From: black.hole.4.spam{at}gmail.com (Don Hills)

In article , Tony Williams
 wrote:
>
>The pictures suggest that you're right. The one at the top of the page I
>linked to seems to show several fronts moving out in different
>directions from a common centre.

From nuclearweaponsarchive.org:

Cause of the surface mottling.
At this point in the explosion, a true hydrodynamic shock front has just
formed. Prior to this moment the growth of the fireball was due to
radiative transport, i.e. thermal x-rays outran the expanding bomb debris.
Now however the fireball expansion is caused by the shock front driven by
hydrodynamic pressure (as in a conventional explosion, only far more
intense). The glowing surface of the fireball is due to shock compression
heating of the air. This means that the fireball is now growing far more
slowly than before. The bomb (and shot cab) vapors were initially
accelerated to very high velocities (several tens of kilometers/sec) and
clumps of this material are now splashing against the back of the shock
front in an irregular pattern (due to initial variations in mass
distribution around the bomb core), creating the curious mottled
appearance.

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Don Hills    (dmhills at attglobaldotnet)     Wellington, New Zealand
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