Cloud PointField Guide

Special — fire-storm cloud

Pyrocumulonimbus

When a fire grows its own thunderstorm

Written by Laurence WayneEntry No. 34Rare

Pyrocumulonimbus: When a fire grows its own thunderstorm
Plate XXXIVPhotograph by Cal Fire, Public domain.

A pyrocumulonimbus is a full thunderstorm cloud generated by the intense heat of a large wildfire or volcanic eruption. The fire's super-heated updraught lofts smoke, ash and moisture upward; as that air rises and cools, moisture condenses and the plume builds into a towering cumulonimbus. The WMO names it cumulonimbus flammagenitus — 'fire-made'. It can produce lightning, fierce gusts, hail and even fire tornadoes, yet often gives little rain.

Worth knowing

  • During Australia's 2019–20 'Black Summer', an outbreak of pyrocumulonimbus injected roughly a million tonnes of smoke into the stratosphere — comparable to a moderate volcanic eruption — and the aerosols circled the globe for months.
Note

Still unsure it's the same cloud? Our app, Cloud Point— on iOS and Android — reads a photo and returns the likely type with altitude, useful when a formation is fading faster than you can look it up.

Test yourselfThree levels

Choose a difficulty

A pyrocumulonimbus is a thunderstorm cloud created by…

  1. aA large wildfire (or a volcanic eruption)
  2. bA passing cold front
  3. cEvaporation from the ocean
  4. dMelting mountain snow
Show answer

AnswerA large wildfire (or a volcanic eruption)

The intense heat of a fire drives the updraught that builds the storm cloud above it.

Why can a pyrocumulonimbus make a wildfire worse?

  1. aIt cools the fire with heavy rain
  2. bIt produces lightning but little rain, which can ignite new fires
  3. cIt blocks all wind from reaching the fire
  4. dIt always drops hail that smothers the flames
Show answer

AnswerIt produces lightning but little rain, which can ignite new fires

PyroCbs tend to be dry, so their lightning can spark fresh fires while giving little rain to put anything out.

Why are the largest pyrocumulonimbus events compared to volcanic eruptions?

  1. aThey erupt molten lava from the ground
  2. bThey can loft smoke into the stratosphere in amounts rivalling a moderate volcano, with global effects lasting months
  3. cThey make the ground shake like a quake
  4. dThey release sulphur straight out of bedrock
Show answer

AnswerThey can loft smoke into the stratosphere in amounts rivalling a moderate volcano, with global effects lasting months

The strongest pyroCbs inject roughly a volcano's worth of smoke into the stratosphere, where the aerosols spread worldwide and linger for months.

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