In very large wildfires there’s a phenomenon that can sometimes happen called “pyrocumulous clouds.” They can be greyish to a dirty white and look like a thunderstorm hovering over the fire area. According to the Web site the Weather Notebook, the fire basically creates its own weather system. Moisture is released from the burning vegetation and is carried aloft along with the smoke and ash by the rising heat to altitudes up to 50,000 feet. Sometimes it can look like a mushroom cloud from a nuclear explosion, which are also considered pyrocumulous clouds.
Erupting volcanoes experience a similar effect and “thunderstorm” is an apt description because the clouds can create their own lightening as well. At times rain can even occur which can be helpful in fighting the fire.
By any measure the Rim Fire near Yosemite National Park is a very large wildfire. According to the U.S. Forest Service incident information Web page it has consumed 105,620 acres of oaks, pine and brush, forced the closure of Highway 120 and is threatening about 4,500 structures.
I haven’t been to the fire but I’ve seen the images that have come across our photo desk and some show the classic pyrocumulous cloud formations looming over the ravaged fire area.
From a distance the pyrocumulous clouds have an ominous beauty, like a Steven Spielberg-esque special effect, but they can also create winds and down drafts that can cause the fires to switch directions unpredictably and make a dangerous situation even more so.