What is the reason for the appearance of a mushroom cloud in a nuclear explosion? In short, this phenomenon occurs due to the so-called Rayleigh-Taylor instability, or convection - a type of heat transfer in which internal energy is transferred by flows of matter. In order not to go into all the subtleties, let's try to give a simple explanation.
It all starts at the moment of explosion, at which a cloud is formed. It is a ball of incandescent gases, rapidly expanding in all directions. At the same time, the core of these gases has a higher temperature and lower density compared to the surrounding air, which leads to expansion. As a result, a cloud is formed that resembles a mushroom cap.
As the fireball rises, the air behind it heats up. This creates a chimney effect, which forms the "mushroom stem" in the form of a column of dust and smoke sucked in by the incandescent cloud. In short - convection in action.
Thus, the "mushroom cap" and its "leg" are a consequence of the temperature difference inside and outside the "cap". The red-hot kernel grows faster, leaving behind the cooler outer edges, which are then pulled into the inside of the kernel to form this bizarre mushroom shape.