Every August, when hazy orange skies roll in and air quality apps turn angry shades of purple, someone at a cookout inevitably asks me: “So Cory, you have a forestry Ph.D… why is the entire West suddenly on fire every summer? Did trees forget how not to burn?”
The short answer is no: trees are doing exactly what physics dictates. What changed is that we spent a century creating the ideal conditions for a blowtorch.
1. The 10:00 AM Hangover
Historically, dry western conifer forests (ponderosa pine, mixed conifer) saw frequent, mild surface fires every 5 to 15 years. Lightning and Indigenous cultural burning cleared out the brush, pine needles, and baby saplings, leaving open, park-like stands with maybe 40 to 60 mature trees per acre. You could literally ride a horse at full gallop through them.
Then came the “Big Blowup” fire of 1910, and the U.S. Forest Service instituted the famous 10:00 AM Policy: every reported fire had to be fully suppressed by 10:00 AM the following morning.
For nearly a century, we were remarkably good at it. But putting out fires didn’t make the fuel go away; it just stockpiled it. Today, those same forests frequently hold 600 to 1,000 stems per acre. The understory is packed with dense saplings acting as literal ladder fuels, allowing a small ground fire to climb up into the crowns and incinerate whole mountainsides.
2. The Atmosphere Turned Into a Giant Sponge (VPD)
The second ingredient is something meteorologists call Vapor Pressure Deficit (VPD).
Don’t let the technical name intimidate you: VPD is simply a measure of how thirsty the air is. When summer temperatures tick up by even a couple of degrees, the air’s capacity to hold water expands exponentially. The atmosphere essentially turns into a giant vacuum cleaner, actively sucking water out of living pine needles and fallen logs.
When heavy fuel loads meet an atmosphere pulling moisture out of wood at maximum speed, a single spark doesn’t just start a fire; it starts an uncontrollable thermal runaway.
3. Matchstick Physics
When relative humidity drops into the single digits and dead twigs hit less than 6% moisture content, you aren’t burning wood anymore. You are burning cured matchsticks. At that point, windblown embers can fly two miles ahead of the main fire front and ignite new spot fires faster than ground crews can cut containment lines.
What Space Sees
Working with satellite data (VIIRS, GOES-18) and airborne LiDAR gives you a clear, if sobering, perspective: you cannot suppress your way out of basic thermodynamics.
The only long-term fix is doing the hard, proactive work: mechanically thinning out those ladder-fuel dog-hair thickets, and re-introducing good fire (prescribed burns) during cool, damp spring weather so the forest gets what it historically needed all along.