One of my favorite albums is U2’s “Under A Blood Red Sky”. This album kept coming to mind as I saw photo after image of the skies in California this week. Although not completely red, the red-orange hue of the sky was undeniable and eerie. Many people have compared the scenes to something from one of NASA’s Martian rovers. The western United States is experiencing an apocalyptic fire season fueled by heat, extensive dry vegetation and even lightning. According to UCLA weather expert Daniel Swain, over 2 million acres have burned (and are in progress). So why were the skies orange?
You may be tempted to conclude that it is only the color represented by all the burning fires, but it is much more complex. We need to talk about smoke, aerosols, dispersion and something called the marine layer to adequately explain what’s going on. I’ll start with the marine layer, which according to the National Weather Service “represents a difference between a cool, moist air mass and a warmer air mass.” The marine layer can persist for weeks along the west coast of the continents. It can also act as a low-level barrier due to the atmospheric stability of this weather feature. The marine layer, as seen below, is a relatively shallow feature in the atmosphere that creeps in from the ocean.
Most of the fires are inland, but the prevailing wind flow blows smoke towards the Bay Area (see below). Smoke, a type of aerosol particle, moves up and over the sea layer and if the winds are weaker in an area, it can literally sit on top of this stable, moist marine layer that has come from the Pacific Ocean. This trend also explains why many surface residents don’t actually smell smoke. It is on top of the air mass. However, if the marine layer deteriorates, ash can settle on the surface and cause a significant deterioration in air quality.
Ok, that was a great explanation, but why the color orange? The answer lies in the way in which particles of different sizes scatter light. The sky is typically blue due to something called Rayleigh scattering. Molecules in the atmosphere most effectively disperse the shorter “blue” wavelength in all directions, so this is what we see. Relatively larger smoke particles disperse even more of the color spectrum, including purples, blues, and greens, but not evenly. Guess what it lets eyes see? You guessed it, orange and red. Sunsets are reddish – orange for similar reasons. Light has to travel through the atmosphere much more (more dispersion) when the sun is low on the horizon. Dispersion by larger particles enters the realm of Mie dispersion. For more information on Rayleigh, Mie and other scattering properties, this link is helpful.
I hope this quick manual was helpful. Stay safe.