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The idea of what a nuclear explosion looks like is likely cemented in your memory. Either from watching documentaries and movies depicting an explosion, or from seeing the images in pop culture, an explosion of the nature and size of a nuclear detonation isn't something that is easily forgotten.
Perhaps the most noticable aspect of these explosions is the trademark large mushroom clouds that they create. Most bombs create similar clouds, but not quite like those seen following a nuclear detonation.
So, what causes these clouds to form?
In short, it's because the bomb suddenly releases large amounts of energy. This energy creates a very hot bubble of gas, which interacts with the cooler surrounding air, making it less dense.In the case of a nuclear detonation, the bomb emits a blast of x-rays, which ionize and heat the surrounding air; that hot bubble of gas is known as a fireball.
The hot fireball rises very rapidly, creating a powerful updraft that is then filled by the surrounding air and dust. It's this that creates the cloud.
However, that was the quick answer, to understand it further, we need to dive a little deeper.
What are mushroom clouds?
In order to understand why nuclear explosions create mushroom clouds, we first need to define just what these clouds are.
Mushroom clouds are clouds of smoke and debris that move through the air following an explosion. These types of clouds are formed not only after nuclear explosions, but also after any event that creates heat very rapidly. An example of this might be the eruption of a conventional bomb or even a volcano.
Why do nuclear detonations cause large mushroom clouds?
The answer might seem simple at this point considering we've basically already addressed it in this article, but there's more to the story.
Of course, the powerful nuclear explosion causes a sudden release of heat, reacting with the surrounding air, making that air less dense – as we've discussed.
The interaction between two materials (fluids or gasses) of different densities when they are forced together is known as Rayleigh-Taylor instability.
This principle primarily characterizes the movement of two fluids with different densities. Fluids with different densities are affected by any given force in different ways, as a matter of their varying properties. Explained simply, RT instability occurs when a heavy fluid is supported by a lighter one. The fluids will trend towards equilibrium, causing the less dense fluid to shoot through the more dense fluid.
In the case of explosions where the less dense hot air is centralized, this "shooting through" of the less dense hot air through the more dense colder surrounding air, occurs at a centralized point. The interactions of these gases causes the mushroom shape to form.
One thing to note is that this interaction is present in all fluids where a less dense fluid supports a heavier one, say, for example, the interaction of oil and water in a cup. In the case of nuclear explosions, the interaction would persist without the presence of smoke or debris. The smoke and debris are simply what allows us to more easily observe the mushroom cloud formation.
The less-dense hot air will rise from the initial fireball and create a vacuum in its wake. This causes the denser cold air to get sucked in as the fireball continues to rise.
The rising hot air meets resistance from the denser cold air, which acts as resistance to its upward motion. It's this resistance that flattens the rising cloud, transforming it into a mushroom shape.
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The edges of the cloud seem to be curling constantly. This is due to the fluid movement, as a result of this resistance. The air on the surface of the fireball is slowly pulled back, only to roll around and get sucked back into the bottom of the fireball again.
This entire process continues until equilibrium is reached. The fireball will only stop rising until it reaches a point where the surrounding air is the same density. In the case of nuclear explosions, this is rather high in the atmosphere, normally in the ozone layer.
According to an article in Scientific American, "All atomic bombs produce a bulge and a stem, but the really huge, mushroom clouds are produced by the very high-yield explosions of thermonuclear weapons (hydrogen bombs). The fireball from an H-bomb rises so high that it hits the tropopause, the boundary between the troposphere and the stratosphere. There is a strong temperature gradient at the tropopause, which prevents the two layers of the atmosphere from mixing much. The hot bubble of the fireball initially expands and rises. By the time the bubble has risen from sea level to the tropopause, it is no longer hot enough to break through the boundary. ... At that point, the fireball flattens out; it can no longer expand upward, so it expands to the side into an exaggerated mushroom cap."
How big are nuclear mushroom clouds?
We can all now visualize what a nuclear explosion looks like, but what is more difficult is to understand the scale of the explosion. Since it is unlikely that we have ever seen a nuclear explosion in-person, the scale can be difficult to grasp.
In general, the mushroom clouds can rise up to tens of thousands of feet in minutes. For reference, most passenger planes cruise at around 33,000 feet, or 10,000 meters.
Looking back at a historical explosion, let's take a look at what happened after the nuclear explosion in Hiroshima in 1945. Within the first 10 minutes, the mushroom cloud rose to more than 60,000 feet, or roughly 20,000 meters.
That doesn't give us the whole picture though. While it was more than 20,000 meters high in the first 10 minutes, within the first 30 seconds the cloud had risen over the cruising altitude of the Enola Gay, the plane that dropped the bomb. That means that the cloud had risen 10,000 meters in 30 seconds. Averaged out, this means the cloud expanded upwards at 333 m/s initially, and then slowed to rise at just a 100 m/s average after 10 minutes.
In the end, mushroom clouds aren't specific to nuclear explosions, rather they're specific to Rayleigh-Taylor Instabilities in fluids – a principle we see in action around us every day.