People here in Sacramento, California, learned a new word last week: “pyrocumulus.” It’s a type of cumulus cloud that can form over a wildland fire when conditions are right. Well, conditions were right on September 17 when the King Fire more than doubled in size in one day. (It’s called the King Fire because it started near King of the Mountain Road in Pollock Pines, about 60 miles east of Sacramento.)
The Wikipedia definition of a pyrocumulus is:
produced by the intense heating of the air from the surface. The intense heat induces convection, which causes the air mass to rise to a point of stability, usually in the presence of moisture…
Pyrocumuli contain severe turbulence, manifesting as strong gusts at the surface, which can exacerbate a large conflagration. A large pyrocumulus…may also produce lightning. A pyrocumulus which produces lightning is actually a type of cumulonimbus, a thundercloud, and is called pyrocumulonimbus.
One person from Calfire said in a news report that the clouds can collapse quickly, too, sending embers out in several directions.
Conditions in the Sierra Nevada foothills are so dry, after three years of drought and almost no snow last winter, that the fire just took off, increasing from about 28,000 acres to over 70,000 acres in one day. The winds shifted in the days after the pyrocumulus, slowing the fire’s expansion and sending smoke out over the Sacramento Valley and the foothills. Still, in less than a week the fire burned more than 80,000 acres (or 120 square miles) of forest, as well as a number of homes.
After decades of fire suppression in California, the forests are thick with brush (where they haven’t been clear cut). It will take crews from Calfire and the US Forest Service weeks to put this fire out. The worst part? The fire was apparently deliberately set. A man has been arrested for arson.
Fire is part of the natural cycle in California. People who live in the foothills know it could happen in any year, dry or no. And we could manage the forests better, leave the oldest trees, which are most fire-resistant, and either burn or cull the understory more. The forests used to burn every decade or so. But we can’t really let these fires burn now—there’s too much fuel. Just like the King Fire.
Do we also have climate change? I think so, though no one can say for sure yet. National Geographic speculated on this recently.
Have you been affected by fire in the West? Have you seen a pyrocumulus cloud?