Why does anime often feature giant, perfectly spherical sci-fi explosions??  Eg, consider this explosion from the movie "Akira", pretty typical of the genre:

These seem inspired by nuclear weapons, often they are literally the result of nuclear weapons according to the plot (although in many cases they are some kind of magical / etc energy).  But obviously nuclear weapons cause mushroom clouds, right??  If no real explosion looks like this, where did the artistic convention come from?

What's going on?  Surely they are not thinking of the spherical fireball that only occurs for a handful of milliseconds after a nuclear detonation??  Is it just that spheres are easy to draw??

The answer seems to be that large explosions in humid air (like, say, Japan) cause exactly this sort of rapidly-expanding spherical pulse, as shockwaves cause a pulse of condensation.  Check out these videos of the (non-nuclear) 2020 Beirut explosion; it's a dead ringer for the matching scene in Akira.  Here is an especially far-away view of the Beirut explosion where you can see how the shockwave interacts with nearby clouds in an interesting way.

The USA's cultural image of nuclear explosions comes mostly from early nuclear tests conducted in sunny Nevada, which only generates dry mushroom clouds.  (Hence also, perhaps, our cultural associations of a post-nuclear-war world as a dry, desiccated wasteland like those of Fallout or Mad Max?)  In Japan, I imagine the more salient reference points would have been 1. eyewitness descriptions of the bombings of Hiroshima and Nagasaki, which happened on partly cloudy & overcast days and presumably looked more like the Beirut video (but bigger), and 2. the later tests of hydrogen bombs at Bikini Atoll, which became somewhat of an international incident regarding the deaths of some Japanese fishermen, and whose explosions similarly had an "expanding white sphere" look to them.

(For a particularly HD, live-action hollywood representation of the spherical-anime-explosion motif, see this scene from Pacific Rim, which interprets the anime motif through a kind of confused mix of vacuum cavitation in water, the back-and-forth wind effects seen in lots of nuclear test footage, and the first-few-milliseconds spherical fireball of a nuclear explosion.)

Thank you for your attention to this matter.