En Plein Air: The Physics of the Sky

23 Apr 2014

Yesterday, I was struck by how many high-quality artists are working today and what valuable advice they offer readers through their teaching. I was especially taken with the landscape artists (I always have my plein air painting eyes peeled for you guys!) I know, including Ben Fenske, Don Demers, Curt Walters, Junn Roca, and John Cogan. I wanted to share a very interesting piece by John Cogan, written by Bob Bahr. In it, Bahr relays the explanation for why the sky appears the way it does, as explained by Cogan, who received his doctorate in physics. Interesting stuff, enjoy!

 A Physicist Explains Why the Sky Is Blue

Maricopa Point 2006, acrylic painting, plein air painting, 48 x 60. Private collection.
Maricopa Point
2006, acrylic painting, plein air painting, 48 x 60.
Private collection.

John Cogan earned a doctorate in physics before he decided to paint full time, so one would expect the acrylic painter to know the science behind the visual phenomena he depicts. A question most artists have probably asked at some point in their life is one of the ponderables that made Cogan want to pursue science as a child: Why is the sky blue?

An artist doesn't have to know why the sky is blue to paint a blue sky. But the reasons why the sky is blue can help artists depict other parts of the landscape, because the sky is blue due to natural light effects that impact other elements in a scene. Cogan explains the science of a blue sky in depth in his workshops; here's a synopsis.

Sunlight is white light. White light is a combination of lights of all colors. Different colors of light behave differently, including the ways they reflect off of surfaces. The warmer colors can travel more easily through atmosphere, whereas cooler colors scatter. Blue light is scattered the most easily. Warmer colors only scatter when there's a lot of atmosphere to get in the way and scatter them. Red scatters least of all, relatively speaking. So when the sun is directly overhead, we see mostly white light if we glance at the sun, and mostly blue light scattered about in the sky. When the sun is low, yellow light is scattered, giving us the so-called golden hour. When it sinks lower, the orange is scattered, too. When the sun is on the horizon, its light is going through the most atmosphere it ever goes through, and only the red light is fighting its way through and staying in our line of vision—the other colors were scattered hundreds of miles before they got to us.

What's important to painters is how the abundantly scattered blue light affects other elements of the landscape. For example, a distant tree-covered hillside is green, yet it looks blue to us. This is not because the hill is blue at all. It is because the blue light from the sun is being scattered by the atmosphere between us and the hillside. In effect, mountains look blue because there is so much sky between us and them. You aren't seeing blue mountains, you are seeing blue sky in the way of the mountains.

Cogan also emphasizes that painters should be aware that in this case, the mountains are a "sink," not a source of light. In other words, they are merely the dark backdrop that lets us see the scattered blue light of the atmosphere, as opposed to clouds, which are reflecting light and thus acting as a secondary source of light. Thus, those painters who glaze a blue-grayish white over distant elements may be mimicking the effects of nature in the most accurate way, in terms of mechanics.

About the Artist

John Cogan earned a doctorate in physics in 1981, but within two years he had quit his job to paint full time. A largely self-taught painter, Cogan quickly gained success in national competitions such as Arts For The Parks, in which he has placed 12 paintings and won three awards. Cogan was commissioned to paint a 60"-x-48" painting for the Henderson Fine Arts Center on the campus of San Juan College, in Farmington, New Mexico, as well as a 10'-x-4' painting for the courthouse of Albuquerque. Recently he was commissioned to paint 15 pieces for the Sultan of Oman, and his work appears in numerous private and public collections around the world. Cogan is represented by the online gallery Traditions Fine Art; Southwest Gallery, in Dallas; Gallery West Fine Art, in Jackson, Wyoming; The Gallery 1876, in College Station, Texas; and El Prado by the Creek, in Sedona, Arizona. He lives in Farmington with his family. For more information on Cogan, visit www.johncogan.com.

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k.gray wrote
on 28 Oct 2009 7:42 AM

You are so right......an artist DOESN'T need to know why the sky is blue to paint a blue sky......but how much richer life is for the knowing.  Hope to see more articles like this in the future,  the back-story on pigments used to chase the color BLUE is interesting in itself.