Open-Source Fire Science

Blog Post When the Wind Fans the Embers

Understanding Smoldering Fire Behavior in Wildland Fuels

By Pyregence Team

In wildfire science, much of the focus falls on towering flames and dramatic fire fronts. But beneath the surface – literally and figuratively – lurks a slower, often-overlooked threat: smoldering combustion. Unlike flaming fires, smoldering can persist for days or even weeks, burning through duff, roots, and organic soils. It’s also harder to detect and extinguish and contributes disproportionately to smoke and carbon emissions.

A 2022 study completed as part of the Pyregence Project and published in Combustion Science and Technology sheds light on one of the least understood aspects of smoldering fire behavior – the role of wind.

Experimental Insights

Researchers conducted controlled laboratory experiments using pine needle fuel beds under varying wind speeds. What they found challenges some conventional assumptions:
  • Low to moderate wind speeds (around 0.3-0.7 m/s) increase the rate of smoldering spread by enhancing oxygen supply and slightly boosting fuel temperatures.
  • However, at higher wind speeds (above ~1.5 m/s), the smoldering front is destabilized, and the fire may extinguish altogether. This is due to convective cooling - the same breeze that feeds the fire with oxygen also steals heat needed to sustain it.
This “Goldilocks zone” of airflow – where wind is just right for sustaining smoldering – is especially relevant to forest floor and peat fires, where limited ventilation often controls fire behavior more than fuel moisture or heat.

Real-World Relevance

Smoldering fires are a major component of residual fire activity in wildland fire events. They:
  • Complicate mop-up operations for fire crews,
  • Pose reignition risks long after the main fire front has passed,
  • And drive high particulate emissions that can significantly impact air quality.
Understanding how wind modifies smoldering behavior can improve fire danger models, inform post-fire monitoring, and support prescribed burning protocols – especially in ecosystems with thick litter layers or organic soils.

Future Directions

With climate change increasing the frequency of high-wind events and extreme fire weather, this research helps us anticipate not just how fires ignite and spread, but how they linger. It also underscores the need to integrate smoldering dynamics into wildfire simulation models like FARSITE, FlamMap, GridFire and ELMFIRE, which currently focus more heavily on flaming behavior.

Dig into the Details:

Cobian-Iñiguez, J., Richter, F., Carmignani, L., Liveretou, C., Xiong, H., Stephens, S., … & Fernandez-Pello, C. (2023). Wind effects on smoldering behavior of simulated wildland fuels. Combustion Science and Technology, 195(13), 3212-3229. https://doi.org/10.1080/00102202.2021.2019239