The atomic bombing of Nagasaki on August 9, 1945

A mushroom cloud is a distinctive mushroom-shaped cloud of smoke, flame, or debris resulting from a very large explosion. They are most commonly associated with nuclear explosions, but any sufficiently large blast will produce the same sort of effect. Volcano eruptions and impact events can produce natural mushroom clouds.

Mushroom clouds form as a result of the sudden formation of a large mass of hot low-density gases near the ground creating a Rayleigh-Taylor instability. The mass of gas rises rapidly, resulting in turbulent vortices curling downward around its edges and drawing up a column of additional smoke and debris in the center to form its "stem". The mass of gas eventually reaches an altitude where it is no longer less dense than the surrounding air and disperses, the debris drawn upward from the ground scattering and drifting back down (see fallout).

The largest mushroom clouds to be photographed resulted from the impact of fragments of Comet Shoemaker-Levy 9 on the planet Jupiter, some of which rose hundreds of kilometers above the cloud layers.

The reasons why the cloud has been described as a "mushroom" are not entirely understood. The original descriptions of the first nuclear test explosion at Trinity Site contained many different associations with the cloud which formed after the blast, including a "multi-colored surging cloud," a "giant column," a "chimney-shaped column," a "dome-shaped" column, the "parasol," the "great funnel," and "geyser," the "convoluted brain," and even the "raspberry." The cloud over Hiroshima was described by a Japanese witness as a "pillar of black smoke shaped like a parachute." In 1945, the Operation Crossroads tests were described as having a "cauliflower" cloud, but a reporter present also spoke of "the mushroom, now the common symbol of the atomic age." Mushrooms have traditionally been associated both with life and death, food and poison, making it a more powerful symbolic connection than, say, the "cauliflower" cloud.