“Fire science is not rocket science—it’s way more complicated.”
Robert Essenhigh, Professor Emeritus, Mechanical and Aerospace Engineering, Ohio State University.
A paper has been written recently that sheds light on the role of vapor pressure deficit (VPD) in wildland fire behavior and forest mortality in the Southwest. Most firefighters have not spent much time studying VPD. Typically they are taught that temperature, relative humidity, and wind are important weather variables to monitor, and sometimes dew point comes into the discussion. But this research indicates that VPD is a very important factor that influences fire behavior.
Below are some excerpts from the paper titled “Climatology, variability and trends in United States. Vapor pressure deficit, an important fire-related meteorological quantity”, by Richard Seager, Allison Hooks, A. Park Williams, and Benjamin Cook.
Unlike the commonly used relative humidity, vapor pressure deficit (V P D), is an absolute measure of the difference between the water vapor content of the air and its saturation value and an accurate metric of the ability of the atmosphere to extract moisture from the land surface.
To our knowledge this is the first comprehensive study of vapor pressure deficit (VPD) which was recommended by Anderson (1936) as a more useful measure of the moisture state of the atmosphere than relative humidity (RH). Unlike RH, for which the same value can be associated with very different moisture conditions depending on the air temperature, VPD is an absolute measure of the moisture deficit of the atmosphere. Hence, VPD, is more closely related to the water stress on vegetation. Indeed, prior work has shown the relationship between VPD variability and burned forest area in the southwest U.S (Williams et al. 2014b).
A case study of conditions in advance of the June 2002 Rodeo-Chediski and Hayman fires in Arizona and Colorado, respectively, shows very high VPD that was caused by precipitation drops, an increase in Bowen ratio and anomalous subsidence in the preceding months. This reveals the complexity of meteorological processes that can increase drying of the land surface and vegetation and set the stage for serious fires.
Since 1961 VPD has increased notably across the western U.S. with the strongest increases in the southwest. These trends have been primarily driven by warming that increases the saturation vapor pressure but have also been contributed to by a decrease in actual vapor pressure. Actual vapor pressure has increased elsewhere in the U.S. [but] has declined in the northern Plains and midwest.