Pyrocumulonimbus Clouds

A pyrocumulonimbus cloud, or pyroCb, is a fire-generated thunderstorm produced when intense surface heating drives a smoke-rich plume high into the atmosphere. It develops from a pyrocumulus cloud when sufficient moisture and atmospheric instability allow the rising plume to deepen into a cumulonimbus cloud.

Conditions required

Formation requires a very intense heat source, commonly a large wildfire, combined with an atmosphere that permits vigorous vertical motion.

• Extreme fire heat creates a strongly buoyant convective plume of hot gases, smoke and ash.
• The plume entrains surrounding air and moisture; combustion and evaporation from vegetation and soil can add water vapour.
• A sufficiently unstable atmosphere allows the plume to continue rising rather than being stopped by a stable layer.

Formation process

• As the plume rises into lower-pressure air, it expands and undergoes adiabatic cooling.
• On reaching saturation, water vapour condenses on airborne particles, including smoke aerosols, producing a pyrocumulus cloud.
• Condensation releases latent heat, which strengthens buoyancy and supports further vertical growth.
• If the cloud rises above the freezing level, ice particles develop and the cloud becomes a deep cumulonimbus capable of producing lightning.

Feedback with the wildfire

A mature pyroCb can create its own hazardous weather and alter the behaviour of the fire beneath it.

• Powerful updrafts can draw additional near-surface air towards the fire and loft smoke into the upper troposphere, sometimes reaching the lower stratosphere.
• Electrical charge separation within the cloud produces lightning, which may start new fires away from the original front.
• Cloud downdrafts and outflow winds can cause abrupt changes in fire direction and rate of spread.

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