Forest Fire Thermal Dynamics: Daytime Absorption and Nighttime Emission

Large burnt and blackened forest areas in Canada's North create significant thermal effects through altered surface energy processes. During the day, these fire-scarred landscapes absorb substantially more solar radiation than intact forests, while at night they release this accumulated thermal energy back to the atmosphere through enhanced longwave radiation emission.

Daytime Solar Heat Absorption

Enhanced Surface Heating

Burned boreal forest areas experience dramatic increases in daytime surface temperatures, with warming effects reaching up to 4.7°C in the first decade after fire disturbance in mid-May, decreasing to 1.0°C by October. Research shows that Canadian boreal forest wildfires cause an overall annual warming effect of 0.27°C during summer months. This warming occurs because the blackened, charred surfaces fundamentally change the forest's albedo - its ability to reflect solar radiation.gfzpublic.gfz-potsdam+1

Albedo Reduction and Energy Absorption

Fire-induced surface darkening leads to significant albedo decreases, particularly in the near-infrared band where reductions can reach -0.16. In burned forests, surface albedo decreases by approximately 0.0096 in summer, leading to increased shortwave radiation absorption of 2.97 W m⁻² at high latitudes. The dark charcoal and carbon residue covering fire-affected areas create surfaces that are exceptionally effective at absorbing solar energy rather than reflecting it back to space.acp.copernicus+2

Reduced Evapotranspiration

The warming effect is further amplified by the destruction of forest canopy, which eliminates the cooling effect of evapotranspiration. Intact forests release water through transpiration from leaves and direct evaporation from soil and canopies, which cools the surrounding air. After fires, this evaporative cooling is drastically reduced, leaving more absorbed solar energy available to heat the ground and warm near-surface air.bbc+2

Nighttime Thermal Radiation Emission

Enhanced Longwave Radiation

At night, the accumulated thermal energy from daytime absorption is released back to the atmosphere through increased outgoing longwave radiation. Studies show that upwelling longwave radiation increases after fire, with greater increases following larger fires. This enhanced nighttime cooling occurs because the warmed surfaces emit more thermal radiation according to the Stefan-Boltzmann law - hotter surfaces emit more energy as longwave radiation.nature+1

Nighttime Surface Cooling

Research reveals that burned Canadian boreal forests experience surface cooling effects during nighttime hours. This cooling occurs through two mechanisms: reduced roughness-generated turbulence that normally brings warm air from aloft to the surface during night hours, and increased longwave radiation emission from the warmed surface. The destruction of forest canopy reduces coupling between near-surface air and land surface at night, contributing to surface cooling.francis-press+1

Diurnal Temperature Patterns

The thermal cycle creates distinct diurnal patterns where burned areas are significantly warmer during the day but cooler at night compared to intact forests. Forest fires cause surface warming during daytime but cooling at night, with the magnitude of these effects varying by fire size and forest type. Larger fires amplify both the daytime warming and nighttime cooling effects.agupubs.onlinelibrary.wiley+2

Long-term Thermal Evolution

Persistent Warming Effects

The enhanced thermal absorption and emission patterns persist for decades after the initial fire disturbance. Surface warming effects can last for approximately five decades while winter temperatures remain slightly cooler. Peak warming decreases with time but remains detectable - studies show warming effects dropping below 1.0°C only in the fifth post-fire decade.gfzpublic.gfz-potsdam+1

Recovery Timeline

The thermal effects evolve as vegetation slowly reestablishes. During the first three decades, energy used for evapotranspiration gradually increases before returning to lower values. Surface albedo changes create contrasting seasonal effects - decreased summer albedo causes warming while increased winter albedo (due to better snow reflection on open areas) contributes to cooling.pmc.ncbi.nlm.nih+1

This thermal dynamic represents a significant feedback mechanism in Canada's North, where extensive burned areas create large-scale heat islands that persist for decades, fundamentally altering the regional energy balance through enhanced daytime absorption and nighttime emission of thermal radiation.

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