ms after detonation: (source) As anna v pointed out, several similar images can be seen in a Google image search for "high speed nuclear explosion photos".
The spikes at the bottom are know as "rope tricks" - they're caused by cables being heated by the radiation. But what causes that weird-looking structure in the fireball itself?
My suspicion is that the explosion at this stage is an expanding ball of air that's being ionised by the radiation coming from the nuclear chain reaction. If some regions of the surrounding air are ionised more easily due to differences in their temperature (or density or moisture content) prior to the explosion then perhaps this would explain the structure. The explosion is probably taking place in a desert, meaning that there could be quite a bit of turbulence in the ambient air due to thermal convection. However, this is just a wild guess, and I'd be interested to know if anyone has any knowledge or insight about this structure.