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Advanced Practice in Art

The Sky is the Limit- Understanding Warm/Cool Theories

In the first two articles we explored the basic principles of light, color, value, and reflections. In this article we will review the atmosphere and its significance in painting light and shadows.

Of all of the modifiers of light, the atmosphere is the most complex and varied in its effects. The sky can simultaneously reflect and filter different wavelengths of light. As some wavelengths are filtered the remainder penetrate the atmosphere. This phenomenon is the underpinnings of our understanding of warm light – cool shadow and cool light – warm shadow theories.

The description of cool and warm light is a relative term. On a sunny day it is usually used to describe the difference between light in shadow (pure reflected light), which is cooler, and direct sunlight, which is relatively warmer. Since the sky is generally our greatest reflector, its blue reflection down into the shadows is the source of cool light. Direct sunlight is described as warm in nature because it is the product of the visible light spectrum with less of the bluer wavelengths.

Penetration of the Sky

As light enters the atmosphere the high energy blue wavelengths bounce off of the air molecules in all directions and are removed from the spectrum of light. This process is diffusion. The remaining wavelengths are resistant to diffusion and penetrate the atmosphere. These wavelengths present to us as typical sunlight that has more of the full spectrum minus the blue wavelengths.

Variations in Blue Sky

On a clear day, as the sky approaches the horizon, there is much more atmosphere for light to traverse. In this case, diffusion of the blue wavelengths multiplies and some of the blue intensity is lost. Thus, we have a dome of sky that transitions from low intensity blue at the horizon to high intensity blue/purple overhead where there is less atmosphere.

The Sky as a Reflector

In the case of a clear day with the sun overhead we have two illumination processes with very different qualities. The first is the primary illumination of the sun which is very strong and directional. At distance, the sun is relatively small in size, but strong in intensity. The second process is the reflection from the sky. While the sky is a very diffuse reflector, it is unique in that it presents the broadest expanse of reflection. The shorter wavelengths of light, blue and violet, have more energy and tend to bounce around in the atmosphere and scatter, or reflect. Some of that blue reflected light is eventually directed back towards us. The sky reflection (primary reflection) is lower in intensity than direct sunlight. Consequently, its qualities are most evident in the shadow areas. Even in areas of direct sunlight the blue sky retains its ability to influence and has the additive effect of restoring some of the blue light that was lost through filtration in the atmosphere. The result of this interaction is the sunlight areas contain more of the full spectrum of light, revealing more of the true color of the objects it falls upon.

Warm and Cool Light

Since the concept of the sky as a reflector as well as a filter is a such a important factor in understanding warm and cool light, it benefits from more examples.

If one can view the atmosphere as a filter of light, one can also determine that more of the full spectrum is seen when there is less atmosphere between the sun and the objects it shines upon.  

In the case of a clear day at sunset we have a slightly different comparison but the same principles apply. The sun penetrates the atmosphere across the horizon and loses most of its blue qualities. The remainder is a warm red light. The sky above remains very blue in its reflective qualities. The warm light of the sun contrasts with the blue reflected light of the sky and produces beautiful lavender and orange colors on the landscape.

“Ecola Sunset” 15 x30 Oil

In this painting we can see the penetration of warm light through the atmosphere onto the rocks on the left. The blue wavelengths from the sky are reflected to the viewer from the water. The rocks in shadow on the right also reflect some blue from the sky.

Another way to view the effects of warm and cool light is to examine the same subject under different lighting conditions.

We will use two photographs of the same subject taken in morning light and at sunset to explore some of the ways direct light and reflected light affects the landscape. For the sake of simplicity we will call one the sunlight photograph and the other the shade photograph.

You can see the nature of the subject in both photographs. It is a spring day. The lawn is freshly mowed and the trees and shrubs appear to be lush and green. The outlines of the house are also clear and distinguishable. In the sunlight photograph you can see by the shadows that the bright sun is above and to the left of the house on a clear cloudless day. In the shade photograph, although it is harder to tell, the sun is almost setting on the right side of the house. In this case the sun is lower in the sky and cast a redder light.

Let us explore some general properties of the sunlight photograph. In this image we can see that there is stronger contrast of light and dark. This illustrates the different illumination quantities between the direct illumination source, the sun, and the lower illumination of the reflected lights from the sky and trees nearby. At first glance one might assume that the roof shadows are blue because of the sky reflecting on them but closer examination reveals the presence of the green influences from the surrounding trees also. The temperatures of the light, warm and cool, can also be seen very well in the image. The facade of the house in sunlight reflects much more of the full spectrum of light which is warmer compared to the blue-green reflections from the sky and trees on the roof in shadow. Notice the difference between the shaded wall color on the left and the shaded wall under the porch. The wall in shadow under the porch receives very little reflection from the sky and takes on more of the warmer lawn color reflected into it.

Now, examine the shade photograph. There is generally less contrast in this photograph because almost all of the light is lower intensity reflected light. The metal roof acts as a very good reflector of the sky which is blue/violet above it. The lawn is no longer a direct reflector of warm light and it takes on more of the cool color of the sky. As a result, the shadows under the porch do not contain strong warm influences from the lawn.

Let us also explore the difference between the quality of light in the photographs. Examine the left portion of the roof in both photographs. This comparison gives us the best illustration of the influences of reflected light. In the shade photograph the roof has a much purer reflection of sky that is violet in color. In the shade photograph very little light is reflecting from the surrounding trees onto the roof. In the sunlight photograph the shaded roof reveals the influences of the sky and trees reflections and takes on more of a blue green hue.  

In preparation for the next articles the viewer should note that these two photographs are a great example of unconscious inferences. In each photograph we get two different transmissions of light data but the brain is able to recognize both as a white house on a beautiful spring day. This is a demonstration of both lightness and color constancy which will be addressed in the following articles.