Atmospheric perspective: The Sky is Blue because it is Black

Things in the Distance are not Just Blue

Myth: things in the distance become more blue and lighter
Reality: things in the distance lose contrast, and if they are blacker they become more blue/cold, if whiter they become more red/warm.

Atmospheric perspective has two effects:

  1. Moisture — act like very thin fog — reduces contrast
  2. Prismatic effect of moisture and dust in the air making the air act as a turbid medium. A turbid medium acts like a prism. So if light is falling on an object that is more black, it will shift to the violet/blue end of the spectrum, but if it is falling on an object that is more white, it will shift to to the red/orange/yellow end of the spectrum.

Painting is subtractive color system — the paint subtracts all wavelengths of light except it’s own color. If it subtracts all visible light, it is black paint. If it only subtracts blue light, then it is orange (red and yellow light is left). In contrast the atmospheric effects of color are additive, since the atmosphere is changing the light as it passes through the atmosphere.

The value and hue of the precipitate effects the color. Seen through a dark fog or smoke or sand storm the observed object looses contrast and takes on the value and hue of the precipitate.

A leafy green forest hillside illuminated by the sun seen at a distance through humid/dusty air will reduce in contrast bc of the humidity of the air and change in color as if violet/blue light were falling on green would make the light leafy green darker and bluer. However, where the light falls on the green and whitens it, that same green will actually be more reddish or yellow.

The same leafy green forest hillside at the same distance and illumination but seen through dry, desert air would still have the contrast effect of moisture and a turbid medium, but less so because the air is clearer, so the same effects happen at a greater distance.

Snow is an example of this. The snow in the shadow is blacker, the snow in the light is whiter. If the myth of atmospheric perspective were true, the whiter snow would be a cold blue color. But if it is illuminated, it is not.

The white snow is not blue at a distance, it is ever so slightly warmish/yellow. This hue is only ever so slight because the intervening air has reduced the contrast a lot, but the illuminated white snow has not turned blue!

Most often things in the distance are black or dark (bc upright things in a landscape are bluer) and usually people are in a place with water vapor in the air as the precipitate which is white when illuminated, hence mostly things in the distance do seem “bluer and whiter,” but don’t make the mistake of just making everything bluer and whiter. If it is a dark vapor, the object will be darker, or the object is white, its hue will actually appear more yellow/red.

There is a third effect to consider. Simultaneous contrast or color context. In the distance things are smaller and usually surrounded by other tones especially the sky and dark tones in upright planes like the dark or shaded sides of hills or mountains. We have to take this effect into account also if we want accurate or deliberately exaggerate colors.

If you put what looks to your eye to be the prober value and hue, you will probably be off relative to the dominate value and hue of the field. It will shift optimally towards the hue’s compliment or value.

E.g. a light spot in a dark field will look lighter than it is when put on a light field and vise versa. Hence why the whites of eyes are actually quite grey/tan/blue. Or why artists often use neutral tones canvases and pallets so they can see the value of their colors keyed to a mid tone instead of keyed to white (or black for that matter). A good way to make a keyed up painting is to paint on a white canvas with a white pallate. Vise versa for a keyed down painting.

For hue dark and light are replaced with compliments. An orange dot on a blue field will look more orange than on a neutral background. A green dot on a blue field will look green-orange. A red dot will look red-orange.

Hence if a little illuminate snow is on a large mountain with a blue sky, if you put the orange you see, it will look too orange. Instead it must be a little less orange and more grey. It is possible that a true grey will work since it will still look slightly orange bc of the effect of the blue sky and cool-blue shadows on the mountain around it. The tone will be easier to mail bc the sky is light and the mountain will be dark, so the color context will be easy to see. The snow will be a little lighter (and warmer) than illuminated snow in the foreground.

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Braus

Educator, Founder, Engineer. Interested in Evidence Based Education and Solving BIG Problems.