Dry thunderstorms are “high elevation” thunderstorms, in which the cloud base is so high that — especially in the West, where conditions often are warm and dry — rain evaporates before it hits the ground. That means there’s dry lightning and thunder, but no rain hitting the ground, said Craig Clements, San Jose State University professor of meteorology and director of the school’s wildfire research center.
The National Weather Service noted concern in a Tuesday fire weather outlook about dry thunderstorms in a swath of the West, including large parts of Oregon, Idaho and Montana.
Because the phenomenon seems counterintuitive, there’s an “air of mystery surrounding dry thunderstorms for folks who’ve never really encountered them,” said Daniel Swain, a climate scientist at the University of California at Los Angeles.
Dry thunderstorms are rare in most places east of the Rockies, he said, because there’s “always enough moisture for rain to fall from them or for precipitation in some form to fall from them anyway, in the eastern two-thirds of the U.S.”
He added: “That’s not always the case in the western third of the U.S.”
These dry thunderstorms develop much higher in the atmosphere than normal thunderstorms — sometimes 10,000 to 15,000 feet above the surface.
“So instead of having 2,000 or 3,000 or 5,000 feet to fall. … The rain has to fall three times the distance,” Swain said. “And if that layer under the cloud base is somewhat dry, as it often is out West, a lot of that water can evaporate before it actually hits the surface. And in the case of dry thunderstorms, it can evaporate completely.”
That leaves just the lightning.
Whether a dry lightning strike can spark a flame depends on the environmental conditions, Clements said.
“The dry lightning is just a source of ignition,” he said.
In the West, conditions have been ripe for wildfires. Red-flag warnings — which denote high fire danger — have been issued in much of the northwestern part of the United States. More than 80 large fires are already active, mostly in Western states, according to the National Interagency Fire Center. The region has been plagued this year by historic drought conditions and major heat waves exacerbated by climate change.
Swain said more research could be needed to determine whether climate change is affecting the likelihood or frequency of dry thunderstorm events. But where the changing climate does have an effect, he said, is on wildfire risk.
“If the vegetation is drier due to climate change, as it is, then that increases both the likelihood of igniting a fire given a dry lightning strike, and it also increases the potential extremity of the fire behavior that might occur,” he said.
And if there are “hundreds or even thousands of strikes” in an area, which can sometimes happen during dry thunderstorm events, “you’re almost guaranteed to have many new fires,” Swain said.
In its fire outlook, the National Weather Service warned Tuesday that it was highlighting dry thunderstorms because of “critically/record dry fuels across the region.”
Dry thunderstorms can also exacerbate existing fires.
The storms, Swain said, can produce “downdrafts and sometimes very strong, gusty microburst-type winds.”
“If there’s already a fire burning and suddenly you get very high winds out of completely different directions … that can be really dangerous for firefighters and for populated areas near where and when that’s occurring,” he said.
Swain said additional fires may ignite because of “extremely dry” vegetation in areas of the West where there’s concern for more dry thunderstorms.
“There’s already a lot of fires burning, and this will just add to that,” he said.