The Earth Story

A unique view of a fire

We’ve discussed how fires can trigger cloud formation multiple times here at The Earth Story. Clouds form above fires and volcanic eruptions when massive amounts of heat cause a mixture of air and hot ash to be pushed upward to the level in the atmosphere where water begins condensing on the particles, creating a fully formed cloud in the sky where otherwise there would not have been one. Because of this unique method of formation, these clouds were named flammagenitus clouds in 2017.

This view of a flammagenitus cloud was taken out the window of an Oregon National Guard F-15c fighter jet flying near a wildfire in 2014. You can clearly see how the ash forms a column heading upwards through the atmosphere; the heat of the fire has warmed so much air that it is rising straight up through the atmosphere to the level where water begins condensing and the cloud changes to a white color.

-JBB Image credit: Oregon National Guard/Nasa EOhttp://earthobservatory.nasa.gov/NaturalHazards/view.php?id=84129 Reference: https://www.theweathernetwork.com/news/articles/cloud-atlas-leaps-into-21st-century-with-12-new-cloud-types/80685/

Pyrotornadogenesis

This incredible video was shared by an ABC News affiliate in California earlier this year during the Carr Fire, which hammered the community of Redding, California during one of many devastating wildfires in that state this year. During many intense wildfires, small “fire tornadoes” or “Fire whirls” are observed when rapidly rising hot air begins rotating in a column. Here’s one video I shared of one a few years ago: https://tmblr.co/Zyv2Js2Av4RcN. However, there’s a huge difference between the thin column of fire in that video and what happened during the Carr Fire. The Carr Fire produced a huge vertical column, with winds comparable to an F3 tornado. A new study of the data collected during this event suggests that in fact it was something very different from a simple fire whirl. It was in fact a pyrotornado, produced by the same type of cloud dynamics and wind shear that give rise to regular tornadoes. This event is only the 2nd on record like it – the first occurred during a firestorm in New South Wales, Australia, in 2003.

Most fire whirls are fairly thin columns of hot gas, they rotate as they rise because of angular momentum being conserved in the thin column of rising gas, but they are confined to layers of air near the surface. In this case, this fire set up a full weather system, which created properties in the whole atmosphere much more similar to a tornado.

When this fire tornado began, air was rising rapidly above the area, triggering the formation of clouds known as pyrocumulonimbus clouds. These clouds are like thunderstorm clouds – high, towering, containing rising air and forming ice crystals – but they are triggered by hot air rising above fires. In this case, the forming pyrocumulonimbus cloud took a small fire whirl and stretched it out vertically into a column just like a normal tornado. It rose to a height of 12 kilometers (over 6 miles) above the surface within about 15 minutes of it first being detected, and its rotation was fed by the same sort of shearing winds at the edge of the cloud that feed tornadoes from thunderstorms.

The combination of shearing winds and a thunderstorm cloud forming created this spectacular vortex, which tore through the city of Redding as a burning column in the air. This fire tornado occurred after a multi year drought was followed by one record wet season and another record dry season – leading to lots of growth of vegetation in the record wet year and a huge amount of burnable vegetation when it dried out. That volatile mix combined with a specific set of weather conditions to trigger this event – by studying the radar signatures of pyrotornadogenesis in Redding, weather forecasters may be able to recognize when those ingredients are present in the future.

-JBB

Video source: https://twitter.com/abc10/status/1022862397780852739?lang=en https://www.youtube.com/watch?v=x1fV5Uq_b4A

Original paper: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL080667?fbclid=IwAR3c6ubet7GxGdirZfcmpvoBQcP_9HqwTVWxfNUj38yEeS1lS76JGVKRXpM Press versions: https://blogs.agu.org/geospace/2018/12/06/scientists-find-causes-of-firenado-in-deadly-carr-fire/?fbclid=IwAR03ZH6bq7Imh1GAZmtE38ZdKwWKEiQHZxUaUrxakvkZG9Qk6iOeD_zdWLQ https://www.sacbee.com/news/state/california/fires/article222687285.html?fbclid=IwAR0Z24UYk4HwQnPfYyIYOxPBs2aACC5g8Vs0HMteos0EhXxpDKOK_0fFjDo

Pyrocumulus

I can honestly say I never heard the term “pyrocumulus” until the 2009 Station Fire in California. If it was used beforehand, maybe it just bounced into and out of my ears, but that’s the first time I really heard it. That fire in the L.A. Basin was enormous and occurred on a day without strong winds, sending thick soot and ash clouds straight up into the atmosphere, impacting breathing conditions as far away as Denver.

This image literally is a textbook-quality illustration of what a pyrocumulus cloud is and how it forms. It was taken by the MODIS instrument on NASA’s Aqua satellite, and shows 4 wildfires burning in the state of Idaho. Each of the 4 fires shown here was started by a lightning strike on August 8, and by August 10th when this image was taken they had grown considerably.

The largest fire of this group is the westernmost Pony Complex Fire, the second largest is the Elk Complex Fire. Take a close look at the smoke clouds billowing from each fire. First you notice they’re all being deflected to the North by the current wind, but there’s something unique about one of them.

The Elk Complex Fire is putting out a much larger cloud that appears white in this image, while the other 3 are putting out thinner, grayish clouds.

The Elk Complex Fire has generated a pyrocumulus cloud. This cloud type is formed when air is heated by a fire (or volcano, etc.) and rises through the atmosphere fast enough that it reaches the level where condensation begins. The ash and dust from the fire serve as seeds on which clouds can form, creating a cloud that is filled both with condensed water, as a normal cloud would be, and ash.

Generally pyrocumulus cloud formation depends on the wind speed; slower winds allow the clouds to form because the hot air from the fire rises up farther before being spread out by the wind. However, here, the 4 fires each are seeing similar wind patterns. The Elk Complex fire must therefore be burning particularly hot, generating enough hot air to push its ash cloud up past the condensation level, whereas the other 3 fires are not.

To finish defining terms, a pyrocumulus cloud can become a pyrocumulonimbus cloud if it produces either rain or lightning, which is rare but can happen (particularly common with volcanoes I believe).

-JBB

Image credit: NASA image courtesy Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team at NASA GSFC http://earthobservatory.nasa.gov/IOTD/view.php?id=81841