Mosquito fire doubles in size west of Lake Tahoe, sending a pyrocumulus cloud skyward. (Credit: NASA/University of California Santa Barbara) In August 2013, firefighters at the American Canyon Lake near the city of Placerville, California responded to a large pyrocumulus cloud — a layer of water and ash that rose from a fire in Lake Tahoe’s North Shore. The fire, which was burning around a volcanic plug, caused a lightning-like phenomenon, creating a pyrocumulus cloud that was almost 100 feet (30 meters) thick and reaching nearly 50 miles (80 kilometers) away into the sky above.
The U.S. Forest Service’s National Interagency Fire Center (NIFC) reported that the fire was estimated to have burned over 20,000 acres (80 square kilometers) and had covered some 2,800 homes. As the smoke cleared, the U.S. Air Force sent a pair of Air Force C-130 Hercules aircraft to drop a pair of 500-lb. (227-kilogram) fire retardant bombs onto the fire. The first two of these bombs fell on the fire while the second came down on the lake near American Canyon. The following morning another fire retardant bomb landed near the Placerville Municipal Airport. After nearly a week spent using chemical retardant to fight the fire, the fire was finally declared out on September 8, 2013. No homes were destroyed, but the fire damaged approximately 3,000 homes.
Pyrocumulus clouds can also occur in a number of other contexts during natural lightning-caused fires, most notably lightning storms. When lightning strikes a forest canopy, it creates highly charged particles that can travel through the trees and reach a water vapor source or other moisture source. The particles also transfer a charge to any moisture source in their path, like rainstorms, clouds, or the soil below a forest floor. In a lightning-caused fire that occurs near a forest canopy, pyrocumulus clouds can occur when the charge from lightning reaches a forest canopy and transfers through the trees and ultimately to a water source, such as a lake or river. This particular fire was also unusual because a pyrocumulus cloud formed in a lightning-caused fire without any warning, which