What is the relationship between fire and weather? Usually, it is the weather that dictates the conditions for wildfires, but in the case of exceptionally intense flames, the roles can reverse, and the wildfire itself begins to create its own weather, generating highly spectacular and dangerous clouds called pyrocumulonimbus. What may look like an ordinary “smoke plume” to a layman is, for an experienced and aware firefighter, a signal that the fire has reached a scale where it starts shaping its own atmospheric conditions – effectively becoming a new generation of fire (6th generation fire). These clouds can make firefighting extremely dangerous and nearly impossible. Understanding this process is crucial for firefighters, as these clouds can dramatically alter fire behavior and directly impact the safety of firefighting operations.
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Terminology: names of fire-related clouds
The terms “pyrocumulus” (pyroCu) and “pyrocumulonimbus” (pyroCb) are increasingly appearing in the media and online, although until 2004 they were not even used within the scientific community. These terms describe an exceptionally violent atmospheric phenomenon associated with the intense heating of moist air by surface-level combustion. In 2017, with the publication of the new edition of the WMO International Cloud Atlas, the World Meteorological Organization introduced an official, though somewhat unwieldy, name for them: “Flammagenitus”. This name refers to clouds generated by fire occurring naturally, for example, as a result of a volcanic eruption, forest fire, or bushfire.
Any object hot enough to produce strong convective currents can generate clouds, provided there is sufficient moisture in the air, for instance, above power plant cooling towers. Clouds whose formation can be clearly attributed to human activity, such as stubble burning, warehouse fires involving flammable materials, or building fires, should formally be called “homogenitus”.
However, despite the official terminology, cumuliform clouds forming above fires are informally and conventionally called “pyrocumulus”, a name derived from the combination of the Greek word pyro (fire) and the name of the underlying cloud type.
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Formation mechanism. Fire, the cloud generator
Fires release enormous amounts of heat and also inject countless microscopic particles into the air. Moist air, strongly heated by an intense fire, rises upward, carried by a powerful convective updraft that transports not only water vapor but also smoke, ash, and dust. As the column rises higher and higher, atmospheric pressure decreases, causing expansion and further cooling of the mixture of smoke and hot air within the column. Moisture from the surrounding atmosphere, as well as vaporized water from burned vegetation, begins to condense onto soot and ash particles, forming a large fluffy cloud resembling a classic cumulus. Finally, the air cools enough for the contained moisture to fully condense, allowing a cloud to form. With such an abundant supply of suspended solid particles, i.e. condensation nuclei, pyrocumulus clouds are usually made up of a vast number of very small droplets. This, combined with larger particles of ash or smoke, gives the cloud a very dense, sometimes yellowish appearance. Due to contamination with dust or soot particles, such clouds can also take on darker hues. The condensation process releases latent heat, which makes the cloud warmer and more buoyant, accelerating the upward motion of the air inside it. Despite the dark color of the smoke plume, a characteristic feature of these fires is the column’s peak crowned by a distinct, sharply outlined white cap of a well-shaped puffy cloud. The entire smoke column takes on an anvil-like form.
If a fire is intense enough, the atmosphere is unstable, and sufficient moisture is available, the cloud may continue to grow vertically and a pyrocumulus can develop into a pyrocumulonimbus. Only a relatively small fraction of pyroCu evolves further into pyroCb. The annual number of pyroCb occurrences worldwide exceeds 40–50 distinct events. When atmospheric conditions are favorable, including a hot, dry air layer near the surface and a cooler, moister layer above, the atmosphere can become convectively unstable. Continued expansion and cooling lead to the condensation of more moisture and even faster upward motion of the air. The rising smoke column along with its cloud top can even extend beyond the troposphere into the lower stratosphere, reaching altitudes of around 12–15 kilometers above Earth’s surface – much like volcanic plumes.
Under suitable conditions, the cloud may accelerate all the way into the lower stratosphere before it loses buoyancy. Collisions of ice particles in the very cold upper parts of these clouds generate electrical charges, which are released in the form of giant sparks – lightning. From that moment, the cloud can produce thunderstorms and ignite additional new fire outbreaks within a radius of tens of kilometers. When the mass of moisture lifted within the pyrocumulus exceeds the strength of the updraft, it may fall as precipitation. However, the rain may evaporate on its way down due to the dry air near the surface. Such precipitation that never reaches the ground is known as virga.
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Hazards for firefighting operations
From the perspective of first responders, the appearance of a pyrocumulus and even more so a pyrocumulonimbus is a warning signal, a true Red Flag. Both types of clouds indicate that the fire has reached immense energy and may begin to behave unpredictably. Such changes in fire behavior make it more difficult and more dangerous to control.
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- Under conditions favorable for the formation of these clouds, fires are accompanied by very strong updrafts that draw in so much air that powerful winds are generated, converging from all directions toward the smoke column. These near-surface air movements can influence fire development. Inflowing winds may cause nearby fires (e.g., from spot fires or lightning strikes, see below) to change direction unexpectedly as they are pulled into the main fire. Sudden shifts in wind direction and strength can trap firefighters in flames or cut off evacuation routes. The rapid and chaotic inflow at the base naturally intensifies the fire, feeding already extreme flames and closing the feedback loop of pyroCb dynamics.
- Strong updrafts rise to great heights with enough force to transport glowing embers. Carried aloft by the wind, they can fall at considerable distances from the fire’s source and ignite new spot fires. During “Black Saturday” in the Australian state of Victoria in 2009, such events were recorded more than 30 km away from the main fire front.
- Violent downbursts also sometimes occur, which can change the direction of fire spread within minutes. The process of evaporation cools the air, making it heavier and accelerating its descent toward the ground. Air saturated with moisture becomes too heavy to be sustained by the updraft and begins to fall. The descending wind strikes the ground with force, disperses, and blows radially – that is, in straight lines in all directions from the point of impact at the surface, similar to water from a faucet hitting a sink. Such irregular, strong winds (>100 km/h) contribute to extreme and unpredictable fire behavior. These winds can be strong enough to carry fire in any direction and have repeatedly been the cause of firefighter fatalities.
- These storms can produce lightning, which may ignite new fires. During the “Black Saturday” fires in Victoria, lightning from pyrocumulonimbus clouds sparked a new fire 100 km away from the fire front.
- Pyrocumulonimbus clouds pose a serious threat to firefighting aviation. They can cause severe turbulence, lightning strikes, and draw vast amounts of smoke and ash into the atmosphere, which can damage aircraft engines in a manner similar to volcanic eruptions. For firefighting pilots, sudden and unpredictable wind changes and dramatically reduced visibility are a major problem, often forcing them to suspend operations in the affected area and close the airspace over the fire.
- Communities and firefighters facing a pyroCb storm often lose orientation and situational awareness due to limited visibility in dense smoke and nighttime darkness caused by the cloud cap blocking out light. Solar radiation can be almost completely obstructed, resulting in exceptionally strong darkening beneath the cloud. Consequently, evacuations during pyroCb storms are particularly dangerous and can lead to tragic outcomes, such as the events in Portugal on 17 June 2017. When pyroCb storms occur at the interface between wildlands and built-up areas, they often cause severe property damage and fatalities, as exemplified by the “Black Saturday” disaster in Australia in 2009, which claimed 173 lives.
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For firefighters, understanding the phenomenon of pyrocumulus and pyrocumulonimbus clouds is an essential part of situational awareness. The sight of such a cloud should trigger vigilance: it signals that a fire may be generating its own weather conditions, and therefore sudden, hard-to-predict changes along the fire line. Knowing how and why these clouds form allows for better risk assessment, informed tactical decisions, and improved team safety.
The pyrocumulus phenomenon is proof that the boundary between fire and weather can be fluid. A fire of sufficient intensity not only destroys, but also becomes a factor shaping the atmosphere itself. In an era when climate change fosters increasingly larger and more violent fires, firefighters can expect to encounter “fire clouds” more often. That is why it is important to know, understand, and learn to recognize their signals—because safety on the fireline may depend on this knowledge
