San Francisco residents woke up Sept. 9 to a world tinted a surreal orange, as shown in these photos taken around the region. All across the Bay Area and other parts of the US West Coast, a layer of smoke from wildfires to the east sat above clear air coming in from the Pacific Ocean. Dust and smoke particles in that smoky layer scattered the sunlight, “the same effect that makes the sky blue and the evening red,” says Sam Trahan, a physicist with expertise in atmospheric physics and modeling. Both Rayleigh scattering, which most strongly affects short-wavelength light toward the blue end of the spectrum, and Mie scattering, which is not as dependent on wavelength, were at play in the effect.
At first, “although our sky was discolored and hazy, surface air quality was not impacted as much,” says Roya Bahreini, a professor of atmospheric science at the University of California, Riverside. But over the next week, she saw “different weather patterns and more subsidence and mixing of the smoke with the surface layer air, and so particulate matter pollution was a lot worse all over Southern California.”
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This story was originally posted at 11:20 a.m. on Sept. 10, 2020. It was updated on Sept. 17, 2020, at 11:14 a.m. to incorporate additional comments.