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A pyrocumulonimbus (pyroCb) is a thunderstorm generated or strongly enhanced by an intense wildfire. An ordinary thunderstorm is powered mainly by warm, moist air rising through an unstable atmosphere. Both can produce lightning, heavy rain, hail and damaging wind, but a pyroCb forms over an already dangerous fire and can feed that fire with new lightning, stronger outflow and erratic wind. That two-way fire–storm feedback is why a pyroCb can change wildfire conditions faster and more unpredictably than a conventional storm.
The World Meteorological Organization (WMO) formally calls a cloud produced by a localized natural heat source, such as a wildfire, flammagenitus; “pyrocumulus” is the unofficial name for cumulus flammagenitus. When that fire-driven convection reaches the cumulonimbus (thunderstorm) stage, it is commonly called a pyroCb.
The decisive difference: what powers the updraft
Every thunderstorm needs buoyant air to rise, condense and release heat. The key question is where the storm’s initial buoyancy comes from.
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- Pyrocumulonimbus: A wildfire heats the air and injects water vapor and smoke into a rising plume. The plume can become deep, organized convection and develop into a thunderstorm.
- Ordinary thunderstorm: Warm, moist environmental air rises because of atmospheric instability, a front, terrain or another lifting mechanism. The storm is not powered by a wildfire.
In a pyroCb, the fire and storm can reinforce one another. The storm’s downdrafts and outflow can spread flames and embers, while lightning can ignite additional fires. An ordinary storm can still be severe and destructive, but it does not normally have an active wildfire supplying its heat and moisture or responding directly to its wind field.
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What the terms mean
Pyrocumulus, pyroCb and flammagenitus
The WMO International Cloud Atlas says clouds clearly originating from localized natural heat sources—including forest fires, wildfires and volcanic activity—receive the special cloud name flammagenitus. Cumulus flammagenitus is also known informally as pyrocumulus.
A pyrocumulus cloud can remain a tall cumulus without becoming a full thunderstorm. A pyrocumulonimbus is the more extreme stage: the fire-driven convection has developed into cumulonimbus, with deep updrafts, electrification and thunderstorm outflow. The WMO explains that cumulonimbus can develop from convection initiated by wildfire heat and is then classified with the supplementary name flammagenitus. See the WMO’s cumulonimbus remarks.
What “ordinary thunderstorm” means
“Ordinary” describes a storm’s basic single-cell type, not a promise that it is safe. NOAA describes an ordinary, or “pulse,” cell as having one main updraft followed by one main downdraft. Falling precipitation eventually cuts off the warm, moist inflow that feeds the updraft, so the cell is generally short-lived. NOAA’s overview is at Types of Thunderstorms.
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Some ordinary cells become severe. The National Weather Service explains that severe thunderstorms generally last longer and can produce very strong winds, flooding, hail and/or tornadoes, while ordinary thunderstorms are usually short-lived and generally do not produce very strong winds or large hail. NOAA’s severe-storm explainer uses a wind gust of at least 58 mph, hail at least 1 inch in diameter, and/or a tornado as severe-storm thresholds in its classification context: Severe storms.
How a pyroCb forms, step by step
- Extreme fire heating: A large or intense wildfire transfers heat to the air immediately above and within the flaming and smoldering area.
- Moisture and smoke enter the plume: Combustion releases water vapor and carries smoke and ash upward. The heated mixture is less dense than surrounding air and rises rapidly.
- Condensation creates a cloud: As the plume expands and cools, its water vapor condenses. A rising pyrocumulus cloud forms over the fire.
- Deep convection develops: If the plume remains buoyant and the surrounding atmosphere is sufficiently unstable and moist, the cloud grows vertically. The WMO recognizes this as wildfire-initiated cumulonimbus convection.
- Electrical charging and precipitation begin: A mature cloud can develop the turbulent mixed-phase region needed for lightning, along with rain, hail or graupel and strong up- and downdrafts.
- Outflow feeds back on the fire: Descending air spreads outward as gust fronts or downbursts. Those winds can change the fire’s direction and rate of spread, loft embers and alter the plume that is feeding the storm.
The Australian Bureau of Meteorology describes the same chain in practical terms: a fire plume rises and cools, moisture condenses, and the plume can become a thunderstorm whose downburst rain and lightning reach the ground. Its explanation, How fires make thunderstorms, warns that these clouds can move unpredictably and are extremely difficult to forecast.
How an ordinary thunderstorm forms
- Moist, unstable air is lifted: Heating, a boundary such as a front, terrain or another lifting process pushes air upward.
- The rising parcel cools and condenses: A cumulus cloud forms as water vapor turns into cloud droplets and ice.
- The updraft strengthens: Latent heat released by condensation keeps the rising parcel warmer than its surroundings, allowing the cloud to grow.
- Precipitation loads the cloud: Rain and ice become heavy enough to fall, creating a downdraft.
- The downdraft cuts off inflow: Rain-cooled air spreads out near the ground and undercuts the updraft. In a typical ordinary cell, the storm then weakens.
NOAA’s thunderstorm life-cycle description details the developing, mature and dissipating stages. Multicell clusters and supercells can last much longer than a single ordinary cell, so the word “ordinary” should not be confused with “non-severe.”
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Pyrocumulonimbus vs. ordinary thunderstorm: side-by-side
| Comparison point | Pyrocumulonimbus (pyroCb) | Ordinary thunderstorm |
|---|---|---|
| Initiation | Intense wildfire heat and moisture drive a buoyant smoke plume. The WMO classification for the heat-source cloud is flammagenitus. | Warm, moist environmental air rises in an unstable atmosphere because of lifting from heating, fronts, terrain or other weather processes. |
| Main energy source | The active fire supplies concentrated heat and moisture in addition to atmospheric instability. | Atmospheric instability, moisture and lift; no wildfire is required. |
| Storm–surface feedback | Lightning can ignite new fires, while outflow and downbursts can fan the existing fire, transport embers and alter its spread. | Rain-cooled outflow can produce gusts and boundaries, but the storm is not normally coupled to a burning fuel bed. |
| Typical hazards | Lightning, erratic inflow and outflow winds, downbursts, ember transport, smoke impacts, pyrogenic lightning and possible tornadoes. | Lightning, gusty or severe winds, hail, heavy rain, flash flooding and, in severe cases, tornadoes. |
| Life-cycle behavior | Can strengthen or reorganize as fire intensity, terrain, plume structure and atmospheric instability change. | An ordinary cell commonly weakens when precipitation cuts off the updraft; its single-cell life cycle is comparatively well characterized. |
| Forecasting challenge | Fire intensity and plume behavior interact with the atmosphere, making movement and wind changes difficult to predict. | Forecasters use established measures of instability, moisture, lift and wind, although severe storms can still evolve rapidly. |
| Primary exposure | Firefighters, evacuees and communities near an active wildfire, including areas downwind where embers or lightning may start new fires. | People beneath or downstream of the storm, with risk depending on severity, rainfall, wind, terrain and drainage. |
Why pyroCbs can make a wildfire worse
Lightning creates new ignition points
Lightning from a pyroCb is produced by a thunderstorm, but it occurs in a landscape that may already be extremely dry and actively burning. A strike outside the main fire perimeter can start a separate ignition. NOAA NESDIS states that pyroCb clouds can produce lightning that sets off even more fires: Wildfires spawn severe weather.
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This is different from the usual thunderstorm risk of a lightning strike damaging a structure or injuring a person. In a pyroCb, each strike can also increase the number of fires that emergency crews must locate and contain, potentially far from the original front.
Outflow winds can shift the fire suddenly
Rain-cooled air descending from the cloud can spread outward as a downburst or gust front. The resulting wind may arrive from a new direction, push flames into unburned vegetation and loft burning material. NOAA NESDIS reports that pyroCb storms can generate stronger winds that fan a fire, make it hotter and help it spread.
For crews working near the perimeter, the danger is not simply a higher average wind speed. A rapid change in direction can invalidate an escape route or cause a fire to run toward an area that had appeared sheltered. The same outflow can also reshape the smoke plume and reduce visibility.
Powerful updrafts and downdrafts alter the plume
The rising column that builds the cloud can pull smoke, ash and embers high above the fire. When air later descends and spreads, material can be carried back toward the surface or transported downwind. The exact path depends on the fire’s intensity, terrain and surrounding winds, which is one reason pyroCb movement is difficult to forecast.
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NOAA NESDIS notes that in some cases the air in a pyroCb can rise so rapidly that a tornado forms. A confirmed pyroCb tornado occurred near the Bootleg Fire on July 18, 2021. This is not a claim that every pyroCb produces a tornado; it demonstrates that fire-generated convection can reach a level at which tornadic winds are possible.
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What hazards do both storm types share?
Lightning, heavy rain, strong wind and hail are common to the broader thunderstorm family. The difference is the setting and the consequences that follow.
- Lightning: Both storm types can strike the ground. Around a wildfire, a new strike may create another ignition.
- Wind: Ordinary storms can produce damaging straight-line winds; pyroCbs can add fire-driven inflow and outflow to the storm’s wind field, making shifts especially dangerous.
- Rain: Heavy rain from either type can reduce visibility and create runoff. Over a recently burned landscape, rain can also interact with unstable soil and drainage, so official local guidance matters.
- Hail: Hail is possible in both. A documented pyroCb associated with the Mallard Fire produced hail about 1 inch in diameter, the same size used by NOAA in its severe-storm threshold, but that event-specific measurement is not a universal pyroCb average.
- Tornadoes: Severe ordinary thunderstorms can produce tornadoes, and rare pyroCb cases can as well. The Bootleg Fire event confirms the latter possibility.
The National Weather Service’s Severe Weather guidance describes the hazards associated with severe thunderstorms. “Ordinary” therefore means a cell category, not a guarantee that people underneath it face little risk.
Behavior and forecasting: why pyroCbs are harder to anticipate
Ordinary-cell forecasting starts with the atmosphere
For a conventional storm, forecasters examine instability, moisture, lifting mechanisms and wind shear, then track radar and satellite signatures as the cell develops. The ordinary single-cell cycle—updraft, mature updraft-and-downdraft stage, then downdraft-dominated decay—provides a useful framework for estimating when precipitation will undercut the storm.
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A wildfire is not a fixed, steady heat source. Flame intensity changes as fuels, slope, wind and fire behavior change. The plume can lean, split, merge with neighboring convection or become blocked and redirected by terrain. Smoke also changes the way sunlight is absorbed and how the plume interacts with surrounding air.
That means the storm’s “engine” can intensify or weaken independently of the ordinary atmospheric ingredients. A forecast that correctly identifies instability may still miss the timing or location of a rapid fire-driven transition.
Outflow can become the next hazard before rain arrives
People often associate a thunderstorm with the arrival of rain, but a pyroCb’s first major impact may be a wind shift, a pressure surge, blowing embers or a sudden change in smoke. Downburst outflow can reach the ground and spread ahead of the heaviest precipitation. The Bureau of Meteorology therefore describes pyro clouds as extremely difficult to forecast and warns that their movement can be unpredictable.
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No simple “more dangerous” ranking applies everywhere
A pyroCb is generally the more dangerous storm for people near the fire because it combines thunderstorm hazards with an active wildfire and possible new ignitions. Far from a fire, however, a severe ordinary supercell or a storm producing flash flooding may present the greater immediate threat. The correct comparison is situational: identify the hazards present, the distance from the fire, the terrain and the official warnings rather than assuming one storm type is always stronger.
What documented events show
Mallard Fire: hail in a fire-generated storm
NOAA documented hail approximately 1 inch in diameter from the Mallard Fire pyroCb in 2018. The observation shows that a fire-generated thunderstorm can produce hail meeting NOAA’s severe-storm size threshold. It does not establish a typical hail size for all pyroCbs.
Carr Fire: an extreme vortex
NOAA reported estimated winds of up to 143 mph in the vortex associated with the Carr Fire in 2018. This was an exceptional event-specific estimate, not a standard intensity or expected wind speed for pyroCbs in general.
Bootleg Fire: confirmed pyroCb tornado
NOAA NESDIS confirmed a tornado near the Bootleg Fire on July 18, 2021. The case is important because it demonstrates that fire-generated convection can produce a tornado, while still offering no basis for assuming that every pyroCb will do so.
These examples are best used as indicators of possible outcomes—hail, extreme vortices and tornadoes—not as averages. There is no directly comparable global frequency statistic establishing how often pyroCbs occur relative to ordinary thunderstorms.
Can a wildfire really create its own weather?
Yes, within limits. A wildfire cannot manufacture every ingredient of a regional weather system, but a sufficiently intense fire can substantially modify the air above it. Heat makes the plume buoyant; combustion adds water vapor; smoke and ash provide material that rises with the plume; condensation releases additional latent heat; and the resulting cloud can develop its own precipitation, lightning and downdrafts.
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Once the cloud becomes a thunderstorm, its winds and precipitation feed back on the fire. This is why “fire-generated weather” is more than a metaphor: the fire is actively helping initiate the convection, and the convection is then changing the fire environment. The WMO’s flammagenitus classification exists to identify that natural-heat origin rather than treating the cloud as an ordinary thunderstorm with no surface cause.
How to respond when a pyroCb is possible
Treat a pyroCb as a wildfire emergency that has become a thunderstorm. The cloud’s appearance alone is not a reliable warning system; a dark, tall plume may be visible before the most dangerous outflow, lightning or smoke change reaches you.
If you are in or near the fire area
- Follow evacuation orders immediately. Do not wait to see whether the cloud produces rain. Evacuation zones and routes are set by local incident managers who have access to fire behavior and road information.
- Monitor official fire-weather and severe-weather alerts. Use local emergency management, fire authorities and weather services. A pyroCb can create overlapping wildfire, lightning and wind warnings.
- Expect rapid wind shifts. Secure loose outdoor items only if it is safe to do so, and do not delay departure for property protection.
- Plan for smoke and visibility changes. Keep required medications, respirators or other personal protective equipment available according to local health guidance, and avoid driving into dense smoke.
- Assume new ignitions are possible downwind. Do not move toward a fresh smoke column or investigate lightning strikes unless directed by authorities.
If you are a firefighter or other responder
- Use the incident command system and current fire-behavior, aviation and weather briefings; do not rely on a visual estimate of the plume alone.
- Recheck escape routes and safety zones when wind direction, smoke movement or plume shape changes.
- Account for downbursts and gust fronts that may arrive before heavy rain.
- Expect lightning exposure and the possibility of spot fires beyond the established perimeter.
Specific tactics belong to trained agencies with local observations, maps and communications. The general rule for the public is simpler: follow the official order and get out of the hazard area early.
How to stay safe under an ordinary thunderstorm
Do not dismiss a storm because it is not fire-generated. Move indoors when thunder is heard, stay away from windows during strong wind or hail, and avoid flooded roads and low-lying areas. Pay attention to severe-thunderstorm and flash-flood warnings, especially when rainfall is intense or terrain channels water.
NOAA’s severe classification of at least 58 mph wind, 1-inch hail or a tornado is a useful warning threshold, not a complete safety boundary. A storm below one of those thresholds can still produce dangerous lightning, poor visibility, falling branches or localized flooding.
Practical signs and limits of visual identification
A towering cauliflower-shaped cloud or a dark smoke column can suggest strong convection, but appearance cannot tell you whether a cloud is a pyroCb, how quickly it is moving or which direction its outflow will take. Identification is most reliable when satellite, radar, lightning data and fire reports are interpreted by weather and fire agencies.
For residents, the actionable signal is an official warning or evacuation message, not a personal attempt to classify the cloud. If you can see a rapidly growing plume over an active wildfire, increase your distance and check authoritative alerts rather than waiting for a visible lightning flash.
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A pyroCb is a wildfire-driven thunderstorm, while an ordinary thunderstorm is an atmosphere-driven convective cell. The pyroCb’s defining danger is the feedback loop: fire powers the storm, and the storm can return lightning, downbursts, embers and sudden wind changes that intensify or spread the fire. Ordinary thunderstorms remain capable of severe wind, hail, flooding and tornadoes, but their life cycle and forecasting framework are comparatively familiar. Near an active wildfire, treat a pyroCb as a rapidly changing fire emergency and follow local evacuation and weather warnings without relying on the cloud’s appearance.
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