How Wildfire Smoke Affects the Daylight Spectrum - A Technical Analysis

In recent months, wildfires have ravaged the western United States, causing widespread devastation and damage. Smoke from the wildfires has affected virtually every town and city along the west coast, obscuring the sun and turning blue skies into an eerie, brownish-orange hue.

As lighting experts obsessed with the spectral properties of natural daylight, we couldn't help but be curious about how the smoke and haze would affect the color metrics and spectrum of daylight.

The following post discusses actual spectral measurements taken in September 2020 in the Portland, OR metro area. We also follow with a theoretical analysis that ties together potential explanations for the spectral and color changes we observed.

Qualitative and Anecdotal Observations

Before we share our spectral test data and discuss our results, we want to share, to the best of our abilities, a qualitative description and perception of the lighting conditions we observed when these measurements were taken.

Different smoke conditions will most certainly lead to different spectral data, and since there does not exist a standardized technical definition of "smoky daylight," the measurements we took can only be taken as a single set of data points in the context of our visual observations. Our hope is that our explanation below will help our readers understand what we saw visually, compared to what the spectrometer data indicates.

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As smoke from the wildfires rapidly began to fill the skies in early September, we began to lose sight of the crisp, late-Summer blue sky that is typical during this time of year. The smoke began to cast a soft and diffused yellowish, brown light everywhere.

We observed unusually intense or vibrant sunsets, where the sun appeared much redder, and yellow, orange and red colors in the sky are especially pronounced.

As the smoke continued to thicken, we completely lost sight of the sun, even in the middle of the day. The photo above was taken around noon on a day that did not have any cloud cover.

In terms of color temperature, the perception was definitely a shift towards the "warm" direction. At the same time, however, the color was certainly not reminiscent of natural daylight observed in early morning or late afternoon sunshine where color temperatures are lower. While difficult to describe, the smoke-filtered daylight had an extra amber tinge, markedly different from the warm, golden glow that we see from low-angle sunlight.

A Quick Rundown of Spectral Measurements

Using our handheld spectrometer, we took several measurements around mid-day (click here to download the full report). A spectral distribution diagram of the measured light is shown below.

While the spectrum is complete and smooth across the entire visible spectrum, a very noticeable reduction in violet, blue, cyan and green wavelengths can be seen.

The color temperature was measured to be 3440K, with CIE 1931 xy coordinates of (0.4134, 0.4046). This color point corresponds to a Duv value of +0.0032, which indicates a significant shift above the black body curve.

When plotted on the CIE 1931 chart, we can verify that a significant color shift has occurred, from what would typically be expected to be an approximately D65 color point at mid-day, to a color point significantly altered with a strong amber color that corresponds to a wavelength of 580 nanometers.

Comparing the Spectral Power Distributions

By plotting the measured SPD against that of a typical D65 spectrum, we can clearly observe a significant reduction in the amount of shorter wavelength energy, such as violets, blues and even some green wavelengths.

Once we normalize the two charts, we can create a relative transmittance ratio, which shows a clear relationship between wavelength and transmittance. In other words, the longer the wavelength, the more light passes through what is essentially a giant light filter covering the entire sky.

A Theoretical Look at Wildfire Smoke and its Effect on Light

Wildfire smoke is composed of primarily carbon-based ash, and is made up of neutral-colored, grey particulates. Yet, if that is the case, how would that explain such a significant shift of the daylight color in the amber direction?

To reconcile this, we turn to the Rayleigh scattering effect, which explains changes in light color as it passes through microscopic particulates whose diameters are significantly smaller than visible light wavelengths. Specifically, the shorter the wavelength, the higher the scattering of photons, and therefore it would follow that violet and blue wavelengths are scattered the most, while orange and red wavelengths are not scattered as much.

The Rayleigh effect is dependent on the amount and diameter of particulates in the atmosphere, rather than the actual color of the particulates themselves. Therefore, even though the wildfire smoke particulates themselves do not exhibit any amber color via reflectance, when daylight passes through significant mounts of it, the resulting light rays have a strong amber hue due to the attenuation of shorter wavelength energy (i.e. violets and blues).

In taking a look at our empirically derived "transmissivity curve" we can certainly see the positive relationship between wavelength and transmission, that maintains consistency with the Rayleigh scattering principle.

For further reading, on theories and applications of wildfire smoke on daylight, we recommend referencing "Color of smoke from brush fires" by David K. Lynch1 and Lawrence S. Bernstein.

In Conclusion

Our tests and data measurement were taken somewhat casually, but we have been able to verify consistency between our visual observations, spectrometer measurements and scientific theory and principles.

In various other blog posts, we have discussed the difficulties in defining and standardizing around a commonly agreed upon definition of natural daylight.

Although uncontrolled wildfires can pose significant threats to life and property, wildfires themselves, and wildfire smoke by extension, are indeed a natural part of our planet and its various ecosystems.

One could certainly make the argument that daylight under wildfire smoke is indeed natural, and it would only further our case that defining the term natural daylight is certainly no easy task.