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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