Weather Blog

Red sun at night, firefighters must fight

Red sun at night, firefighters must fight
Sunset near Edmonds from YouNews Contributor gmcconaghy

The sunsets over the past few nights have featured a brilliant red sun, and we've also had reports the moon was red as it was near the horizon as well.

What gives? Blame it on Canadian smoke.

A wildfire is burning in southern British Columbia, and the upper air flow Monday and Tuesday evenings was from the north, blowing smoke down into Western Washington.

Here is a visible satellite image from Monday evening. You can see the smoke as nearly-transparent clouds.



Both smoke and -- whenever there's big volcanic eruptions in the hemisphere -- volcanic ash are good ways to get red suns at both sunrise and sunset.  That's because the smoke and ash particles scatter away parts of the visible light spectrum. 

The colors that correspond to shorter wavelengths of light -- purple, blue, green and yellow -- are more easily scattered by particles. On the other hand, the red and orange wavelengths are longer and thus aren't as prone to bounce off particles.

Hard to visualize? Let's try this: Imagine a square yard with fences at the north and south ends, with four holes dug out in random spots. The challenge is to get from one end to the other while blindfolded without stepping in a hole.

In real life, the wavelengths travel in a pattern akin to this: ~  .  Longer red wavelengths have much longer gaps between the peaks and valleys, than the shorter blue wavelengths, which might travel a path more like ~~~ for the same distance.

Or, for our example, the red wavelengths can make it from one side to the other while maybe bouncing off the top and bottom fences once. Meanwhile, the blue wavelengths have to bounce off each fence five times as they move forward to the other end.  Over the long run, the red wavelengths are going to have a better chance of getting through without falling into a hole than the blue ones.

What's that? Still having trouble visualizing? Well then, let's put my 3rd grade Photoshop artistic skills to the test!



So you can see, the shorter blue wavelengths have a better chance of hitting the holes -- the holes, in this case, are the smoke particles and why the red light survives through the smoke while the other colors do not.

(Yes, I am fully aware that there are drawings on my fridge that look better than that. There's a reason I went into science...)

It's a somewhat similar reason why sunsets are pretty oranges and reds. When the sun is low on the horizon, the light is traveling a longer path through Earth's atmosphere than when the sun is overhead.  The longer trip through air gives more chances for the shorter wavelength light to scatter out, plus there's the added effect that pollutants or salt (over and near ocean water) near the horizon can weed out colors too.