In many pics of Atomic explosions, what are those white streaks you often see near the ‘Mushroom Cloud’ that appear to be going up and down from the ground to the sky? (anyone know?)
Ground launched rockets used to help measure the height and speed of rise of the mushroom cloud.
I used to wonder for years about the smoke trails, then read that they were to get a reading on shock waves. There are actually two shock waves of interest to weaponeers figuring out optimum burst height, an air wave and ground wave. These join up and interesting things happen. The smoke gives them an idea what is going on.
As many have answered here, they are markers to help gauge the yield.
More significantly, back in those days, say early 50s, guys like Feshbach and Morse out of MIT were considered experts in “Acoustics”. Acoustics was the 50’s buzzword for studying atmospheric phenomenon associated with atomic/nuclear blasts. While much could be calculated, the clouds and trails provided hard evidence of how the detonations actually effected the physical domain.
Explosions tend to be many rates of reaction occuring in very short periods of time, with the products of one reaction also effecting subsequent reactions, with many having exponentially rapid rates of reaction.
While theoretical calculations indicated what blast yields might evolve, it wasn’t known if other rates of reaction of targeted areas wouldn’t exponentially further the initial detonation. Nor was it clearly known how the atmosphere at various elevations, temps and pressures might react to the event.
This became more prolific with thermonuclear tests. I had read some at the Bikini tests had posited the detonation could trigger a self sustaining reaction consuming all the planet’s oxygen, though not shown to be the case.