ANSWER I: When a nuclear weapon explodes, there is a rapid release of a large amount of energy within a small volume. This results in significant increase in temperature and pressure. The temperature may be a few tens of million degrees and pressure a few million times the atmospheric pressure. At this temperature, all the material present in the weapon will be converted into hot compressed gases.
Within a fraction of a millionth of a second of the explosion, the weapon's residues emit large amounts of energy mainly in the form of X-rays. The surrounding atmosphere absorbs this energy. This results in the formation of a blazing, highly luminous, spherical mass of air and gaseous weapon residues called the fireball.
Within an extremely short time after the explosion, the fire ball from a high yield nuclear weapon will be about 130 metres across increasing to about 1700 metres in ten seconds.
The fireball expands rapidly engulfing the surrounding air. The ball of hot air is less dense than the surrounding air. It rises swiftly like a hot air balloon.
This rising column pulls up debris of the weapon, dust and moisture along with it forming a cloud. As it moves up, it cools gradually and reaches about 10 km where the atmosphere is extremely stable.
The ball of air mass moving up does not have enough energy to penetrate this stable layer. It flattens out. As the relatively warmer layers at the bottom push up, the top layers spread laterally and equally in all directions, and the cooler denser layers descend at the edges, giving a distinct mushroom shape.
ANSWER II: Atmospheric nuclear explosion leads to sudden formation of a massive fireball near the ground, setting aflame whatever is in its vicinity. Since the fireball is very hot and thus less dense than the surrounding air, it rises very quickly. The massive updraft due to the rapidly rising fireball leaves a column of low-pressure. This acts as a chimney, sucking in smoke, dust and debris from the surroundings. This forms the stem of the mushroom.
At first the mixture of hot air and dust rises vertically, forming the column of the cloud. But as the hot cloud meets the colder air at higher altitudes, it slowly cools. Eventually the cloud reaches the temperature of the surrounding air and ceases to rise, but spreads horizontally along air levels at the same altitude, which are at the same temperature. This forms the cap of the mushroom.
The smoke, dust and debris gushing into the central column cause toroidal eddy currents in the horizontally spreading hot cloud. This introduces curling under the cap of the mushroom. Mushroom clouds are most commonly associated with nuclear weapons. However, any massive explosion capable of creating the same conditions would produce a mushroom cloud. Volcanic eruptions are typical natural mushroom clouds.
Courtesy : The Hindu