This month, we look at snow. Sounds simple enough - snow is white, right? Well... sometimes. And sometimes not.
Figure 1: Munsell colors for January 2013
Before we begin to look at it, this does raise the very interesting question of what white IS to begin with. It’s not a spectral color, in the sense that it’s not a color in the rainbow. You may recall Sir Isaac Newton’s famous prism experiment, in which he proved that sunlight can be refracted into all of the colors of the rainbow, and that it can be re-combined into white light again.
(Photo credit: Wikipedia)
When we look at normal objects that don’t emit their own light, we see the light that’s reflected from their surface. Examining this slightly more deeply, we see whichever wavelengths come from the light source(s) that are not absorbed by the object itself. A red object, for example, reflects mostly red wavelengths and absorbs mostly non-red wavelengths.
A white object reflects all of the wavelengths of light that strike it, more or less equally. When we perceive all of the spectral colors simultaneously, we see white.
So back to the question: is snow white? The answer depends on a few factors.
It is more accurate in Munsell terms to describe the color of snow as NEUTRAL. Neutral colors range from black (value 0) through even steps of gray up to pure white (value 10).
(A neutral is defined as a color that has no hue or chroma, only value. White and black are both neutral, as are all of the grays in between. Munsell neutrals are written with the letter “N” replacing the hue name (for “Neutral”), along with a value indicator only, and no number after the value where chroma goes in non-neutral Munsell notations. Neutral value 8 is written “N 8/”. The trailing slash is left in in order to emphasize that there is no chroma, since chroma requires hue.)
However, our eyes (and brains) are remarkably adaptive, and this can lead to very interesting situations for painters who wish to capture what they see with accuracy.
If we look at a typical winter snow scene on a sunny day, we see at least two distinct colors: the white of snow under an afternoon blue sky, and the gray of snow in shadow:
In the image above, the backlit, sunlit snow is almost white, at value 9. Snow in direct sunlight will be value 10, pure white at the highest possible value. The snow in shadow on the pond surface comes in at value 6 to 6.5. The darker shadows at the far shore are around value 5, and the darkest shadow values on the snow deep in the woods is just over value 4, which is about as dark as it gets outdoors in daylight.
You may have heard it said that snow in the shadows has a blue color. There is some truth to that, but only when the overhead sky is blue, and the effect is very slight. The areas in shadow are not being lit directly by the sun, so the primary source of light within the shadows is the scattered blue light from the overhead sky. The influence is very subtle: the color of the blue-sky-influenced shadows in this scene is 2.5PB 6/1 to 2.5PB 7/2.
White snow, gray sky?
We sometimes see paintings depicting white snow under a cloudy, gray sky. This is impossible in nature: an object or surface cannot be brighter than the light source that illuminates it. Thus, the value of a snowy surface cannot be higher than the value of the cloudy sky above it. This can be proven with a simple test: place a mirror face-up in the snow on a cloudy day, and compare the snow on the ground to the reflection of the sky within it. You can easily see that the snow is darker than the sky.
One final phenomenon that’s interesting to consider is the beautiful effect of yellow morning light on snow. As we’ve said, all objects reflect the light that illuminates them (minus the wavelengths they absorb), and snow reflects all wavelengths. If the light hitting the snow is predominantly yellow, then the snow will appear yellow. However, we are only able to perceive this if there is a color contrast between the snow and other objects in the scene. In a field of nothing but snow, as far as the eye could see, our eyes would acclimate to the yellow light and the snow would still look white. This is a complex psychological phenomenon that has some fascinating effects, and is part of the reason we perceive color constancy when moving between areas with vastly different lighting conditions (such as going from indoors to outdoors, and still perceiving the color of your shirt as the same, despite an enormous and measurable difference in the amount and composition of light being reflected from it).
In a case like the unadjusted scene above, where the presence of shadows breaks up the directly-illuminated area, it is easier to perceive the effect of light. However, it does depend what our eyes and brain expect to see as white, and how we acclimate. This can be simulated by setting the white balance in the photograph. The hand is holding a white-balance card, half in morning light and half in shadow. If we sample the shadow portion of the card as neutral, then we see a bright yellow effect (7.5YR 9/2):
If we sample the sunlit part of the card as neutral, then the snow reverts to white, and the shadows, by comparison, look purple-blue:
This is due to two factors: the (slight) purple-blue influence, within the shadows, of the overhead blue sky, and the psychological effect that compared with yellow, neutral itself appears purple-blue.
So, in this case, whether you choose to paint the snow yellow or white depends on what you want to emphasize in your painting. If you choose yellow, you’ll be rendering the neutrality of the shadows and calling attention to the lit areas; if you choose white, then your shadows will be purple-blue, and the snow will look yellower by comparison, but the emphasis will rest on the slight purple-blue hue of the shadows.
Clearly, we’re only scratching the surface here of fascinating and complex visual effects that painters can use to render light and color with accuracy and beauty in their work. There is much more to come in the months ahead.