A mushroom cloud is the ominous formation often seen in the sky following a nuclear explosion or volcanic eruption. This type of cloud derives its name from its resemblance to an actual mushroom, complete with a bulbous top and a centralized stem. Following the dropping of atomic bombs on the Japanese cities of Nagasaki and Hiroshima, the image of a mushroom cloud has become a grim icon for the destructive power of nuclear weapons.
When an atomic bomb is dropped over its target, it does not hit the ground and explode. This would force much of the bomb's destructive force into the earth, not through the buildings above it. Instead, an atomic bomb is detonated several thousand feet in the air. The force of this blast instantly pulverizes anything in the blast area into dust and debris. Meanwhile, a ball of super hot gases forms instantly in the sky, which destabilizes the surrounding air and creates powerful currents.
All of this happens within milliseconds of the explosion itself. The superheated ball of gas forms the top of the mushroom cloud as it rises in the atmosphere. Meanwhile, dust and debris are drawn upwards as a column of air rushes in to fill the gap created by the rising fireball. This is essentially the "stalk" of the cloud. There is little to no actual water vapor in the cloud; it is almost entirely composed of dust and debris. Eventually, however, water vapor may combine with the remnants of a mushroom cloud to form a deadly form of radioactive rain called nuclear fallout.
As the head of the mushroom rises in the atmosphere, it eventually reaches a point where the air above is more stable and less penetrable. This causes the fireball to spread out in all directions, forming the very distinctive shape of a mushroom cloud. Convection currents created by the fireball as it rose in the air also force the outer edges of the cloud's cap to curl under and be drawn back into the central stalk.
A mushroom cloud generally forms only when a significant amount of explosive energy and heat are released. A few large conventional bombs can create small mushroom clouds under the right conditions, but generally the only events capable of producing a visible mushroom cloud are above-ground nuclear explosions and major volcanic eruptions. Fortunately, most of us should be able to avoid experiencing either one of those options during our own lifetimes.