"Fire" in the Sky


Sunset at Cedar Falls, Iowa, Photo by Craig Johnson, Copyright 2018

This yellow-orange glow is not from a California fire. Rather, it's caused by light from the setting Sun streaming through a gap in the clouds between two thunderstorms. Selective screening of the light by the atmosphere is blocking the shorter wavelengths of light (violet, blue, and green) in favor of the longer orange and red. This effect is visible during sunset and sunrise. It was particularly intense in this photo because of the contrast between the bright light reflection in the background and the back-lit clouds in the foreground. 

When the sun is near the horizon its light must pass through much more atmosphere to reach our eyes than when the Sun is higher in the sky. The longer path length causes the light to be scattered by more air molecules, dust and other particles, and water vapor. The scattering becomes more intense as the Sun sinks to the horizon. Its light is eventually blocked from view leaving a dimming glow in the sky. Eventually the light disappears entirely as our location rotates to the side of the Earth that is away from the Sun allowing stars and planets to become visible.

The daytime sky is naturally blue. That's because the blue in sunlight is most effectively scattered by air molecules. However, dust, pollution, or other particles are able to change the intensity of blue sky or in severe cases create a milky color to the sky. Looking at the sky during the day or night is a completely different experience depending on what is in the air.  

As an aside, if you have ever experienced a total eclipse of the Sun you have seen a dramatic change to the sky in a matter of minutes. The shadow of the Moon can be seen approaching in the distance as a dark diffuse shadow. Then the sky overhead turns dark and a glow appears in a circle around the horizon. For a few moments the brightest stars and planets appear in the darkened sky. Then suddenly the Sun returns and the sky color returns to blue. So what color is the sky? It all depends on the lighting and time of day.