Hot air rises.
Air that’s hotter than other hot air will rise faster.
As the mushroom cloud rises, it cools down, meaning the top is moving slower than the air below it. As the faster hotter air pushes past the cooler air, it causes it to swirl, creating that iconic mushroom shape. [Any explosion that is large enough will do this](https://youtu.be/Riauw5UTnW8?si=sduqUMqk-nEoHxIW).
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