http://www.vce.com/grable.html
In many pictures and films of atomic tests there are a series of vertical smoke trails to one side of the blast. These are identified in one of the frames on the page you posted. I assume that these trails are not caused by the blast, but serve some sort of testing function, perhaps to measure some parameters of the shock wave. Anyone know for sure?
In the test and images that you talked about, the military fired rockets into the air to provide smoke trails.
After the nuclear explosion, they were able to measure the displacement of the smoke trails in an effort to understand the effects of the weapon as a function of distance and altitude.