Red Sky at Night

Another Lens on Life blog by Chuck Kopczak, PhD

10/26/2020
Orange, red and purple clouds in a lavender sky reflect in a slightly rippling lake, with evergreen trees in the background and a log sticking out of the water in the foreground.
Chuck Kopczak, PhD

Sunset over Horseshoe Lake in the Eastern Sierra Nevada Mountains.

Today’s photo was taken at Horseshoe Lake in the Mammoth Lakes Basin with a Canon EF28-135 mm f/3.5-5.6 IS USM lens set to a focal length of 28 mm on a Canon EOS 5D Mk III. The exposure was set to 1/30 sec at f/9.5 and ISO 1600.

The old nautical rhyme, “Red sky at night, sailors delight, red sky at morning, sailors take warning,” associated fiery skies at sunset with the approach of fair weather. But there is little truth to this old saw, or the daily TV weather forecasts would include descriptions of sunrises and sunsets as part of their predictions.

While the color of the sky at dawn or dusk can’t predict weather, the reason the sky takes on those colors can be explained by physics. Simply put, as sunlight passes through the atmosphere it interacts with the gasses that make up the atmosphere, as well as particles that are floating about. On a clear day at high noon, the sky looks blue because those colors at the blue end of the color spectrum are the ones most strongly scattered and reflected to our eye, while the colors at the red end of the color spectrum tend to pass straight through.

As Earth rotates on its axis and the sun appears to sink toward the horizon, sunlight must pass through a greater thickness of atmosphere (for an illustration of this, see the first link below). At this time, the sunlight encounters more matter in the atmosphere, and more of the color spectrum becomes scattered and reflected, thus concentrating the red and orange colors. Of course, anything visible in the atmosphere, like clouds, will reflect that light, causing the amazing sunsets we can observe.

On the day of this sunset above Horseshoe Lake in the Eastern Sierras, the remnants of humid, tropical weather were passing over the mountains, and the clouds picked up the vibrant reds and oranges of the last remaining sunlight in the sky. Even Horseshoe Lake took on a blood red color in the fading light.

Sources

http://www.physicsclassroom.com/class/light/Lesson-2/Blue-Skies-and-Red-Sunsets

https://news.nationalgeographic.com/news/2013/10/131027-sunset-sky-change-color-red-clouds-science/