Nuclear bombs are perhaps, the most terrifying instruments of destruction, capable of turning rich ecosystems into heathen wastelands. Unlike ordinary explosions, they appear as a glaring ball of fire, leaving behind a thick trails of curling smoke in the form mushroom cloud. But what's the reason behind it's shape? Why doesn't it blast off like any other explosive?Science has an interesting answer to this question.
It starts off with blinding flash of light, followed by a trail of red smoky ball of pyrocumulus cloud, typically found above volcanic eruptions. The extreme heat generated immediately after the explosion causes the surrounding air to rise above rapidly, which forms the mushroom cap . The mushroom cap is a combination of a burning ball of flame and thick smoke .
The rising mushroom cap, much like a launched rocket, releases smoke through its tail responsible for the formation of mushroom stalk . The curling effect in the mushroom cap however is a little more interesting as it is governed by a less known principle of mixing fluids. The periphery, being relatively immobile compared to the rapidly accelerating centre , creates a torque, causing the smoke to rotate outwards.
This differential in heat and its consequential movement, known as convection, is nothing special,as it occurs every single day without us being aware of it. When air becomes hot, it rises above due to lowering of its density, whereas it settles down when cooled. The principle of convection is what makes the water cycle possible. It is why air conditioners are installed close to the ceiling and heaters, close to the floor. However, the convection as seen in nuclear bombs is more prominent due to the rapid change in ambient temperature, and unlike volcano, is man-made. That being said, it doesn't take a nuclear bomb for the formation a mushroom cloud.
Source
Rayleigh-Taylor instability is a property that governs the strange mixing behaviour of two accelerating fluids(liquids and gases) of different densities. Any process that can be described by the above, can be used to exhibit a similar scene at a smaller scale, and of course, without the destructive effects.