Smoke and heat from a huge wildfire in south-eastern Oregon are creating giant “fire clouds” over the blaze – dangerous columns of smoke and ash that can reach up to six miles (10km) in the sky and are visible from more than 100 miles (160km) away.
Authorities have put these clouds at the top of the list of the extreme fire behavior they are seeing amid the Bootleg fire, the largest wildfire burning in the US. The inferno grew on Friday to about 377 sq miles (976 sq km), an area larger than New York City, and was raging through a part of the American west that is enduring a historic drought.
Meteorologists this week also spotted a bigger, more extreme form of fire cloud – ones that can create their own weather, including “fire tornadoes”.
Extreme fire behavior, including the formation of more fire clouds, was expected to persist on Friday and worsen into the weekend. There are currently at least 70 wildfires burning in the western United States and dozens more in Canada.
Devastatingly high temperatures are also expected to bake the west through the weekend, from the central Rockies into southern Canada, as the region braces for the fourth heatwave in five weeks. Forecasts estimate that highs across the region will be 20 to 30F higher than average for this time of year. By Monday, Bozeman, Montana may see temperatures reaching up to 107F (41.6C) – the hottest temperature the city has ever recorded.
The heat will complicate firefighting efforts and increase the threat of new ignitions.
Firefighters were scrambling on Friday to control the raging Bootleg fire, which is spreading miles a day in windy conditions. Authorities have ordered a new round of evacuations amid worries that the inferno, which has already destroyed 21 homes, could merge with another blaze that has also exploded in size.
Pyrocumulus clouds – literally translated as “fire clouds” – look like giant, dirty-colored thunderheads that sit atop a vast column of smoke coming up from a wildfire. Often the top of the smoke column flattens out to take the shape of an anvil.
In Oregon, fire authorities say the clouds are forming between 3pm and 5pm each day as the sun penetrates the smoke layer and heats the ground below, creating an updraft of hot air. Crews are seeing the biggest and most dangerous clouds over a section of wilderness made up mostly of dead trees, which burn instantly and with a lot of heat.
For four days in a row, the Bootleg fire has generated multiple fire clouds that rise nearly six miles into the atmosphere and are “easily visible from 100 to 120 air miles away”, authorities said on Friday. The conditions that create the clouds were expected to worsen over the weekend.
When a pyrocumulus cloud forms over a fire, meteorologists begin to watch carefully for its big brother, the pyrocumulonimbus cloud. Nasa has called the latter the “fire-breathing dragon of clouds” because it is so hot and big that it creates its own weather.
In a worst-case scenario, fire crews on the ground could see one of the monster clouds spawn a “fire tornado”, generate its own dry lightning and create dangerous hot winds below. The clouds can also send particulate matter from the smoke column up to 10 miles above Earth’s surface.
So far, most of the clouds on the Bootleg fire have been the less-intense fire clouds, but the National Weather Service on Wednesday spotted a pyrocumulonimbus cloud forming in what it called “terrifying” satellite imagery.
“Please send positive thoughts and well wishes to the firefighters ... It’s a tough time for them right now,” the weather service said in a tweet.
Meanwhile, a fire near the northern California town of Paradise, which was largely destroyed in a 2018 wildfire that killed 85 people, worried homeowners who were just starting to return to normal.
And in Washington, a wildfire threatened more than 1,500 homes near Wenatchee, growing to 14 sq miles (36 sq km), and crews had little control over it, the Washington state department of natural resources said.
Extremely dry conditions and heatwaves tied to climate change have swept the American west, making wildfires harder to fight. Climate change has made the region much warmer and drier in the past 30 years and will continue to make weather more extreme and wildfires more frequent and destructive.
This article was amended on 19 July 2021. An earlier version incorrectly stated that the 2018 fire in Paradise, California was the deadliest in US history.