Sky Blue

Most people know why the sky is blue: small particles in the atmosphere scatter the shorter (blue and violet) wavelengths from the white sunlight so that the blue light bounces around in all directions, covering the black void of space with a veil of sapphire.

As artists, we need to know a little more than that. How does the sky color change as we look in different directions? Where is the blue darkest or most chromatically saturated?

Here are two photos that were both taken at 4:10 in the afternoon of April 12 in Germantown, New York. The one on the left was taken facing the sun, and the one on the right was taken seconds later facing away from the sun.

The clouds are completely different. Near the sun, clouds have dark centers and light edges. With the sun shining directly on them at right, they are lightest at the tops or centers and they get darker at the sides and bases. Smaller clouds are not as white because they have a lesser mass of vapor to reflect back the light.

The color of the sky is different, too. Around the sun is a region of warm glare which weakens the chroma of the blue, making it more of a dull grey-green. Looking away from the sun, the blue is more saturated, tending a bit more toward violet.

How do we know the camera isn’t deceiving us? Is there another way to check these observations?

I went to my local hardware store and picked up a bunch of blue paint swatches. Here’s how the sky looks compared to the paint swatches while facing away from the sun. One of the blue swatches (A) is a close match to the adjacent area of the sky.

I wanted to photograph the swatches while facing toward the sun, but I had a problem. There was no way to angle them so that the sun could shine directly on them. So I set up a mirror on the windshield of my car to bounce the light back onto the swatches.

But we can’t trust this comparison, because those color swatches are lit by the warm light bouncing up from the ground and from my t-shirt. That’s why the swatches look a little warmer than they should.

Here’s another experiment to demonstrate how the sky gets lighter near the sun. I took a single paint swatch, cut it in half, and taped the symmetrical halves on a mirror to make a device that we can call a "cyanometer." None of the swatches matches the sky exactly. The hue and chroma are different, but the values come close in a couple places.

Clearly the value of the sky darkens as we shift our gaze horizontally away from the sun. The left arrow, nearer the sun, matches the value of the lightest swatch, while the right arrow, just a little bit farther from the sun, matches a swatch that’s two steps darker. The mirror is reflecting another section of the sky, which also gets darker from left to right.

The sky color shifts in value from zenith to horizon, too, as we can see when the cyanometer is arrayed vertically. At (A) there’s a close match of value between the darkest swatch and the distant sky, even though the chroma is different. Higher in the sky at (B), the same swatch looks much lighter than the sky surrounding it.

The mirror reflection shows an instant comparison of what’s going on behind us in a region nearer the sun. This area of the sky is much brighter in value, as well as being duller in chroma, confirming the first pair of photos in this post. The swatches at (C) and (D) show us that the sky in the region of the sun stays about the same value as it goes up from the horizon, because the increasing solar glare offsets the effect of lightening near the horizon.

Let’s draw some general conclusions from these observations.

• First, there are two separate but overlapping systems of color gradations in a daytime sky. One system, "solar glare," is governed by the proximity to the sun. The other, "horizon glow," depends on the angle above the horizon.
• In each of these two systems the sky color gradates in value, hue, and chroma. These two systems interact with each other so that that every patch of sky gradates in two different directions at once. This means that the painter in opaque colors needs to mix at least four separate starting colors to paint any given segment of clear blue sky.
• As we move from the zenith to the horizon, the sky generally tends to get lighter, because we’re looking through more atmosphere. A poet might say we’re peering through more veils drawn across the void of heaven. Near the horizon, depending on the time of day and the direction of view, the sky color can range from a pale cerulean to a warm grey to a dull orange, but usually it’s a whitening.
• As we approach the sun, the color gets lighter and warmer because a great volume of white light is scattered at shallow angles by large particles in the atmosphere. You can see this best by standing near the edge of the shadow of a building with the sun just hidden behind the roofline. A weaker but noticeable lightening also occurs at the “antisolar point,” 180 degrees opposite the sun.
• The point of the darkest, deepest blue, which I call the “well of the sky,” is at the zenith only at sunset and sunrise. To be precise, the well of the sky is actually 95 degrees away from the setting sun across the top of the sky. At other times in the day, the well of the sky is about 65 degrees away from the sun.

Notes and recommended reading

Why is the sky blue? Link.
Some ideas in this post are based on Light and Color in the Atmosphere by M. Minnaert (1954) in Dover edition, link
Minnaert and scientists as early as Humboldt built instruments that they called cyanometers for observing sky colors, but they were of a different design, link.
Photos were taken without polarizing filters.