Across much of the United States Northwest, the last several summers have been hot and smoky. While hot and smoky may be the ideal environment for cooking savory victuals inside the contained inferno of the grill, living through hot and smoky conditions week after week is no picnic. This year has been a different story. This summer the Northwest has been mostly free from incessant smoke and burning. Mostly, but not entirely.
As of August 26th, a total of 470 thousand acres have burned in 2019 across the Northwest. (Here we define our region as encompassing the states of Washington, Oregon, Idaho, and Montana.) If we examine the statistics of burned area, comparing a few years with very large totals to other years with small totals, it would be foolish to label as ‘normal’ or ‘average’ any individual fire season. So, while we won’t call this a ‘normal’ or ‘ average’ fire season, we can certainly call it a very welcome reprieve from the hot and smoky summers we’ve seen in recent years.
The largest fires we’ve seen this summer have occurred primarily in the sparsely populated, drier parts of our region, including the Williams Flat Fire on the Colville Reservation in northern Washington that burned 45,000 acres of grass-pine savanna, the Sheep Fire that burned 112,000 acres of grasslands in eastern Idaho, and the Cold Creek Fire that burned 42,000 acres of grasslands and sagebrush in central Washington.
While these numbers may look really big, in reality they are pretty typical for the large wind-driven fires that tend to occur in the drier, grassier reaches of our region. Nonetheless, these fires left impacts.
Smoke from the Williams Flat Fire brought Spokane the “worst” air quality in the country for a brief period in the first week of August. But things could have been worse.
For proof look no further than last summer when over 2 million acres burned in our four-state region and over 3 million additional acres burned in British Columbia. Or consider the summer prior to that when over 3 million acres burned in our four states and over 3 million additional acres burned in British Columbia.
So, why was this year so mellow for fires compared to years past? To get that answer, let’s talk climate. (After all, this is Northwest Climate Currents.)
In a previously installment of this wonderfully entertaining newsletter, we described the Energy Release Component (ERC), a measure that is frequently used to determine fire potential. ERC essentially tells us if conditions are dry enough for a fire to ignite given the right ignition source (for instance, a lightning strike, or human negligence personified in the form of a stray, still-smoldering cigarette butt).
As it happens, CIRC with support from the National Integrated Drought Information System (NIDIS) program has been developing different tools for assessing fire potential. You can, for instance, examine current (and projected future) ERC conditions using CIRC’s Climate Mapper Tool, part of our Northwest Climate Toolbox. Our hope is that the Toolbox and posts like this one will help disseminate climate information that will be used by foresters, firefighters, water resources managers, farmers, you name it. Our latest contribution to this effort has been to add to the Climate Mapper a variable called Vapor Pressure Deficit.
Okay, okay, the name Vapor Pressure Deficit is admittedly a bit jargony, but the concept is quite simple and it has direct relevance to our discussion on fire potential and why this summer seemed like such a breath of fresh air compared to the smoke-filled days of recent yester-summers.
Vapor Pressure Deficit (VPD) like ERC is a way to determine if conditions are dry enough for a fire to start given an ignition source. ERC is currently widely used by multiple agencies to assess fire danger. VPD is not. However, it could be. So, for our readers in fire management, consider what follows as a kind of pitch for using VPD when monitoring fire conditions.
In a nutshell, VPD represents how large of a difference there is between the moisture content of the air and how much moisture the air can hold when saturated. Think of it as a way to measure atmospheric dryness. The higher the deficit, the drier the air. The concept here is similar to that of relative humidity, which is a ratio of the moisture content of the air and how much moisture the air can hold, but there are some important differences between VPD and relative humidity that should be explained.
The amount of moisture the air can hold increases exponentially with warming. Warm air can hold more water vapor than cool air can. For instance, 90 degree Fahrenheit air holds approximately twice the moisture of 70° F air and four times the moisture of 50 °F air. An air mass with a constant relative humidity will therefore have escalating VPD with warming. But here’s the catch. Relative humidity fails to reflect this effect, meaning in some instances the air can actually be drier in reality than it appears to be if looked at using relative humidity alone. By contrast, using VPD gives you something closer to the actual dryness of the air.
All this matters because VPD has direct effects on fine fuel moisture and fire potential. VPD also varies daily, meaning daily VPD values can give you a pretty good sense of how high the fire danger is on any given day.
For the purpose of this explainer, we’ll focus on the cumulative effects of VPD leading into this year’s fire season, which here in the Northwest generally runs from June to September. The cumulative effects we will be examining have been shown in several studies to exhibit strong correlations with burned area in the forests of the western US, California, and the globe.
Similarly, a recent analysis we performed for the state of Washington found that VPD was a good indicator of fire potential. Our analysis looked at annual burned area in large fires (1000+ acres) in forested portions of Washington from 1984 to 2016. Our research revealed that VPD averaged for the period May to September explained about half of the year-to-year variability in burned area.
(By the way, the units of Vapor Pressure Deficit are, not surprisingly, pressure (Pascals). If you are a person who goes outside on a dry day and talks about dryness in Pascals, good on you! For everyone else, we translate this data into more usable information using percentiles.)
Another way to slice the VPD data is to bin the data by ranking VPD and looking at how burned area shaped up for the bottom third, middle third, and top third of years. The figure below tells the story.
If we take those same years used in our study (1984–2016) and rank the VPD measurements from those years, we find that the top third (tercile) of years shown here in red (e.g., 2015, 2014, and 2003) burned on average a whopping 75 times more forested land in Washington than years in the bottom third of years shown here in blue (e.g., 2010 and 2011). VPD for the years that fell into the top third was very warm and dry producing conditions ripe for wildfires. The opposite was true for the bottom third of years, VPD was low, temperatures were cool, and conditions were not conducive to wildfires.
Okay, you’re probably thinking, so, warm seasons with high VPD start more fires, right? Not so much. Lightning and humans start fires. But weather and climate do strongly influence how dry fuels are, how challenging fires are to control, and how much of the landscape fires can carry over.
So, given what we know about VPD, let’s look at this year’s VPD conditions. (Again we’ll give the deets in percentiles not Pascals.)
The figure above shows the percentile of VPD for the last 90 days relative to the same 90-day period covering the 1979–2015 observational record. Note that much of the Northwest—outside of a stretch from northeast Washington state to northwestern Montana— is ranked in the bottom third for VPD. To spell it out—that is if the color scheme reminiscent of the underwater sequences in the recent “Aquaman” movie weren’t clue enough—conditions during this 90-day period weren’t exactly conducive to drying out fuels. The air during this period was moist enough to make a randomly sparked wildfire unlikely. Or, to put it in terms of the VPD measure, our regional air during this period wasn’t exactly suffering from a water vapor deficiency.
Interestingly enough, precipitation totals over this same 90 day period have generally been below normal across the broader region, which would typically favor fire activity. The difference this year has been how much cooler temperatures have been, particularly for daytime highs, relative to our recent torrid summers.
This year’s cool summer has halted drought progression across the region and staved off widespread fuel desiccation. Which is some welcome news given our recent summers in general. The Northwest has seen few coolish summers in recent decades, and they’ll become increasing rare in the decades to come. However, this year’s fire season is not done across our region, and we can’t close the book on it just yet. So, as we honor Smokey Bear’s 75th year of existence, let’s be sure to blow out those celebratory candles and heed his wisdom and leave the starting of wildfires to lightning. And as we do so, let’s be sure to savor summers like this one. It has been a very welcome reprieve.
This post is part of Northwest Climate Currents, an ongoing series that uses the Northwest Climate Toolbox and the data it collects to help us understand and prepare for our region’s climate events. The Northwest Climate Toolbox is a suite of free online applications designed by CIRC researchers and intended to help foresters, farmers, and water managers respond to and prepare for climate variability and change and related impacts.
Acknowledgements: The Northwest Climate Toolbox is funded in part through the NOAA Regional Integrated Sciences and Assessments (RISA) program and National Integrated Drought Information System (NIDIS).
- “Northwest Climate Currents, March 2019”
- “Drought Returns to the Pacific Northwest”
- “Spring Snowpack Numbers”
- “Winter Snowpack Numbers”
- “Northwest Climate Recap”
- “Fire Season is Here!”
- Vapor Pressure Deficit (VPD)— a measure that represents how large of a difference there is between the moisture content of the air and how much moisture the air can hold when saturated. Essentially a way to determine the actual dryness of the air, VPD is used to determined fire potential.
Pics and Figures:
Figure One: Annual forest area burned (in thousands of acres) in the state of Washington for the years 1984–2016 binned by summer Vapor Pressure Deficit (VPD). Here years are ranked into thirds. The top third of years with the highest VPD are shown in red. The middle third of years are shown in yellow. The bottom third of years with the lowest VPD are shown in blue.
Figure Two/Featured Image: Percentile of VPD for the last 90 days (5/28/2019–08/25/2019) relative to the same 90-day period covering the observational record (1979–2015).
Citation: Hegewisch, Katherine C., John T. Abatzoglou, Bart Nijssen, Liz Clark, Hordur Bragi, Oriana Chedwiggen, Nathan Gilles, and Holly Hartmann. The Climate Mapper web tool, The Northwest Climate Toolbox. The Pacific Northwest Climate Impacts Research Consortium, a NOAA RISA Team. Imaged created on May 28, 2019.
John Abatzoglou has been a CIRC team member since 2010. A climate and meteorology researcher at the University of Idaho and self-described “weather weenie,” John leads CIRC’s Northwest Climate Toolbox effort. He has participated in the creation of several CIRC-related Climate Tools, including Climate Engine and Integrated Scenarios. More Posts by this Author. Research by this Post’s Author, John Abatzoglou.
Nathan Gilles is the managing editor of The Climate Circulator, and oversees CIRC’s social media accounts and website. When he’s not writing for CIRC, Nathan works as a freelance science writer. Other posts by this Author.