To understand how smoke from Canadian wildfires is coloring the sky orange in many places this week, it helps to first ask: why is the sky blue most of the time?
The scientific jargony answer: atmospheric scattering.
On the moon and other places where there is no air, the sky is black. But on Earth, we have air and some light from the sun bouncing off the air molecules in the atmosphere. The higher energy colors with shorter wavelengths – that is, blue light – scatter more easily, and as a result the entire sky is suffused with blue.
Smoke particles also scatter light, and because they are larger than air molecules, they also scatter the orange and red colors. That results in much more red and orange in the sky than we are used to during the day, giving it a surreal hue.
The color of the sky during periods of heaviest haze is reminiscent of the colors seen at sunset. That’s because what happens in the sky is somewhat similar to what happens when the sun sets.
When the sun is close to the horizon, light travels through more air and nearly all of the shorter wavelengths are removed from the original sunbeam. Only the longest wavelengths – the reddish colors – are likely to travel undisturbed through the atmosphere, which is the color of the sun at sunset.
And with more of the longer wavelengths scattered at sunset, the sky looks more orange, too.