Pyrocumulonimbus Cloud: When Wildfires Spit Storms, Lightning
Context:
The intense wildfires burning across the United States and Canada have generated pyrocumulonimbus clouds, which can produce thunder and ignite additional fires.
More on the news:
- The development of Pyrocumulonimbus clouds has increased in frequency in recent years.
- Before 2023, the global average was 102 pyrocumulonimbus clouds recorded per year. Out of these, 50 were typically seen in Canada.
- In the extreme wildfire season of the previous year, 140 pyrocumulonimbus clouds were recorded solely in Canada.
What are Pyrocumulonimbus clouds & Their Formation:
- Pyrocumulonimbus clouds are thunderclouds created by intense heat from the Earth’s surface. They form similarly to cumulonimbus clouds, but the heat source is fire from large wildfires or volcanic eruptions.
- The prefix ‘pyro’ means fire in Greek, reflecting their formation mechanism.
- Clouds occur with extremely hot wildfires or volcanic eruptions. Example: Australian bushfires of 2019-2020 when temperatures crossed 800°C.
- Intense heat from the fire warms the surrounding air, which moves upward into the atmosphere.
- As the hot, buoyant air carrying water vapour , smoke, and ash rises, expands and cools down. Hot air condenses on ash, forming a pyrocumulus cloud (fire cloud).
- With sufficient water vapour and intensified upward movement, pyrocumulus clouds can evolve into pyrocumulonimbus clouds, also known as pyroCbs.
Impact on Atmosphere:
- Pyrocumulonimbus clouds are linked to aerosol pollutants such as smoke and ash in the stratosphere and upper atmosphere.
Reasons for Increased Occurrence:
- Climate change is believed to play a role in the increased frequency.
- Rising global temperatures, wildfires are becoming more common and intense, potentially increasing the occurrence of pyrocumulonimbus clouds.
Characteristics of Pyrocumulonimbus Clouds:
- Fire clouds are greyish or brown due to ash, smoke, and particulate matter, unlike fluffy white cumulus clouds.
- They can generate fire tornadoes, more devastating than regular thunderclouds.
- These clouds typically produce lightning with a positive charge, making the storms last longer. They can spark new wildfires many kilometres away from the main blaze.
- Trigger strong winds, making the spread of the wildfire faster and more unpredictable.
- They rarely produce rain, which would help extinguish the wildfire.
- These clouds can reach heights of 50,000 feet and generate their own thunderstorm systems.
Wildfires, Climate Change, and Air Pollution: A Vicious Cycle:
- Wildfires are uncontrolled fires that burn in wildland vegetation, including forests, grasslands, savannas, and other ecosystems.
- Wildfires can start from natural events like lightning strikes or human-made sparks.
- Weather conditions, such as wind, high temperatures, and low rainfall, significantly influence wildfire growth.
- Natural Causes: Lightning (particularly hot lightning) is the most common natural ignition source for wildfires.
- Climate change triggers more extreme lightning storms by warming the land and increasing carbon emissions, which create stronger updrafts.
- Fuel Sources: Dry vegetation, due to elevated temperatures and low precipitation, is a significant fuel for wildfires.
- Human-Induced Causes: Human activities are also major sources of wildfires, either accidentally or intentionally.
- Common human-induced ignition sources include open burning, equipment failure, debris burning, negligent cigarette disposal, and arson.
Case Study: Wildfire Impact of the “Black Summer” Bushfires
The Vicious Cycle:
- Wildfires impact climate and air quality by releasing large amounts of carbon dioxide, carbon monoxide, nitrogen oxides, black carbon, brown carbon, ozone precursors, and organic materials contributing to climate change, which affect radiation, clouds, and air quality on regional and global scales.
- The resulting climate changes further increase the frequency and intensity of wildfires, creating a self-perpetuating cycle.
- This cycle impacts both local and global environments and public health.
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Ecological Benefits of Wildfires:
Some plant species depend on wildfires for survival:
Wildfires help maintain ecosystem health by:
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