August 03, 2010

Why did smoke cause the a reddish sun?

On Sunday a number of you noted the reddish color of the sky and the orange-red hue of the sun. I mentioned in my blog that day that this was the result of smoke from wildfires, blowing in from BC (the main origin) and eastern Washington.

But why does smoke cause the red coloration?

The reason? The scattering of light by small particles in the atmosphere--also known as Rayleigh Scattering. It also explains why the sky is blue!

The light from the sun contains all wavelengths and is essentially white. White light possesses contributions from all wavelengths (or colors) in the visible spectrum (see figure). On the shorter wavelength side there is blue and on the longer wavelength side there is red.
It turns out the small particles in the air (small compared to the wavelength of light) can scatter incoming light into various directions and that they scatter short wavelengths (purple and blue) much more than longer wavelengths (orange and red).

So the light coming in from the sun is white with all wavelengths, but the molecules in the atmosphere scatter blue light more than other wavelengths. That blue light then bounces around between molecules and down to earth and when you look at the sky in a direction other than the sun, you see this scattered light (see figure below). That is why the sky is blue.

Now if you add a lot more small particles--like a nice dense smoke plume--you get scattering on steroids. Not only blue and purple get scattered, but so does a lot of the greens and yellows. So what does that leave? Oranges and reds. And thus the sun's rays have a red and orange color...and that is what we saw on Sunday. A similar effect occurs near sunset, when the sun's rays have to pass through a greater distance in the atmosphere.

Finally, a number of you have asked about a potential aurora tonight....this is a bit out of my field...but from my checking out the latest aurora websites it does not appear that we will see any here in the Northwest.


  1. Good news for aurora fans:

    "Forecast:Auroral activity will be active. Weather permitting, active auroral displays will be visible overhead from Inuvik, Yellowknife, Rankin and Igaluit to Juneau, Edmonton, Winnipeg and Sept-Iles, and visible low on the horizon from Vancouver, Great Falls, Pierre, Madison, Lansing, Ottawa, Portland and St. Johns."

  2. No northern lights, but the lightning from the thunderstorms around Maple Valley was pretty impressive even from Seattle.

  3. Tried to see if any aurora activity would be visible from Lake Forest Park (a long stretch given the forecast, urban light pollution and an obscured horizon.) Well, no aurora for me... but many many flashes from thunderstorms in the Cascade foothills. Took a look on radar... wow, there were some powerful cells dumping there last night!

  4. What was weirder was the blue shadows.. all day long.

  5. We came pretty close with Auroras, I was watching the map on When the big red area rotated over us, the thing kind of fizzled quickly, and sucked back up north. Nothing to see.
    Lightning was kind of fun though.

  6. So small particles scatter short wavelengths, and larger particles scatter longer wavelengths? What is it about a particle size that "attracts" a particular wavelength of light? I mean, I guess it's intuitive but science doesn't proceed by intuition -- when short wavelength light encounters a particle that's about the same size as one cycle then it somehow can deflect (scatter) that light more easily than if the same light hit a larger particle...? I guess that's the mechanism I don't understand. But light, to me, remains an amazing mystery.

    1. That is a great question. Particles much smaller than light act like a radiating dipole. As a particle gets bigger and bigger it starts to behave more like a glass ball: almost all light goes through, very little sideways scatter. Inbetween there are some interesting physics known as Lorenz-Mie scattering.


    this is a helpful link

  8. check out the radar animation from 12:00 utc to 15:15 utc over Vancouver Island this morning (8/5) I've never seen a rotation like that

  9. Hi Cliff,

    The UW probability forecast has seemed off lately--with predicted high temps seemingly higher than the NWS forecast, news organizations and actual high temps. Is something amiss with the prediction program?? (I haven't actually charted the data--so this could be impression error.....)



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