Forest fires can release more energy than an atomic bomb

On the first full weekend in September, when the Line Fire burned 20,000 acres and was only 3% contained, a San Bernardino County resident described the sky as “looking exactly like a nuclear warhead had been detonated.”

On a basic level, it makes sense: At this point, the Line fire has already released more energy into the atmosphere than a dozen nuclear bombs. And just as nuclear explosions produce a distinctive mushroom cloud, uncontrolled fires can be powerful enough to generate their own weather.

When wood and other vegetation burn, they produce four main compounds: carbon dioxide, smoke (itself a mix of toxic compounds, including carbon monoxide, methane, benzene, and many others), heat, and water vapor. Of these, carbon dioxide is the least important to local weather—although it plays an important role in global climate, it is more because of its long lifespan than its direct power.

The most noticeable consequence of smoke emissions is its dangerous impact on human health.

The smoke column can extend for hundreds or thousands of miles as it is carried by wind currents. In addition, smoke aerosols block and scatter sunlight, causing the surreal “red sun” effect that appears in apocalyptic-looking photos on social media; their optical properties also tend to suppress precipitation in upwind locations, which can (in the long term) fuel more fires due to drier conditions.

Another byproduct of fire is heat—like a hot-air balloon burner, a forest fire causes the lower atmosphere to become less dense and therefore rise. As the air above the fire rises, outside air rushes in to replace it, providing the fire with oxygen to continue burning.

If a fire is strong enough, it can create a “firestorm.” This happens when all the winds surrounding the fire are directed toward the center of the fire, leading to a feedback effect: more oxygen causes more intense flames, which in turn draw in even more oxygen.

These winds have a mixed effect on a fire’s ability to spread—on the one hand, the gusts are directed inward, meaning sparks are less likely to be pushed outward. On the other hand, strong updrafts can catch burning embers, lifting them into unburned material, where they can cause “spot fires” even miles from the fire line.

Moreover, a firestorm can emit such intense heat that firefighters cannot operate near it. Firestorms have been observed not only during forest fires but also during World War II, when bombed cities—such as Dresden, Germany, and Hiroshima, Japan—suffered far greater damage from fires than from the original bombing.

The final ingredient is water vapor.

As hot air rises higher in the atmosphere, water vapour released during combustion will condense, aided by the presence of smoke particles that act as “condensation nuclei” and allow the water to form droplets. This condensation produces more heat, leading to even stronger convection, and the end result is known as a pyrocumulus cloud (or in more extreme cases, pyrocumulonimbus).

These clouds often signal trouble to firefighters trying to contain a fire—not only because they indicate that the fire is gaining strength, but also because the hazardous conditions and poor visibility in the clouds make it impossible to use aircraft to fight the fire. In addition, these clouds can cause frequent lightning strikes, which ignite new fires in the area.

One advantage is that pyrocumulus clouds can produce rain, which in some cases smothers the very fire that caused them. However, depending on wind conditions, this rain sometimes evaporates before it reaches the ground, due to the hot, dry environment surrounding the fire.

If this happens, it can cause a “downdraft” as cold, dense air quickly descends from the cloud. Like updrafts, this feeds the fire with fresh, oxygenated air; unlike updrafts, downdrafts cause gusts that spread away from the center of the fire, causing it to spread quickly in many directions at once.

What does this all mean for Southern California?

Fortunately, large-scale firestorms are virtually nonexistent in the region, in part because the region’s narrow canyons and high winds direct blasts—and therefore fires—in specific directions. Worse still, both factors can accelerate fire spread and promote the formation of pyrocumulus.

Developments on hilltops and ridges are at greater risk because fires can spread up to eight times faster up steep slopes than on flat ground, and lightning strikes from pyrocumulonimbus clouds are more likely to occur in higher locations.

With the National Interagency Fire Center forecasting a higher-than-normal wildfire risk for the rest of the year along the Southern California coast, there is a strong possibility that more fires will break out in the region in the coming months.

The relationship between wildfires and the environment can cause rapid and unpredictable changes in fire direction and intensity, so it is extremely important for residents to remain vigilant during periods of high risk.