Supplemental Data: Climatology, variability and trends in United States vapor pressure deficit, an important fire-related meteorological quantity

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. V P D has been shown to be closely related to variability in burned forest area in the western United States. Here the climatology, variability and trends in V P D across the U.S. are presented. V P D reaches its climatological maximum in summer in the interior southwest U.S. due to both high temperatures and low vapor pressure under the influence of the northerly, subsiding eastern flank of the Pacific subtropical anticyclone. Maxima of variance of V P D are in the southwest and southern Plains in spring and summer and are to a large extent driven by temperature variance but vapor pressure variance is also important in the southwest. La Niña-induced circulation anomalies cause subsiding, northerly flow that drive down actual vapor pressure and increase saturation vapor pressure in fall through spring. High spring and summer V P D can also be caused by reduced precipitation in preceding months, as measured by Bowen ratio anomalies. Case studies of 2002 (the Rodeo-Chediski and Hayman fires) and 2007 (the Murphy Complex fire) show very high V P D caused by antecedent surface drying and subsidence warming and drying of the atmosphere. V P D has increased in the southwest U.S. since 1961, driven by warming and a drop in actual vapor pressure, but decreased in the northern Plains and midwest, driven by an increase in actual vapor pressure.