The earth’s atmosphere can make things wet or dry them out depending on the water vapor pressure. When there is a Vapor Pressure Deficit, it dries the earth out. VPD is defined as “a measure of atmospheric water demand defined as the difference between the amount of water vapor in the air and the amount of water vapor that air would hold at saturation.” If there is a lot less water vapor than the atmosphere would normally hold, all other things being equal, you have a bigger deficit. A dry atmosphere also makes the earth beneath it drier. Dry land conditions are conducive to wildfires. The dryness of the atmosphere can also be seasonal. But more than 66% of the summertime vapor pressure deficit in the West has been caused by human activity in burning fossil fuels.

Source: 2/3s of the Extra Dry Air that encourages Wildfires comes from Humanity burning Fossil Fuels