On Monday, the National Weather Service in Medford, Ore. shared stunning images of pyrocumulus clouds over the Oregon Gulch wildfire. The photos were captured in-flight by the Oregon Air National Guard.

Pyrocumulus clouds form when intense heat at the ground causes air to rise rapidly. This usually happens in the event of a wildfire or volcanic eruption. The clouds themselves form in the way that all clouds do — as the air rises, water vapor condenses into tiny droplets, which form the cloud. While typical cumulus clouds appear puffy and white, pyrocumulus can take on a grey appearance due to the ash and smoke within the cloud. In the case of exceptionally strong updrafts, pyrocumulonimbus clouds are possible, which can produce rainfall and lightning.

The Oregon Gulch Fire is part of the larger Beaver Complex fire, located near the Siskiyou Mountains along the Oregon-California border. The fire has burned over 36,000 acres since it was started by lightning in late July. 30 percent of the complex is contained as of Tuesday morning. A red flag warning has been issued for the area through Tuesday night as scattered thunderstorms — which could produce lightning and 30 to 40 mph gusts — could combine with very dry ground to ignite more fires.