Bay Area residents awoke to a thick layer of wildfire smoke blanketing their cities on the morning of Wednesday, Sept. 9.
The ominous orange haze resembled an apocalyptic landscape, or perhaps the surface of Mars. While the sky was merely a dusty yellow at 7 a.m., it soon turned into a dull orange by 8 a.m., and darkness started creeping in. The eerie skies are now being ruled as a scientific phenomenon.
The smoke comes from the numerous wildfires that continue to burn up and down the coast of California. Most of the smoke that emerged that day was coming from the August Complex Fire in the Mendocino National Forest and the Bear Fire, which started in the mountain terrain of the Plumas National Forest.
So what caused the orange sky?
Only seven colors of the electromagnetic spectrum are visible to the human eye.
Blue light, which has a short wavelength, is scattered by small particles like nitrogen and oxygen. When sun rays hit these particles, it gives off the blue color of the sky that we see in the daytime. This is known as Rayleigh scattering, which is the dominant type of light in the Earth’s atmosphere.
The smoke particles that clogged the air in California early September are much bigger than the molecules in the atmosphere and similar in size to visible light.
These particles absorbed the blue and green light of the atmosphere, exposing a red hue to the sky. This process is known as Mie scattering.
The smoke particles also absorbed the sunlight, which created a darkening effect on the sky.
Despite the smoke in the air, the Air Quality Index (AQI) remained “moderate” until later in the day.
Suspended smoke stayed higher in the atmosphere above breathing levels. However, shifting winds and the sun later burned away the marine layer keeping the trapped smoke at bay.
The AQI readings also remained low due to the size of the falling ash particles. Designed to detect smaller particles, the sensors did not pick up the ash in the sky. Ash can irritate the lungs and cause other symptoms such as coughing, shortness of breath, and chest pain.
When Dani Courtney, a junior at Carlmont, awoke that morning, she knew she was witnessing something strange.
“I figured this might be a once-in-a-lifetime occurrence, so I grabbed a nice camera and took pictures of anything I could,” Courtney said.
The results are striking images of the dark orange skies.