You've probably noticed the headlines: megafires that burn for months, smoke that drifts across continents, entire towns reduced to ash in hours. These aren't ordinary wildfires grown large. They're something different—fires so intense they create their own weather systems, generating lightning storms and tornado-strength winds.

What changed? The forests didn't suddenly become more flammable by accident. A precise combination of rising temperatures, vanishing snowpack, and prolonged drought has transformed how forests relate to fire. Understanding this shift starts with a measurement most of us have never heard of: vapor pressure deficit.

Think of vapor pressure deficit—VPD—as the atmosphere's thirst. It measures how aggressively the air pulls moisture from everything it touches: soil, leaves, tree bark, dead wood on the forest floor. When VPD is high, the air is essentially drinking the forest dry.

Here's how it works. Warm air can hold more water vapor than cold air. When the actual humidity falls short of what warm air could hold, that gap creates suction. The bigger the gap, the harder the air pulls moisture from its surroundings. As temperatures rise, this gap widens dramatically. A forest that stayed safely damp through summer a few decades ago now desiccates by early July.

Scientists have found that VPD explains fire behavior better than temperature or humidity alone. Forests don't burn based on how hot the day feels—they burn based on how dry they've become inside. A log that looks unchanged on the outside might have lost half its internal moisture. The fuel is there, waiting. VPD determines when it's ready to ignite.

Takeaway

Forests burn based on internal moisture, not air temperature. The atmosphere's thirst—vapor pressure deficit—determines when vegetation becomes fuel.

Once a large fire gets burning, it stops being a passive consumer of weather and becomes a weather-maker. The mechanism is straightforward but spectacular: fire heats air, hot air rises violently, and that rising column of air reorganizes the atmosphere above it.

A major wildfire can send a plume of superheated air miles into the sky. As this column rises, it cools and condenses, forming what's called a pyrocumulonimbus cloud—essentially a fire-generated thunderstorm. These clouds can produce lightning that ignites new fires miles from the original burn. They generate downdrafts that slam into the surface, creating horizontal winds exceeding 100 miles per hour. They've even spawned fire tornadoes.

The fire feeds itself. Those violent winds pull in fresh oxygen from surrounding areas, intensifying combustion. They throw burning embers ahead of the main fire, starting spot fires that merge and expand. What began as a manageable burn becomes a self-sustaining engine of destruction. This is why modern megafires behave so unpredictably—they've stopped responding to weather and started making their own.

Takeaway

Large fires generate their own weather systems, creating feedback loops of wind and lightning that make them largely uncontrollable by conventional firefighting.

Forests are supposed to be carbon sinks—they pull CO2 from the atmosphere and lock it in wood and soil for centuries. When they burn, that stored carbon releases back into the atmosphere almost instantly. A single severe fire season can release more carbon than some countries emit in a year.

This creates an uncomfortable feedback loop. More atmospheric carbon means higher temperatures. Higher temperatures mean higher VPD. Higher VPD means drier forests. Drier forests mean more severe fires. More severe fires release more carbon. The system accelerates itself.

There's another layer. Burned forests don't always grow back as forests. In some regions, repeated severe fires are converting woodlands into shrublands or grasslands that store far less carbon and burn more frequently. The ecosystem shifts to a new state. What took centuries to accumulate can vanish in days, and the conditions that allowed those forests to exist may not return. We're watching carbon sinks become carbon sources, feeding the very conditions that destroyed them.

Takeaway

Megafires convert forests from carbon sinks to carbon sources, releasing stored carbon that accelerates the warming that makes future fires worse.

These three mechanisms—vapor pressure deficit, fire-generated weather, and carbon feedback—work together to explain why modern wildfires behave so differently from those of previous decades. The forests haven't fundamentally changed. The atmosphere has.

Understanding this helps us move past surprise at each new fire season. The physics is clear: warmer air creates drier fuel, drier fuel burns more intensely, intense fires make their own weather, and the carbon released makes the next fire more likely. Recognizing this pattern is the first step toward decisions that might slow it.