Pyrocumulus clouds are a special class of cumulus clouds, the most recognizable type of cloud and the the ones that children draw like piles of cotton balls. Cumulus clouds, which are usually a sign of fair weather, can be identified from their flat bottoms and the rounded, bumpy outlines that look like popcorn or other, more complicated shapes. They appear where rising warm air cools and condenses. Cumulus clouds form across broad areas as warm and cool air masses move across the landscape.
In places where there is a powerful heat source on the ground, that rising hot air can create a very local kind of cumulus cloud called a pyrocumulus cloud. The prefix "pyro" comes from the Greek word for fire, which tells us that pyrocumulus clouds are most likely to be found above a large fire. The heat source may be natural, such as a volcanic eruption or forest fire, or man-made.
A pyrocumulus cloud forms in the same way as an ordinary cumulus cloud: warm air is lighter than cool air, so it will rise into the atmosphere. As the warm air rises, it cools when it comes into contact with colder air high above the surface. When this air cools, the moisture it carries condenses into water droplets, much the same way that water droplets condense on the outside of a glass of ice water. Vast collections of water droplets in the sky form clouds that are visible to us on the ground.
Pyrocumulus clouds form above large forest fires because the extremely hot air rising from a fire contains the moisture that was in plants burned by the fire, plus water from streams and ponds. A similar cloud can form above an erupting volcano because lava brought to the surface from deep underground contains moisture, which is added to any moisture from plants consumed by lava flows. Pyrocumulus clouds have also been seen above the biggest fires in cities or at large industrial sites like refineries.
Unlike ordinary cumulus clouds, which are often cottony white or light gray, pyrocumulus clouds are likely to be brownish or grayish. This is because the rising warm air carries smoke, ash and debris into the air along with the moisture. They are also often connected to the ground by a column of smoke and ash that can be traced back to the source on the ground.The image of one kind of man-made pyrocumulus cloud is familiar to those who lived through the Cold War in the 1950s and '60s. This is the mushroom cloud that rises from a nuclear explosion. A super-hot column of rising air contains the moisture from the surface, which condenses high in the atmosphere as a pyrocumulus cloud. The "stem" of the mushroom is centered on the fire created at the center of the explosion.
Ordinary cumulus clouds can grow into thunderheads, or cumulonimbus, clouds that can produce thunderstorms with heavy rain and lightning. Large pyrocumulus clouds have been known to form a pyrocumulonimbus cloud above the source. A thunderstorm formed in a pyrocumulonimbus cloud might create rain that helps put out the fire, but lightning from the storm might ignite new fires nearby.