The short video above is of LLNL atmospheric test shot Turk in Operation Teapot in 1955 at the Nevada Test Site. The test was of the primary for the XW-27D thermonuclear weapon, giving a 43 kt yield.

What struck me about the footage was that it captured detail of the expanding fireball as it contacted the ground. Turk was a tower shot with the explosive sitting at 150 m above the ground. As the roughly spherical fireball expanded, jets of roiling material protrude radially from the growing incandescent ball. In particular a conical extension of incandescent material protrudes from and is overtaken by the expanding fireball.

A commenter below offered what seems to me a quite plausible explanation of the conical “jet” observed in the footage. It is a cable from the tower caught in the act of cooking off into plasma from the much faster radiant energy.

Addendum 6/28/17. Thanks to a link supplied by commenter K, we find there is a name for this phenomenon- it is called the “rope trick”. See this link for more information.