Vapor Pressure Deficit - basically, this measures how dry the air is compared to the ground. No surprise: it's drier than ever, with more fuel to burn than ever.
In the past few months, one in every 33 acres of California has burned.
https://www.theatlantic.com/science/archive/2020/09/most-important-number-for-the-wests-wildfires-california/616359/