why do nuclear explosions create mushroom clouds
This phenomenon all comes down to a little something called the Rayleigh-Taylor instability, and by extension, convection. IБll 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 convectionБs 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 donБt 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. б For instance, the magnetic fields of planets, the jet-stream of winds that help control our planetБs 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. б WhatБs 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). б 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 БWilson CloudБ, named after Scottish physicist Charles Wilson who invented the Wilson Cloud Chamber where similar sorts of things can be observed. If you liked this article, you might also enjoy our new popular podcast, The BrainFood Show (,
), as well as: б 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 RayleighБs Бinterfacial instabilityБ occurs for other differing substance accelerations as well. The phenomenon, and all the equations that describe it, became known as Rayleigh-Taylors. Helder's answer is good, but I feel it doesn't quite answer all your questions explicitly. What causes the mushroom shape? This is the part Helder answered well, no need reason to repeat it. Do conventional explosives like TNT and grenades also make mini-mushrooms? It doesn't matter what causes the explosion, the only thing that matters is how strong it is - mostly, how much energy is released. Conventional explosives are perfectly capable of making a mushroom cloud, although I'd say that it's impossible to pack enough explosive in a grenade. Something like nuclear or anti-matter grenades might be capable of producing a mushroom, but note that it would be wildly impractical - there's no way you'd be able of throwing the grenade far enough to be able to use it safely.
Is it a physical attribute of the explosion, or something specific to nuclear energy? Volcanoes are an example of a natural explosion that can cause a mushroom cloud. And it doesn't even need an explosion in fact - the only thing the explosion does is supply the energy to heat enough of the air fast enough to ensure the resulting air movement is going to behave just so. The same effect occurs when e. g. water is suspended above oil (oil having lower density) and the oil starts to move upwards; ink can also commonly be used, as well as heating coloured water. It's also suggested that mantle plumes behave in the same manner - the less dense rock from deep in the mantle (perhaps even from the core-mantle boundary) is gravitationally unstable (another way to say "lower density than everything around" :D), and when it overcomes whatever is binding it down, it starts rapidly (well, in geological terms; it's still rather slow on human scale) accelerating upwards, forming a distinctive mushroom shape. All of those aren't exactly the same - for example, some depend on continuous heating (think a lava lamp), but they're still relevant.
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