Why Nuclear Bombs Create Mushroom Clouds
The mushroom cloud created by a nuclear blast
is incredibly striking, but have you ever
wondered why the cloud forms this way?
Well, wonder no more.
This phenomenon all comes down to a little
something called the Rayleigh-Taylor instability,
and by extension, convection.
I’ll begin with the somewhat longer, but
less geeky explanation before descending once
again into extreme nerdery.
It all starts with an explosion that creates
a Pyrocumulus Cloud.
This ball of burning hot gases is accelerated
outwardly in all directions.
Since the burning ball of accelerated gases
is hotter, and therefore less dense, than
the surrounding air, it will begin to rise-
in the case of nuclear explosions, extremely
rapidly.
This ultimately forms the mushroom cap.
As the ball rises, it will leave behind air
that is heated, creating a chimney-like effect
that draws in any smoke and gases on the outer
edge of the chimney- convection in action!
Visually, this forms the stipe (stalk) of
the mushroom.
The perception that the mushroom cap is curling
down and around the stipe is primarily a result
of the differences in temperature at the center
of the cap and its outside.
The center is hotter and therefore will rise
faster, leaving the slower outer edges to
be caught up in the stipe convection’s awesome
attributes.
Once that cloud reaches a certain point in
our atmosphere, where the density of the gas
cloud is the same as the density of the surrounding
air, it will spread out, creating a nice cap.
This brings me to the shorter, yet more geeky
answer.
This entire process is something that describes
the Rayleigh-Taylor instability.
This instability is well known in physics
and, in general, describes the merging between
two different substances (mainly liquids and
gases) that have different densities and are
subjected to acceleration.
In the case of an atomic bomb, the acceleration,
and the hotter gases creating the differing
densities of material, are caused by the explosion.
From this, you might have guessed you don’t
necessarily need an atomic bomb to create
a mushroom cloud.
All you need is enough energy delivered rapidly
(in this case an explosion) that creates a
pocket of differing densities of material
(in this case, heated gases).
There are numerous other examples in our world
that create, and are described by, the same
phenomenon that gives us this formation.
For instance, the magnetic fields of planets,
the jet-stream of winds that help control
our planet’s climate, the sound of snapping
shrimp, even our understanding of certain
different forms of fusion can all be attributed
to Rayleigh-Taylor instability.
Now, you might have also noticed that nuclear
explosions, besides producing this frightening
fungal formation, also sometimes result in
a cloud ring around the mushroom cap.
What’s going on here is that a low pressure
area is created via the negative phase of
the shockwave (the phase that follows the
wave of compressed gases at the leading part
of the shock wave).
This results in a drop in temperature, which
along with the low pressure can potentially
lower the dew point sufficiently for a temporary
cloud to form.
This cloud halo around the explosion is known
as a “Wilson Cloud”, named after Scottish
physicist Charles Wilson who invented the
Wilson Cloud Chamber where similar sorts of
things can be observed.
Bonus Fact:
• What has been commonly referred to as
the Rayleigh-Taylor instability was first
brought to light by Lord Rayleigh in 1880.
He was attempting to describe the motion of
liquids when one of higher specific gravity
was supported by one that was lighter.
Specifically, trying to better understand
how cirrus clouds were formed.
In 1950, Sir Geoffrey Ingram Taylor discovered
that Rayleigh’s “interfacial instability”
occurs for other differing substance accelerations
as well.
The phenomenon, and all the equations that
describe it, became known as Rayleigh-Taylors.
