As I shared in a recent post, Vapor Pressure Deficit (VPD) is an index that translates into plant stress.  The darker the color in these illustrations, the more stressed the vegetation is.  The more stressed, the more available to burn.  While these levels vary from day to day, overall VPD levels have been on the rise in recent years.

Utilizing VPD to better characterize the fire environment is new to many of us. It describes fuel conditions in ways similar to fuel moisture and ERCs but because it is an absolute value, it will better reflect the impacts from climate change. I am updating this fairly regularly during August and September to help build reference points and improve an understanding of trends relative to this metric and the fire behavior we see.

Vapor Pressure Deficit Values on Labor Day, 2020 on left with 9/1/2021* on right.
The darker the color, the more available the vegetation is to burn.
What a difference a day makes. One the left is an image from 8/17/2021, and on the right 8/18/2021 after cooler and more moist weather came into the region. Very little rain, if any, had fallen in CA, OR, or WA but the change is enough to see measurable relief in terms of VPD.
  • 9/7/2020
  • 9/8/2020
  • 9/27/2020
Vapor Pressure Deficit (VPD) progression in September 2020, a period of historic fire activity along the Pacific coast. Here you see progression from the Labor Day 2020 period to several weeks later with cooling, occasional cloud cover and increased humidities across the region.

*Map times are UTC, 7 hrs ahead of Pacific Time in the US. These maps are showing 5pm on the date prior to the image heading.