The current wildfires in the United States and Canada are so strong that they are causing “pyrocumulonimbus” clouds, which have the ability to produce thunder and ignite other fires.
Pyrocumulonimbus clouds: Also known as fire clouds, these are thunderstorm clouds generated by extreme heat from the Earth's surface. These clouds form in a manner similar to cumulonimbus clouds, but the significant updraft that drives their formation is caused by the intense heat from large wildfires or volcanic eruptions.
Characteristics of Pyrocumulonimbus Clouds:
Can reach altitudes of up to 50,000 feet.
Capable of producing thunderstorms.
While they can generate lightning, they generally produce little rain.
They can ignite new fires far from the original blaze and generate high gusts, making wildfires spread more rapidly and unpredictably.
Formation of Pyrocumulonimbus Clouds:
Pyrocumulonimbus clouds are not produced by all wildfires; they form under extreme conditions such as exceptionally hot wildfires or volcanic eruptions.
Example: During the Australian bushfires of 2019-2020, temperatures reached 800°C, leading to the formation of these clouds.
The process begins with the intense heat from a fire warming the surrounding air, causing it to rise. This hot, buoyant air carries water vapor, smoke, and ash upwards.
As the air rises, it cools and expands. When cooled sufficiently, the water vapor condenses on the ash, forming a grey or brown cloud known as a pyrocumulus or ‘fire cloud’.
If there is ample water vapor and the hot air continues to rise strongly, pyrocumulus clouds can evolve into pyrocumulonimbus clouds.
Increasing Frequency of Pyrocumulonimbus Clouds:
The precise reasons for the increased occurrence of these clouds remain uncertain, as research on them is still emerging.
Scientists suggest that climate change may play a role in their growing frequency.
Rising global temperatures lead to more frequent and intense wildfires, which could contribute to a higher incidence of pyrocumulonimbus clouds.
These clouds are believed to contribute to the presence of various aerosol pollutants (including smoke and ash) in the stratosphere and upper atmosphere.
In recent years, there has been an increase in the frequency of these clouds developing. Prior to 2023, an average of 102 pyrocumulonimbus were observed worldwide in a single year; of those, 50 were observed in Canada, per a paper published in the journal Nature. But according to the paper, 140 pyrocumulonimbus clouds were observed in Canada alone during the severe wildfire season of the previous year.
