While it is somewhat due to the makeup of the bomb and the environment at the time, fusion physicists are discovering that such instabilities in the explosion are extremely difficult to get rid of. Even in the NIF, which is a controlled environment with no air and a perfectly symmetric fusion target, they'se seeing images that look creepily similar to this one. Unfortunately, that means that creating an efficient fusion reactor is...more challenging that expected.

The mottling is apparently from the bomb casing.

(http://www.damninteresting.com/rapatronic-nuclear-photograph...)

> The resulting extraordinary photographs revealed intricate details of the first instant of an atomic explosion, including a few surprises such as irregular “mottling” caused primarily by variations in the density of the bomb’s casing. It also showed the detail of the “rope trick effect,” where the rapid vaporization of support cables caused curious lines to emanate from the bottom of an explosion.

The camera is interesting - using an electronic not mechanical shutter. This was around 1944. (Bell Lab's transistor was around 1947.)

More interesting still -- the mottling is from the remains of the casing and the shot cab (the little building the bomb was housed in at the top of the tower) splashing against the shock front at the fireball's boundary.

(http://nuclearweaponarchive.org/Usa/Tests/Tumblers.html)

This is what impresses me so much about this photo. I think of nuclear bomb fissile materials as being one of the most precise things ever humans have ever made, yet the the fireball is so incredibly irregular.

It reminds me of when I first learned that irregularity in the early universe is what enabled material to condense (probably not the best word) and eventually form gas clouds.

Edit: I was wondering if there was some connection between the two, but thanks to DanBC's comment, it seems like the actual cause is a lot less mysterious.

Oh no! I'm just quoting from a linked article. We need a scientist to answer your question.

Implosion-type nuclear bombs (which are nearly all of them) use a mind-bogglingly precise sphere of fissile material. If you put a drop of thin oil on top and came back a few hours later, the oil would coat the sphere evenly.