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If you’ve ever put your feet up on a warm summer’s day and played “spot the shape in the cloud,” you might’ve seen a kangaroo, a guy preparing to facepalm, maybe a mushroom... On second thought, hopefully not that last one.

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Mushroom clouds forming over volcanoes and forest fires:

Pyrocumulus above forest fires

Brief overview of Rayleigh-Taylor instabilities

Rayleigh-Taylor instabilities are important in supernovae:

Video of Rayleigh-Taylor instability with water of different temperatures

Video of Rayleigh-Tayor instability with salt and pure water.

General (not primary) resource – University paper

If you've ever put your feet up on a warm summer's day and played "What's that cloud look like?," you might have seen a kangaroo, a guy preparing to face palm, maybe a mushroom. On second thought, hopefully not that last one.

A mushroom cloud is not a thing you want to be seeing because they are usually caused by huge explosions, like from atomic blasts. But there's a reason they look like fungi. The hotter air in the fireball from a strong blast gets pushed away by the cooler, denser air in the atmosphere.

But the source doesn't have to be nuclear; it just has to be really powerful. Regular, non-nuclear explosions and even really violent volcanoes and forest fires can cause mushroom clouds. The insanely high temperatures and pressure caused by a nuclear detonation just help things along a bit.

It's called the Rayleigh-Taylor Instability. Mushroom shapes tend to form whenever two layers of different densities interact, like the hot air from an explosion and the cooler air in the atmosphere. It all starts with an explosion: a sudden release of stored energy that heats up and expands the surrounding air. The gas ignites, creating a giant fireball that can hit temperatures similar to what you'd find in the center of the sun, millions of degrees Celsius. Hot air rises and fireballs rise fast. A fireball from a one megaton blast, which is like 50 Hiroshima bombs going off at once, can rise to over 7 kilometers high in just one minute. 

It starts off the way you'd expect, like a sphere, but then forces transform it into that iconic mushroom shape. The cap of a mushroom cloud is actually shaped like a horizontal donut of rotating winds, a huge smoke ring constantly turning itself inside out. It forms when air resistance from the surrounding atmosphere pushes the flat stuff at the top of the fireball sideways, flattening it slightly. The displaced material trickles down the sides of the fireball as it cools, only to get sucked back into the hot center by the rising air currents in the stalk. That's what makes the mushroom cap look like it's rolling down, even as the cloud itself moves up. Sometimes misty halos will surround the mushroom cloud but they don't have anything to do with the smoke and debris. Instead, they are made of the same stuff as regular ol' clouds: condensed water. They've formed from the low pressure wave that follows the blast, which cools the air. If it's humid enough, the now-chilled water vapor in the air condenses into rings known as Wilson clouds.

But, no matter how big they get, mushroom clouds don't last forever. As it rises, the cloud eventually gets to a height where it's the same density as the surrounding air. Then it tends to spread outward rather than straight up. Soon the cloud dissipates completely, although, if it's from a nuclear blast, the fallout night last a while longer.

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