Why do nuclear bombs form mushroom clouds?

When a bomb goes off, energy is shot out indiscriminately in all directions. So, instead of an expanding ball of fire, why do nuclear explosions result in mushroom clouds?

Although the outburst of energy does initially form a sphere of hot air, that's only the beginning of the story, according to Katie Lundquist, a researcher of computational engineering at Lawrence Livermore National Laboratory in California. Because hot air rises, the larger bulk of the sphere in the middle column — where the core of an apple would be — experiences more buoyancy than the edges do.

"The way that a sphere is shaped, you have the largest column of the low-density fluid in the middle, so that rises the fastest," like the middle of a cupcake rising in the oven, Lundquist said. (Although in vernacular English we tend to use "fluid" and "liquid" interchangeably, for scientists, the term "fluid" can refer to either a liquid or a gas; both lack a fixed shape, are able to flow and can be described by the same suite of mathematical equations, according to Lundquist.)

Although the entire sphere rises, because this middle column elevates with greater urgency, the cooler air outside the sphere begins to "rush in below the bubble that's rising," Lundquist told Live Science.

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