Last edited 20 Sep 2022

Pyro-cumulonimbus plumes

1 Introduction
2 Examples
3 Comment
4 Climate change
5 Related articles on Designing Buildings
6 External references

[edit] Introduction

Pyro-cumulonimbus plumes, often called pyroCb for short or more formally, cumulonimbus flammagenitus clouds (CbFg), are clouds generated by extreme heat released at the surface of the earth, such as from a volcanic eruption, or a wild fire, in some cases enhanced by localised industrial combustion emissions. They can have a distinctive grey colour because smoke particles, which can lead to thunderstorms as water condenses on their surface, and they can rise up high into the upper troposphere or lower stratosphere.

[edit] Examples

One of the earliest photographs of a major pyroCb occurred in Hiroshima, some time after the atomic bomb and its infamous atomic cloud in 1945. It was caused by the devastating ground level firestorm that jointly killed over 30% of the population of the city.

In 1991, sometime after the eruption of Mount Pinatubo in the Philippines, volcanic thunderstorms formed near its summit, but also in areas away from the summit, near smaller localised heat sources. These clouds produced rain or mud fall as it contained ashes from the initial eruption.

In 2003, 2009 and 2019 extreme summer wildfires in Australia created what were reported as fire tornadoes and significant lightening clouds throughout the period when the fires burnt. Similar incidences occurred in the US and Canada and elsewhere in 2020-2022, so much so that pyroCb's are often now referred to more commonly as fireclouds.

[edit] Comment

In 2021 a a meteorologist at the US Naval Research Laboratory noted about fireclouds that:

“You can think of them as like giant chimneys, funneling smoke that's being released by the fire up into a thunderstorm... this extremely dirty thunderstorm, with all these smoke particles for water to condense on.”.. though, the resulting water droplets don’t tend to get large enough to fall as rain. “But it is a cloud that can produce a lot of lightning,” These clouds can then advance across the landscape, sparking new wildfires as they go. So not only can the blaze propagate itself by flinging embers ahead of the main fire line, it can also produce so much hot, rising smoke that it in essence recruits the atmosphere to light more fires.

[edit] Climate change

The Fourth National Climate Assessment (NCA4), Climate Science Special Report Volume I noted that climate change enhances the drying of organic matter in forests (the material that burns and spreads wildfire), and has doubled the number of large fires between 1984 and 2015 in the western United States. The top 10 years with the largest annual acreage burned all occurred since 2004, a period which coincides with many of the warmest years on record across the US and worldwide.

Severe drought, heat and low humidity become more common and extreme as the climate warms, these hot and dry conditions are often referred to as fire weather. Scientists can measure changes in fire weather (temperature, humidity, rainfall and wind) to rate the level of danger of a wildfire striking.

The Fire Weather Index (FWI) is a meteorologically based index used worldwide to estimate fire danger. It consists of different components that account for the effects of fuel moisture and wind on fire behaviour and spread. The higher the FWI, the more favourable the meteorological conditions are for triggering a wildfire.