Mushroom clouds

I suspect that if you're reading this blog right now, you've seen the fear- and awe-inspiring pictures of mushroom clouds over nuclear tests. What on earth causes those? Part of it is a result of all the dust and debris that was surely churned up by such an explosion. The ground underneath the explosion is partially vaporized, but the rising plume of hot air pulls in nearby air (and dust and debris) as it heads upwards. (As an aside, some fireballs can rise as fast as 300 miles per hour!) At a certain height, the air stops uniformly rising and starts falling, creating convection plumes that form the donut-shaped top of the mushroom. Note that all this happens a while after the initial detonation, after the fireball has done some expanding and shock-wave generation of its own.

Another interesting issue is the nice circular rings that sometimes form around the stem of the mushroom cloud. These are actually partial condensation clouds, which are formed in the negative phase of the shock wave, behind the obvious high-pressure shock front. Previously, I've discussed complete condensation clouds, which appear as continuous surfaces behind, for instance, an airplane in supersonic flight. The basic idea here is the same: as the front of lower-pressure air expands outward, the air inside it expands and therefore cools, so if there's enough humidity, the water in the air condenses into a cloud. The difference is that in supersonic flight, the shock wave is localized enough that the whole lower-pressure region condenses. In atmospheric nuclear weapon detonations, though, the shock wave is large enough that the structure of the atmosphere comes into play. The rings you see, then, indicate high-humidity layers in the atmosphere, which condense in rings as the negative phase of the shock wave intersects them.

Thank you to Jonathon Vigh's thesis for much of the information contained here.