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Why Do Nuclear Bombs Make Mushroom Clouds?

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 facepalm,

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're 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 some mushroom clouds.

The insanely high temperatures and pressures

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 at the center of the sun:

millions of degrees celsius.

Hot air rises and fireballs rise fast.

A fireball from a 1 megaton blast, which is like

50 Hiroshima bombs going off at once,

can rise over 7 km high in just one minute.

It starts out the way you'd think a fireball would, 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 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're made of the same stuff

as regular old clouds: condensed water.

They form 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 and the surrounding air.

Then, it tends to spread outward rather then straight up.

Soon the cloud dissipates completely, although

if it's from a nuclear blast the fallout might last a while longer.

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