Rope Trick

Operation Snapper

Taken 1 millisecond after detonation, showing the "Rope Trick"
The spikes seen at the bottom of the detonation are caused by thermal radiation (approx 20,000 degrees Kelvin, 3 1/2 times hotter than the surface of the sun) vapourising cables that held the device in place. This is known as the "Rope Trick"

The mottling effect seen on the surface of the fireball is caused by clumps of vapourised bomb debris, travelling at several tens of kilometres/second, hitting the back of the slower expanding fireball.