Minnesota’s Greenwood Fire Generates ‘Fire-Created Lightning’ and Pyrocumulus Cloud
The aggressively-growing Greenwood Fire near Isabella has been expanding at a scary pace, reaching 19,493 acres in the 9 days that it has been burning. Road closures and evacuation orders have been implemented for the safety of the public in the area, but the size and intensity of the fire has brought about new concerns for the safety of the firefighters battling the blaze as well.
You've probably heard some semblance of the phrase "fire can create its own weather". The Greenwood Fire has exhibited some of that, including the generation of something called pyrocumulus clouds, which can develop "fire-created lightning". The US Forest Service explained that they had to pull crews off the fire Monday due to these threats.
The Duluth office of the National Weather Service explained the phenomenon, using a photo from the Greenwood Fire to illustrate some of what is going on.
What are pyrocumulus clouds?
As the National Weather Service explains, these are clouds formed by fires. Similar to how normal cumulus clouds form, heated air raises into the atmosphere - in this case, over an intensely-burning fire.
The smoke provides more airborne particles for water vapor to condense onto as the smoke particles travel upward. Moisture drawn out of burning vegetation is also drawn into the rising air. As these particles lift into the air and water vapor condenses onto them as the particles and water vapor cool away from the fire, clouds form above the smoke plume.
Why are pyrocumulus clouds potentially dangerous?
As these clouds develop, they continue to draw more air into the fire. Imagine a balloon letting air out against a surface, but only in reverse. The upward draw of the air into these clouds draws more air into the fire. This provides more oxygen, which fuels the fire, similar to blowing on a campfire.
As these clouds grow, they have the potential to become unstable and collapse. This, similar to what happens with some thunderstorms, creates downdrafts. Using the balloon analogy from before, it is like having a balloon full of air, and letting the air out over a table. That air hitting the table's surface and blowing outward across the table is like the downdraft from a collapsing pyrocumulus or pyrocumulonimbus cloud.
As these clouds grow and develop, they also have the potential to generate dry lightning or fire tornadoes, again similar to how thunderstorms function in some ways.
What is 'dry lightning' or 'fire-created lightning'?
As mentioned before, the US Forest Service pulled crews off the attack Monday on the Greenwood Fire over concerns for safety as the pyrocumulus cloud over the fire. One of the concerns included 'fire-created lightning', which is lightning generated by the electric charge generated in growing pyrocumulus clouds.
At its most basic level, lightning is essentially the release of a giant static charge kind of like what you might get from dragging your stocking-covered feet over carpet. As particles move around in a storm cloud, they exchange electrons and generate that charge in different parts of the cloud.
Eventually that charge reaches a point at which it needs to be released, which is when the "grabbing the doorknob" part of the static charge analogy takes over, and lightning happens to balance out the otherwise imbalanced electrical charges. This not only happens within normal thunderstorms, but can also happen within pyrocumulus clouds as they grow as well.
As with normal storm clouds, sometimes this exchange happens within the cloud, but other times it goes from cloud to ground. In the case of lightning generated by a pyrocumulus cloud, this not only creates a danger for firefighters in the air and on the ground, but it can also spark additional fires.
The term "dry lightning" is more specifically related to when there is no accompanying rain, as the name would suggest.
Can these pyrocumulus clouds bring rain?
They can, though not always. In some cases rain may fall and then evaporate as it falls toward the intense heat, further growing the cloud formation cycle and generating wind that then further fuels the fire.