A pyrocumulonimbus cloud, often shortened to pyroCb, is a thunderstorm triggered or intensified by extreme heat from a wildfire. Ordinary thunderstorms grow when warm, moist air rises; a fire can supply an especially powerful source of rising air. If that towering plume reaches sufficient height and develops thunderstorm processes, it can produce lightning and dangerous winds.
Pyrocumulus versus pyrocumulonimbus
The words are related, but not interchangeable. Pyrocumulus describes a puffy, cumulus-like cloud formed by powerful heating from a fire or other hot source. Pyrocumulonimbus is the deeper thunderstorm form: it can develop an anvil top, electrical activity, strong downdrafts and sometimes precipitation. Neither is simply a giant plume of smoke; the white cloud is made of condensed water droplets and often ice.
How does a fire create a thunderstorm?
- Strong heat causes lift: air above the fire becomes buoyant and races upward, carrying smoke, moisture and particles.
- Rising air cools: at lower pressure higher in the atmosphere, water vapor condenses into visible droplets and releases heat.
- Cloud growth accelerates: if enough instability and moisture are available, the convective tower can reach higher, colder levels, where ice forms.
- Thunderstorm processes begin: in a sufficiently developed pyroCb, charge separation can create lightning, while precipitation and downdrafts affect surrounding winds.
Why are these storms dangerous?
Fire-created storms can change the behavior of a wildfire and threaten firefighting operations. A collapsing or shifting plume may accompany powerful gusts and erratic fire spread. Lightning can ignite additional fires. Some pyroCb storms loft smoke high into the upper atmosphere or lower stratosphere, where satellites can track it over long distances.
A pyroCb does not have to produce measurable rain at the fire itself. Some falling moisture evaporates in dry air, and the resulting cool downdrafts can still drive strong surface winds.
Can you see a pyroCb from far away?
A high, cauliflower-shaped white tower or anvil above a darker wildfire smoke column is a clue, but a photograph alone may not establish that lightning-producing storm processes have begun. Scientists use satellite data, radar, lightning observations and meteorological reports to classify events. Do not approach a wildfire to photograph one. Follow evacuation orders and official local alerts.
Frequently asked questions
Does the smoke itself become a cloud?
Smoke particles may help water condense, but the recognizable white cloud is mainly condensed water. The brown-gray smoke plume and white cloud can be seen together.
Can a wildfire create its own lightning?
Yes. In sufficiently deep pyroCb storms, thunderstorm charge separation can lead to lightning. This does not happen in every pyrocumulus cloud.
Sources and further reading
Read NASA Earth Observatory’s pyrocumulus example, NOAA’s research on pyroCb smoke lofting, and the National Wildfire Coordinating Group’s fire-weather field guide.
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