My favorite album by U2 is “Under a Blood Red Sky”. This album came to my mind after seeing a picture of the sky in California this week. While not completely red, the red-orange color of the sky was undeniable and arrogant. Many compared the scenes to some from one of NASA’s Martian rovers. The Western US is experiencing an apocalyptic fire season as fuel from heat, sufficient dry vegetation, and even electricity. According to UCLA meteorologist Daniel Swain, well over 2 million acres have been burned (and counting). So why were the skies orange?
You may be tempted to conclude that it represents all burning fires, but it is far more complex. We have to talk about fumes, aerosols, scattering and something called oceanic crust to clarify what is happening. I’ll start with the oceanic crust, which according to the National Weather Service “represents the difference between a cool, moist air mass and a warm air mass.” The oceanic crust may rot along the west coast of the continents for weeks. It can also act as a low-level barrier due to the atmospheric stability of the meteorological feature. The oceanic crust, as seen below, is a relatively shallow feature in the atmosphere that jumps from the ocean.
Most fires are inland but the prevailing air flow is blowing smoke towards the Bay Area (see below). Smoke, a type of aerosol particle, rises over and above the oceanic crust and if winds are weak in a region, it can literally sit atop the oceanic crust of this steady, moist air that flows through the Pacific Ocean . This trend also suggests that many residents on the surface do not actually smell of smoke. It is on top of the air mass. However, if the marine layer deteriorates, ash can settle on the surface and significantly reduce air quality.
Okay, that was a great explanation, but why the orange color? The answer is how particles of different sizes scatter light. The sky is usually blue due to the thing that scattered Rayleigh. Molecules in the atmosphere more effectively scatter short wavelength “blue” in all directions so that we see. The color spectrum, including violet, blue and green, disperses relatively more smoke particles but not uniformly. Guess what it leaves for the eyes to see? You guessed it, orange and red. Sunsets are colored red – orange for similar reasons. When the Sun is low on the horizon, the light undergoes more atmosphere (more scatter). The scattering of larger particles comes under the scope of mie scattering. For more information about Rayleigh, Mie and other scattering properties, this link is useful here.
I hope this quick primer was useful. Be safe