My wife looked outside as we were planning to run some errands and said, “It’s raining like hell out there.” Actually, the weather was just very wet and cold. Rain from hell would consist of clouds dropping flaming embers on our house. Welcome to the world of pyroCbs.
Cb is an abbreviation for cumulonimbus clouds, hence the term pyroCbs (fire clouds). I am not describing something from a science fiction novel. However, I am describing actual events that are happening now in Australia. The continent of Australia is in flames with some of the worst wildfires in history, and pyroCbs are one of the natural phenomena that help propagate these intense fires. Fire driven storms are increasing in frequency as uncontrolled brush fires and forest fires plague many parts of the world.
Taking a step back to understand cumulonimbus clouds is essential. These clouds are the flat, darkened bottom ones that often form along storm fronts and then send white billows towering up into the sky. Thunderclouds are a common name for them. When fully formed, these clouds can store the same amount of energy as up to 10 Hiroshima-sized atomic bombs.
Atmospheric convection gives rise to these monsters. When moist air near the earth’s surface heats up, it gets less dense and rises. This rising moisture then condenses as the air cools in the upper atmosphere to form clouds. The height of cumulonimbus clouds grows by a continual feed of warm, less-dense air from below.
Storm fronts containing long stretches of cumulonimbus clouds often appear along the line of intersection between two weather systems. When warm moist air encounters a cooler air mass, the warm air is forced to rise over the colder air because of a density differential between the two air masses. This process creates a visible storm front.
The version from hell
The basic physics of convection and updrafts applies to wildfires. Fire heats the air, making it less dense than the surrounding atmosphere, and the hot air rises. Then, the rising air creates an atmospheric void at the surface of the earth, causing winds to form as fresher air moves in to fill the void. If a fire is particularly intense, then air spirals in from all directions creating a vortex or tornado-like phenomenon. Some people refer to these events as “firenado’s.”
Smoke, ash, fire, and water vapor are sucked upward into the atmosphere forming large fire clouds that look and act like thunderclouds. The heat and particulates in these clouds inhibit precipitation, but like thunderstorms, the pyroCbs produce lightning. Lightning strikes then trigger more fires as the clouds move across the landscape.
The other aspect of these fiery clouds that perpetuates an ongoing disaster is their ability to spread burning embers over large areas. The pyroCbs are hot and chaotic inside, with associated wind speeds that can reach tornado strength. When a plume collapses, burning embers shoot out in all directions, literally raining fire on the surrounding countryside. This method of spreading embers wreaks havoc with any ongoing fire suppression efforts. The role pyroCbs play in wildfire disasters cannot be ignored, and research into them is a developing field of interest for both firefighters and climate scientists.
Hot, dry, and windy: Australia’s wildfires explode (Source: ArcheanWeb) – https://archeanweb.com/2020/01/07/hot-dry-and-windy-australias-wildfires/ Also:
Fire-Induced Storms: A New Danger from the Rise in Wildfires (ED STRUZIK – JANUARY 24, 2019 – Yale Environment 360) – https://e360.yale.edu/features/fire-induced-storms-a-new-danger-from-the-rise-in-wildfires Also:
Cumulonimbus clouds (Met Office) – https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/clouds/low-level-clouds/cumulonimbus Also:
Feature Image: Pyrocumulus Cloud or Pyrocumulonimbus ice capping at +-25k feet over the Mazatzal Wilderness during the Willow Fire near Payson, AZ. Shot from Mt Ord. (Eric Neitzel) – (Modified) – This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license. – https://creativecommons.org/licenses/by-sa/3.0/deed.en