WRAL WeatherCenter Blog

Why have California skies turned orange?

Posted September 12, 2020 6:15 p.m. EDT
Updated September 12, 2020 6:19 p.m. EDT

EDS.: RESTRANSMISSION TO CORRECT BYLINE IN METADATA TO JIM WILSON - Deep orange skies shroud San Francisco and the surrounding region on Wednesday, Sept. 9, 2020. Fires continue to blaze up and down the West Coast, destroying neighborhoods and forcing mass evacuations as firefighters struggled to contain them. (Jim Wilson/The New York Times)

The skies above California and Oregon are orange for the same reason the Martian skies take on a tan color. In fact, the same physics can explain a Carolina blue sky.

Sunlight is made up of wavelengths of light across the visible spectrum. Blues in the lower wavelengths, reds in the higher wavelengths.

Comparison of wavelength, frequency and energy for the electromagnetic spectrum. (Credit: NASA's Imagine the Universe)

Normally, the smaller nitrogen and oxygen molecules that make up our atmosphere do a really good job of interacting with the sunlight to scatter those shorter blue light waves.

Smoke particles are just the right size to scatter out that blue light before it reaches your eyes, leaving behind yellows, oranges and reds.

Scenes from San Francisco have been compared to science fiction films like Blade Runner.

This is even more amazing when you consider how bad red light is at scattering. British physicist Lord Raleigh and Sir James Jeans described the amount of light scattered by wavelength (λ) as λ-4. This means the shorter wavelength blue light is more than 9 times better at scattering than red.

California wildfires are visible in satellite images from Sept 12, 2020 (Image: NOAA/NASA)

The wildfires have put a lot of dust in the skies above the western states. The reddening effect is even greater near sunrise or sunset, just as it is without all that ash and dust because the light is passing through so much more atmosphere.

Mars Pathfinder in Ares Vallis (Image: NASA/JPL)

Comparisons to Martian skies aren't far off.

Mars' atmosphere is thin but does a good job at suspending a lot of dust from surface. Martian dust is similar in size to smoke particles from these wildfires filtering out the blues, and leaving behind yellows and reds.

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