Before the Spark

Introduction

A few weeks ago, I was on vacation in Northern California when a wildfire started burning nearby. The fire itself never threatened where I was staying, but the smoke did. For about 24 hours, the air quality became bad enough that I changed my plans and stayed indoors. It was a reminder that you do not have to be anywhere near the flames for a wildfire to affect your life, and this summer, that experience is being repeated across large parts of North America.

Today, we are going to look at how the changing climate leads to greater fire danger, why determining what caused a wildfire is more complicated than it sounds, and how smoke from a distant fire can eventually find its way to you. Welcome back to the podcast. I'm Dr. Mac, and this is The Climate Translation.

What Does a Fire Need?

When we talk about wildfires, we tend to focus on the moment they begin, asking what started the fire: lightning, a campfire, a downed power line, or a piece of equipment. Those are important questions, especially when investigators are trying to determine responsibility. But from a meteorological standpoint, I am interested in what happens after that first flame appears. To understand that, let's revisit the fire triangle: every fire needs heat, fuel, and oxygen, and taking away any one of them prevents the fire from continuing.

Pouring water on a campfire removes heat, while a fire extinguisher smothers a flame by cutting off oxygen. In a wildfire, oxygen is abundant in the atmosphere, and once an ignition source provides enough heat, that part of the triangle is satisfied as well. That leaves fuel, which exists across forests, grasslands, and shrublands as trees, grass, pine needles, fallen branches, leaves, dead shrubs, and organic soil layers. But not all fuel burns the same way. If you have ever tried to start a campfire with damp wood, much of the fire's energy must first evaporate internal moisture before the wood can ignite, whereas dry twigs catch fire from a single spark. Wildland firefighters call this fuel moisture, which is one of the most critical factors in understanding fire behavior.

Small fuels like dead grass and pine needles gain or lose moisture quickly as relative humidity changes, while large logs and living trees respond much more slowly based on weeks or months of rainfall, soil moisture, and drought stress. When assessing wildfire danger, meteorologists and fire managers look at the condition of the entire landscape. Adding wind transforms fire behavior by supplying fresh oxygen, bending flames toward unburned vegetation to preheat it, and carrying burning embers far ahead of the fire front in a process called spotting. Terrain also plays a major role because fire moves faster uphill as rising heat preheats fuels on the slope above. Fire behavior is the interaction between fuel, weather, and terrain, and changing any one of those elements dramatically alters the outcome.

A warming climate alters the background conditions before ignition occurs. Longer dry spells, intense heat waves, and early spring snowmelt give landscapes more time to dry out before summer peaks. An identical spark landing in moist grass produces a very different result than one landing in parched vegetation under low humidity. The spark explains how the fire started, but the condition of the landscape determines what happens next. Meteorologists measure this atmospheric demand for moisture using a metric called vapor pressure deficit.

A Thirstier Atmosphere

Vapor pressure deficit, or VPD, measures the difference between the amount of water vapor currently in the air and the amount needed for the air to reach saturation at a given temperature. You can think of it as atmospheric thirst: when VPD is low, the atmosphere demands little additional moisture, but when VPD is high, water evaporates rapidly from soils and dead fuels, and plants lose moisture quickly through their leaves. As air warms, its capacity to hold water vapor increases substantially, meaning that if temperatures rise without a proportional increase in moisture, VPD rises and the atmosphere becomes thirstier.

Relative humidity alone does not tell the full story. On two days with the exact same amount of atmospheric moisture, a hotter afternoon will have a much higher vapor pressure deficit and demand far more water from the landscape. Plants respond to high VPD by partially closing the stomata on their leaves to conserve moisture, which slows photosynthesis and places the vegetation under severe stress. Dead vegetation responds even faster, losing moisture directly to the dry air and becoming highly flammable in a short period. When a spark occurs during high-VPD conditions after extended drying, it enters a landscape primed to burn.

Across the western United States, research demonstrates a strong relationship between high vapor pressure deficit and total burned area. As temperatures rise, the frequency of high-VPD conditions increases as well. While seasonal snowpack, wind, terrain, and ignition sources remain vital factors, VPD explains why a region does not necessarily need a dramatic drop in rainfall for wildfire risk to escalate: the atmosphere simply becomes more efficient at extracting moisture from the ground.

Who Lit the Match?

When a major wildfire occurs, the immediate question is what caused it, but the answer is layered. If lightning strikes a tree and starts a fire that eventually burns a hundred thousand acres, identifying the spark does not explain why the fire grew so large. We have to separate what started the fire, what allowed it to spread, and what conditions were already present on the landscape when ignition occurred.

In the United States, humans cause the majority of wildfires through campfires, equipment, vehicles, power lines, debris burning, fireworks, and arson, while lightning remains a major source across remote Western areas and Canadian boreal forests. Once ignition occurs, the fire responds to the same winds, humidity, and fuel conditions regardless of how it started. Historical forest management also plays a significant role: for much of the twentieth century, aggressive fire suppression allowed grasses, shrubs, and dead branches to accumulate in ecosystems that naturally rely on periodic burning. Prescribed fires and forest thinning help manage these built-up fuels.

Forest management and climate change are not competing explanations; both influence fire risk simultaneously. A forest with heavy fuel loads from past suppression can experience weeks of extreme heat, high vapor pressure deficit, little rainfall, and strong winds. Every one of these factors contributes to the outcome. Attribution in science is not about finding a single culprit, but about understanding how multiple environmental factors converge to increase the probability and severity of a fire. Identifying what lit the match is only the first step; understanding why the landscape burned requires evaluating everything that was waiting when that match was struck.

When the Fire Comes to You

A wildfire alters both the land and the atmosphere. Burning vegetation releases gases and microscopic particles into the air, most notably PM2.5, which refers to particulate matter smaller than 2.5 micrometers across, roughly twenty to thirty times thinner than a human hair. Because these particles are so small, they remain suspended in the air, travel vast distances on atmospheric winds, and penetrate deep into human lungs when inhaled.

The movement of wildfire smoke depends on atmospheric dynamics. Intense heat creates rising air that lifts smoke aloft, and depending on atmospheric stability, smoke may stay trapped near the ground or rise into higher wind currents. Winds at several thousand feet can transport smoke plumes across states, borders, and continents, as seen when Canadian wildfire smoke degrades air quality across the American Midwest and Northeast. Under extreme conditions, powerful updrafts generate pyrocumulonimbus clouds that inject smoke into the upper troposphere and lower stratosphere, though ordinary tropospheric winds are more than sufficient to carry smoke thousands of miles.

Wildfire risk extends far beyond the immediate burn area. While communities near the flames face direct physical hazards, millions of people hundreds of miles away experience the impacts of degraded air quality. A wildfire begins as an event on the landscape, but once its smoke rises, it becomes a meteorological event governed by wind, atmospheric stability, and regional weather patterns.

Conclusion

A wildfire may begin with a single spark, but climate helps determine the environmental conditions waiting when that spark arrives. As the atmosphere continues to warm, understanding how heat, moisture, and weather interact with fuels becomes increasingly essential. Climate change does not have to light the fire to change the way it burns.

I'm Dr. Mac. This has been The Climate Translation. If you have a question about the climate that you have been too afraid to ask, or if you have a differing opinion, I want to hear from you. I can use your viewpoints in a future episode. You can reach me at TheClimateTranslation@gmail.com. I'll see you next time.