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A pyrocumulonimbus (abbreviated pyroCb; formally cumulonimbus flammagenitus, CbFg, in the World Meteorological Organization International Cloud Atlas) is a cumulonimbus cloud that forms above a wildfire, a volcanic eruption, or another intense heat source, and whose development is driven or strongly enhanced by the heat and moisture that the fire releases. In the strictest sense, a pyroCb is a deep convective cloud that produces ice (and often lightning) because it reaches high altitudes and low temperatures, and the plume of smoke that it lofts can be injected above the tropopause into the lower stratosphere. The term "pyrocumulus" refers to a smaller cloud that forms above a fire without the vertical development, ice, and electrification that a pyroCb has.

PyroCbs are studied in meteorology, atmospheric chemistry, and Earth system science because they are a route by which smoke from the surface reaches the stratosphere, where it can stay for months and affect radiation and chemistry. The events are not new. The earliest documented examples go back at least to the nineteenth and twentieth centuries, but the recognition of their importance for stratospheric composition is recent, and the number of well-documented events has grown. Part of this growth is the result of better satellite observation, and part is due to more extreme fire conditions. The relative importance of the two is not settled, and it is not known whether a trend in the number of events exists independently of the improved detection.

Heat, moisture, and buoyancy

An ordinary cumulonimbus cloud is powered mainly by the release of latent heat when water vapor in rising air condenses. In a pyroCb, a large fire adds sensible heat and also moisture. The fire releases heat at rates that can exceed 10⁵ to 10⁶ kW per kilometer of fire front in extreme cases, and large fires can release energy at gigawatt to tens of gigawatt rates in total. The heated air is less dense than the surroundings, so it rises as a buoyant plume. Water enters the plume from three sources: the vapor in the ambient air that the plume entrains, the water that is released when the fuel is burned (combustion of hydrocarbons produces water), and the moisture that evaporates from the vegetation. The first is usually the largest, and the amount of ambient moisture at middle levels of the atmosphere is an important factor.

The buoyancy of the plume relative to its surroundings is described by the temperature difference or, equivalently, by the difference in virtual potential temperature. For a parcel with temperature T in an environment at temperature T_e, the buoyant acceleration is:

where g is the acceleration of gravity. For example, a parcel that is 10 K warmer than a 280 K environment has a ≈ 9.8 × 10 / 280 ≈ 0.35 m·s⁻². Over a distance of 2 km, an acceleration of this size would give an updraft speed of w = √(2_ah_) ≈ √(2 × 0.35 × 2,000) ≈ 37 m·s⁻¹, which is about 130 km per hour. Updrafts in pyroCb plumes of tens of meters per second are therefore physically reasonable, and speeds higher than this have been inferred from radar and satellite observations of strong events, in part because the release of latent heat adds to the buoyancy. The figure "above 150 km per hour" is of the right order for the strongest updrafts but not for typical ones.

When the plume reaches the level at which the vapor condenses (the lifting condensation level), a cloud forms, and the latent heat that is released can add to the buoyancy, so the plume may continue to rise. If the plume rises to altitudes where the temperature is below about −40 °C, the cloud droplets freeze, and the freezing releases additional latent heat. Smoke particles are efficient as cloud condensation nuclei, and a high concentration of small droplets slows the formation of precipitation. As a result, the cloud contains many small droplets that are carried to high altitudes and freeze, which can be seen in satellite measurements as small ice crystals at the cloud top.

Environmental conditions

A pyroCb usually needs the combination of a large and intense fire, an atmosphere that is unstable or has an unstable layer above the fire, and enough moisture at middle levels. The typical meteorological setting includes a very dry, hot surface layer with a deep mixed layer, and an unstable lapse rate. A measure of the potential for the development of deep fire plumes is the Haines Index, and more specialized indices, such as the C-Haines, are used by fire weather forecasters. Whether a plume produces a pyroCb depends on the interaction of the fire's heat output, the environmental stability, and the humidity, and the same fire can produce pyroCbs on one day and not on another. Strong winds can tilt the plume, which reduces the depth of convection, so that some pyroCbs form in conditions with weaker winds, or with winds that favor a vertical plume.

Fire behavior and the storm's effects at the ground

The updraft draws air in toward the base, which produces inflow winds that can be strong and erratic near the fire. The strength depends on the intensity of the fire and the structure of the plume. The downdrafts from a collapsing pyroCb can produce outflow winds that spread the fire in unexpected directions. PyroCbs can generate lightning, and this lightning can start new fires. Dry lightning is a well-known cause of ignitions in many fire-prone regions, but it is also produced by ordinary thunderstorms, so the lightning from a pyroCb is one of several sources. Heavy precipitation does not usually reach the ground from pyroCbs, because the droplets are small and evaporate in dry air, although some produce rain or hail, and a few have produced precipitation that affected the fire. PyroCbs can also be associated with fire tornadoes (fire whirls with a strong vortex), although most fire whirls are formed by the fire itself and not by the cloud. The claim that the storm "generates hurricane-force inflow winds" is not supported in a general sense. The strongest wind speeds that are associated with fire-induced circulations are those in fire whirls and in the outflows, and they differ greatly among events.

How smoke reaches the stratosphere

The tropopause is the boundary between the troposphere, in which the temperature decreases with height, and the stratosphere, in which it is nearly constant and then increases with height. The stratosphere is stably stratified, so the vertical motion of air across the tropopause is limited. A deep convective updraft can overshoot its level of neutral buoyancy and penetrate into the stable layer, where it is rapidly decelerated. This is the same mechanism by which the strongest thunderstorms produce overshooting tops. The height that a plume can reach depends on the buoyancy it has and on the stratification, and the maximum overshoot above the tropopause is typically of the order of a kilometer or two for severe storms. The terms "rupture" and "punch through" are figurative. The tropopause is not a lid that is broken. It is a stable layer, and air that overshoots it either mixes with stratospheric air or returns downward, and the smoke that remains in the stratosphere is the part that is mixed in and carried by the large-scale circulation. In addition, the smoke can be lifted further by its own heating. Smoke particles that absorb sunlight heat the surrounding air and can rise slowly by this "self-lofting" through the lower stratosphere. A well-known example is the Australian pyroCb smoke of 2019 to 2020, which formed a vortex that rose by several kilometers over a few months in the Southern Hemisphere stratosphere.

The tropopause is higher in the tropics (about 16 to 17 km) than at mid and high latitudes (about 8 to 12 km), so that pyroCbs at high latitudes, in boreal forests, can reach the stratosphere more easily than those at low latitudes. Observations since about the 2000s have documented many events that reached the lower stratosphere. These include the Canadian pyroCbs of 2017, in which smoke from fires in British Columbia reached about 12 km, the 2019 to 2020 Australian fires, which injected a very large quantity of smoke, the 2021 Pacific Northwest and Siberian events, and the 2023 Canadian fire season. Large events have injected amounts of smoke that range from about 0.1 to 1 teragram of aerosol, with the 2019 to 2020 Australian events at the high end of this range, although the estimates differ and depend on the method and the definition of the event.

What the injected plume contains

The plume carries organic and black carbon aerosols, which are the main components of the smoke (the organic fraction is usually larger than the black carbon fraction, so that much of the smoke is "brown carbon" and organic aerosol, which absorbs less than black carbon), water vapor and ice, and gases including carbon monoxide, volatile organic compounds, and nitrogen oxides. The mass of water vapor that is lofted is small compared with the amount that is carried into the stratosphere by the normal tropical circulation, although it can matter locally. Methane and carbon monoxide are emitted by fires in smaller amounts relative to the carbon dioxide. The statement that pyroCbs inject "millions of tons" of material is correct for the largest events if one counts all of the smoke aerosol and gas, but not for all events, and many pyroCbs lift much smaller amounts.

Radiative effects

Smoke in the stratosphere interacts with solar radiation in two ways. The particles scatter sunlight, which tends to cool the surface. They also absorb sunlight, which heats the stratosphere and reduces the radiation that reaches the surface. The balance depends on the black carbon content, the size and age of the particles, and the surface albedo below. Smoke from the Canadian pyroCb of 2017 formed a persistent layer that was observed for many months and reached altitudes above 20 km, because of self-lofting. The Australian smoke of 2019 to 2020 caused a measurable warming of the lower stratosphere, of the order of 1 K in some regions, and a reduction of the radiation at the surface of the order of a fraction of a watt per square meter on average for the Southern Hemisphere, with larger local effects. The Pinatubo eruption of 1991, for comparison, injected about 15 to 20 teragrams of sulfur dioxide, and produced a cooling of about 0.5 K globally. The radiative effect of a pyroCb smoke layer is much smaller on the global scale. The comparison to the "highly reflective sulfate aerosols" is a simplification, because the smoke has both scattering and absorbing components.

The claim that stratospheric smoke has "disrupted planetary Rossby waves and destabilized the polar vortex" is not established. The heating of the lower stratosphere by smoke can change the circulation locally, and it has been proposed as a factor in the change of the stratospheric circulation after the Australian fires, but the effect on the planetary waves and the polar vortex is an active subject of research and has not been shown.

Chemical effects

Heterogeneous chemistry on the surfaces of aerosols can change the balance of reactive chlorine and nitrogen compounds in the stratosphere. In the polar regions, ozone loss is driven by chlorine compounds that are activated on the surfaces of polar stratospheric clouds. The rate of the reaction of chlorine nitrate (ClONO₂) with hydrochloric acid on aerosol surfaces rises when the surface area is larger or the temperature is lower, and it rises with water vapor. The stratospheric smoke of 2019 to 2020 was associated with a decrease of ozone in the mid-latitudes in the Southern Hemisphere of a few percent, and with higher concentrations of reactive chlorine, and model studies suggest that the reactions on the surface of the smoke, together with changes in the circulation, contributed. The ozone depletion in the Antarctic in 2020 and 2021 was unusually large and long-lasting, and the role of the smoke in it is a subject of research. The conclusion that smoke causes "rapid, catalytic destruction" of the ozone layer is too strong. The ozone layer is also affected by other factors, including the polar vortex, the stratospheric temperature, and the decline of ozone-depleting substances because of the Montreal Protocol. The relative importance of pyroCbs for the global ozone is small, but the issue becomes more important if the frequency of large fires increases while chlorine remains elevated. It is not correct to say that the smoke functions "identically" to polar stratospheric clouds. The surfaces are different, and the temperatures in the mid-latitudes are much higher than those at which the polar stratospheric clouds form.

PyroCbs occur in North America (in particular western Canada and the United States), in Australia, in Siberia, and in other regions with extensive forests and dry fire seasons. The Black Saturday fires of 2009 in Victoria, Australia, the Canberra fires of 2003, the Chisholm fire of 2001 in Alberta, and the Black Summer fires of 2019 to 2020 are among the many examples. In the 2017 Canadian and the 2019 to 2020 Australian seasons, dozens of pyroCbs were observed in a few weeks, and in the Australian season, the number was larger than in the whole record before it, although the observational record is short. Some studies project that a warming climate will increase the conditions that favor extreme fires, such as hotter and drier fuels and longer fire seasons, and therefore the occurrence of pyroCbs, but this is a projection and not an observed global trend. The direct observation of trends is limited because satellite detection became routine only in the past two decades.

Whether pyroCbs are a net "positive climate feedback" is not established. Smoke can have a cooling effect on the surface in some conditions, and a warming effect in others, and its main climatic influence is on the stratosphere. The fire itself emits carbon dioxide, which is a feedback to the climate when the burned forest does not regrow, and this is a separate process from the stratospheric injection.

PyroCbs are detected with geostationary and polar-orbiting satellites. Signs include a cloud top with very cold temperatures in infrared imagery (often below −40 °C) located above an active fire, a high reflectance of small ice particles at the cloud top in the shortwave infrared, and, in the days that follow, a plume of aerosol in lidar and limb-sounding data (from instruments such as CALIOP, OMPS, and others). Lightning detectors and radar provide further information. A formal classification of confirmed events usually requires that a cloud has been identified above a fire and that it has the characteristics of a thunderstorm cloud, which can be difficult from the ground because the smoke hides the cloud. Forecasters use fire weather indices, plume-rise models, and coupled fire-atmosphere models, which represent the exchange of heat and moisture between the fire and the atmosphere, to anticipate conditions with a risk of pyroCbs. This is an active area of research, and forecasts of individual events are limited.

PyroCbs are a risk for firefighters and for communities, because they are associated with extreme and erratic fire behavior, large downdrafts, long-distance spotting (the transport of embers that ignite new fires ahead of the front), and sudden changes in wind direction. Fire services use plume-rise forecasts and the early detection of pyrocumulus as an alert. Because the same extreme conditions also hinder suppression, such events are among those that overwhelm firefighting resources.

• Cumulonimbus cloud
• Wildfire
• Stratosphere
• Tropopause
• Ozone depletion
• Fire whirl
• 2019–20 Australian bushfire season