A pyrocumulus cloud is seen during the Aggie Creek Fire near Fairbanks, Alaska, on July 11, 2015.Wildfire season is underway in the West, and there are already several large fires in Arizona, California and Colorado.
Hot, dry weather and increasing drought conditions across the western United States this summer could result in above-average wildfire potential into September, according to the National Interagency Fire Center (NIFC).
Here's a list of facts you might not know about the relationship between large wildfires and weather.
(MORE: Where the Worst Wildfire Activity Is Expected This Summer)
1. Wildfires can create their own weather. Vigorous, fire-generated updrafts and strongly turbulent eddies can form along a fire plume’s edges, and fire-modified winds can extend more than a mile from the blaze, concluded a study published by Neil Lareau and Craig Clements from San Jose State University.
One example of weather created by intense wildfires is the formation of pyrocumulus clouds.
An example of a wildfire smoke plume and pyrocumulus clouds from the Fort McMurray wildfire on May 3, 2016.The heat of the fire can lead to rising air, which cools and condenses as it rises. This results in a puffy cumulus cloud white, gray or brown in color, which can sometimes produce localized strong winds, lightning and little rain. Pyrocumulus clouds can also occur with volcanic eruptions.
In July 2018, a giant fire whirl generated by the Carr Fire in Redding, California, produced wind damage equivalent to an EF3 tornado, a National Weather Service investigation found.
The preliminary damage survey conducted by a team from the NWS office in Sacramento and Cal Fire found that damage produced within the large tornadolike vortex generated by the Carr Fire in Redding suggested its top winds exceeded 143 mph.
2. The West is the U.S. region most prone to wildfires. According to the U.S. Geological Survey (USGS) map below, the number of large wildfires – classified as 300 acres (0.5 square miles) or bigger – was the highest in the West from 1994 to 2013.
Every Western state except Wyoming has at least one county in the category with the highest wildfire frequency (between 101 and 1,308 fires). When looking at the second-highest wildfire frequency category (21 to 100 fires), you can see that the Western states stand out with a large number of counties included.
Wildfire activity by county (1994-2013)The West is so prone to wildfires because many parts of the region have distinct wet and dry seasons. In California, for example, over 90% of the annual average precipitation falls from October through April. May through September is the state's dry season, when little rain falls. This causes the ground to become very dry for nearly half the year, making it conducive for the rapid spread of wildfires.
3. The NWS issues red flag warnings to warn the public of conditions that are conducive to the rapid spread of wildfires. A red flag warning means critical fire weather conditions are either occurring or are imminent. A combination of strong winds and low relative humidity can contribute to extreme fire behavior.
4. Most large wildfires are human-caused. According to the NIFC, 85% of wildfires are human-caused, while only 15% are ignited by lightning.
5. But the small number of wildfires triggered by lightning burn more acreage than those started by humans. According to the NIFC, 60% of acres burned are by lightning-caused wildfires, while human-caused fires burn the remaining 40%.
6. Lightning-caused wildfires have been increasing 2 to 5% per year since 1975, according to a NASA study published in the journal Nature Climate Change.
7. Since 2000, the year with the greatest number of wildfires was 2006, with 96,385 fires, according to NOAA. A total of 15,428 square miles were burned by those fires. Dry conditions plagued the southern, central and western U.S. much of that year, making conditions ripe for the ignition of wildfires.
8. 2015 had 34,463 fewer fires than 2006, but that year had the most square miles burned since 2000: 15,821 square miles. The wildfires raged in places that had a dry winter followed by hot temperatures during the summer and early-fall dry season.
The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.
The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.
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