The mushroom cloud has long been the symbol of what it meant to set off a nuclear weapon. But, as Spencer Weart noted in his Nuclear Fear (I haven't checked to see if it is in the new edition, The Rise of Nuclear Fear, yet), it wasn't always so, nor necessarily so. There are other words in English you could describe such a cloud, though "mushroom" has nice symbolic connotations. But more to the point I want to make here, you don't necessarily have to focus on the cloud in its most mushroom-y stage. There are lots of ways to photograph a nuclear explosion, and the ideal mushroom cloud shape represents just one particular point in time in the evolution of the nuclear effects.
Case in point, the "standard" image of the "Trinity" shot (July 16, 1945), is not of a mushroom cloud, but as a blobby fireball, milliseconds after detonation:1
It's kind of curious, isn't it, that these sorts of images are the most common? I haven't traced out the whole story here, but the obvious reason for this is that these are the images that were first distributed about the test in the wake of the bombings of Hiroshima and Nagasaki. (There's one of the "fireball" photos inside the Princeton University Press edition of the Smyth Report, for example.)
Why the fireball and not the cloud? One might be tempted to say it has something to do with wanting to underplay the fallout, but of course they did distribute photos of the Hiroshima and Nagasaki mushroom clouds. I suspect that the reason isn't terribly deep — that, perhaps, whomever chose the photos was more impressed by the massive fireball with its uncanny spherical shape than they were by a mere mushroom cloud, which, without a sense of scale, is indistinguishable from many other large explosions. But this is just speculation. I've never found much documentation on the release of the "Trinity" photos — just a memo from a year later (the summer of 1946) which said, interestingly, that the decision to declassify all of the tiny-time scale "Trinity" photos (like the ones linked above) was made after consultation with none other than Klaus Fuchs.
The other common photo of "Trinity" is the Jack Aeby photo, perhaps most famous these days as being the cover of Richard Rhodes' Making of the Atomic Bomb. It is later than the millisecond photos, obviously, but is still very early in the cloud formation.
Below are some alternative views of "Trinity," from Los Alamos National Laboratory's archives. They show considerably different stages of the cloud development, and are pretty interesting images for that reason:
I find all of these ones to be much more ominous than either the fireball or the Aeby photos — they are dark, they are smokey, they are not friendly. The last one in particular looks like something ancient and terrible. Which arguably is as correct an interpretation of the "Trinity" test as the more common narrative about inspiring achievement.
- All of these images come from Los Alamos National Laboratory; their file names reflect their index numbers within the LANL system. [↩]