The mushroom cloud that forms after an atomic blast is almost an iconic. People use the terms "mushroom cloud" and "atomic bomb" interchangeably. But why does it form? And does it only go up after a nuclear blast? Let's find out.
You don't need a nuclear weapon to create a mushroom cloud, but it helps. First, you need a sudden release of a great deal of heat. This heat gets the air around the explosion to expand and become less dense. The air rises fast — creating a vacuum around it, that sucks more air toward the source of heat, and warms it until it also expands and rises. This phenomenon is the bane of firefighters, as it keeps pulling oxygen into a blaze that might otherwise put itself out. In most fires and small explosions, it just leads to a rising column of smoke and hot air.
Occasionally, though, if the column of air is large enough or comes from a very specific source, the air in the center of the column is much hotter than the air on the edges. It rises much faster than the rest, causing the edges to appear to curl down to form the edges of a mushroom. Sometimes, the air will either be blown or move inward toward the column, be reheated, and rise again making a little central curl. Since an atomic blast is so huge, it's much more likely to form the mushroom shape than smaller explosions.
Finally, and most dramatically, an atomic blast goes significantly higher than any other explosion. The column of hot air that it makes will rise until it hits the point in the atmosphere where the surrounding air stops being cooler and starts being much, much hotter. At a certain level of the atmosphere, ozone starts making its way into the mix, and the solar radiation that ozone absorbs heats it up. The rising air from the explosions is suddenly not hotter or less dense than its surroundings and so instead of rising, spreads out, making a clear cap for the mushroom.
Pronounced mushroom clouds often are seen in smaller explosions, but conditions have to mimic that of an atomic bomb. There needs to be a very hot column of air, with central heating, a cool atmosphere at the site of the explosion, and a reason for the rising hot air to level off very quickly.
Image: US Department of Energy