Scientists have long known that dry lighting could cause major wildfires. However, until recently, such lightning strikes were thought to pose wildfire risks only if occurring with less than 2.5 mm of precipitation in a day (approximately 0.10 inches).
Now, a team of researchers led by Washington State University (WSU) has found that dry lightning can still be disastrous when weather conditions are not so dry. The experts report that fires can be ignited by lightning even if precipitation is up to 7.7 mm (about 0.3 inches).
These more accurate estimations could help responders detect fires earlier, particularly those known as “holdovers,” which can smolder for many days before turning into full-blown wildfires.
“The rainfall amounts we quantified should help provide a better understanding of just how much rain can fall and still pose a fire risk,” said lead author Dmitri Kalashnikov, a doctoral candidate in Environmental Sciences at WSU.
Kalashnikov and his colleagues analyzed data on over 4,600 naturally caused wildfires compiled by the National Interagency Fire Center across Western U.S from 2015 to 2020, and matched 3,726 of those to the lightning strikes that most likely started them by using National Lightning Detection Network data.
The investigation revealed that 15.3 percent of these were holdover fires, representing more than 100 fires annually.
By analyzing precipitation data at the time of the lightning strikes, the experts found that rainfall was often higher than previously thought among the earlier undetected fires, ranging from 1.7 to 4.6 mm (0.07 inches to 0.18 inches).
Moreover, holdover fires tended to be ignited with even greater precipitation of 3.0 to 7.7 mm (0.12 to 0.3 inches).
Although humans remain the leading cause of most wildfires (either by accident or arson), fires caused by lightning burn the most acreage.
For instance, previous research has shown that almost 70 percent of the wildfire-burned land in the American West was caused by lighting, while the largest wildfire burn area in California history occurred in August 2020 after dry lightning sparked multiple fires at the same time.
Moreover, dry lighting can also lead to wildfires in remote places that are difficult for firefighters to reach, while holdover fires pose an additional problem since they are so difficult to detect in their early stages.
For example, the scientists found that the highest rate of holdovers occurred in the forested mountains of the Southwest, as well as in the middle and southern Rocky Mountains.
Such forested regions are especially vulnerable to these types of fires because lightning often ignites the leaves and twigs on the forest floor that is sheltered from rain (and from view) by the branches above.
“Holdovers are extra sneaky because lightning can start a fire, and it might just kind of smolder for a day or two or sometimes a week or more until conditions are right for fire to spread. So the lightning storm may have passed a long time ago, and you might think there’s no danger, then all the sudden, the fire blows up,” Kalashnikov explained.
Further research is needed to devise more efficient ways to prevent the worst consequences of such fires by better understanding their causes and development and implementing accurate early detection mechanisms.
The study is published in the journal Geophysical Research Letters.
“Dry lightning” refers to lightning strikes that occur without significant accompanying rainfall. Here are some key points about dry lightning:
Dry lightning usually happens in thunderstorms where the lower layers of the atmosphere are too dry to support rainfall. Any rain that does form in the upper atmosphere tends to evaporate before it reaches the ground, a phenomenon known as virga.
Dry lightning is particularly concerning in forested or grassy regions because it can ignite wildfires. Since there’s no rain to dampen and suppress the fires, they can spread quickly.
Certain regions, especially in the western United States, are more prone to dry lightning. In these areas, moisture can get lifted into the upper atmosphere, creating thunderstorms, but the lower levels remain dry due to geographic and climatic factors.
Just like any other lightning, dry lightning is dangerous. Even if it doesn’t appear to be stormy in your immediate vicinity, lightning can strike from a storm several miles away. Always seek shelter in a safe location during thunderstorms, regardless of whether it’s raining.
Meteorologists monitor atmospheric conditions to predict the likelihood of dry lightning, especially in areas prone to wildfires. When conditions seem favorable for dry lightning, warnings may be issued to alert local authorities and residents.