Painting Sky and Clouds.  By Don Finkeldei:   Some tips and rules to paint a realistic sky and clouds.

If you look at the sky near mid day, you'll notice that at the zenith overhead  the sky is a deep blue violet color.  As the sky transitions to the horizon it becomes lighter and  warmer with less color and even transitions to a very light green color.  When the sun is low in the sky the color of the sun turns a bright orange red and  even the sky near the horizon can be very warm with yellows, oranges and reds.

Of course, you don't have to know why the sky has a gradient from blue overhead to a paler, warmer, more neutral color at the horizon.  All you need to know is that it happens and that you absolutely need to paint it that way. 

Clouds behave a lot differently than what happens in a cloudless atmosphere.  Clouds contain droplets and small ice crystals that are very much larger in size than the smaller molecules in a cloudless atmosphere.  They reflect pretty much all light frequencies equally.  That's why the sunlit side of clouds appear whitish.  The larger and more dense the cloud, the darker the shadow underneath.  That's because most of the light can't get through to the bottom.

Clouds can be very hard to paint for some people.  Following these rules will help.  Clouds are smaller in size at the horizon, they are farther away.  That makes sense, right?   Clouds have a sunlit side, a transition areas between sunlit and shadow, and shadow areas.  Clouds don't have defined hard edges, they simply grade softly at the edges and transition boundaries.

The sunlit side of the clouds overhead (nearer you) are more intense and whiter than the sunlit side of clouds at the horizon.  The sunlit side of clouds at the horizon are less intense and  warmer (pinkish). That's because of the scattering effect in the atmosphere described above.

Shadows in clouds are generally gray but the underneath sides of clouds directly overhead can reflect a lot of bounced colors back from the earth.  Shadows of  clouds overhead can be quite warm due to these reflections. 

Clouds aren't just shadowy underneath.  There are usually shadows on the side of a cloud, in the cloud, various places, depending where the sun is -- and what angle the sun is hitting the clouds.   Often, the shadows in a cloud are EXACTLY the same value and the sky behind it.  The only difference is the color.  A gray AT THE SAME VALUE as the sky behind the cloud.  A perfect example of changing color, not value.  Changing color while keeping the value the same can be a very powerful artistic effect and can greatly enhance the attractiveness of your painting.  Most people put way to much value contrast in a sky and clouds.  Values in a sky are much closer together than you think.  Very subtle value changes.  Of course, sometimes clouds can be dramatic with wide value changes.  Like just before a powerful storm.  If it's like that, paint it.  But, be sure that the rest of the painting supports the drama and keep your focus area in mind.  A good painting has a focus area and the rest of the painting is just "supportive and diminutive" to the main focus.

When I'm driving, or just sitting around outside, I try to analyze and evaluate the values in the sky compared to the clouds both overhead and in the distance.  A powerful tool to help you evaluate values in a sky:  Take a digital image and convert it to gray scale in your photo editor... like photoshop.  It'll help you nail down values in a sky and understand what is going on much quicker than trial and error.

You don't need to know why this happens  (this is the physics of the sky on planet Earth).  I'm not sure you want to continue -- but for those of you who want to know - here it is :

The sun contains all wavelenths of light.  Red, orange, yellow, green and blue are all present.  Red is the longest wavelentgh.  Blue is the shortest wavelength.   In outer space (above the atmosphere of the earth) the sun looks intensely white because all the colors are present.  it's color additive meaning that "adding all colors together will show as white".  If the earth had no atmosphere, the sun would look white and the sky black.  Light generally travels in straight lines and is only interrupted and scattered by small  molecules and atoms in the atmosphere.  Our atmosphere is comprised of oxygen, hydrogen, carbon dioxide, smog and pollutants. 

When the light from the sun enters the atmosphere the molecules of gasses scatter the shorter wavelength blue color  much easier than the longer warmer wavelengths.  The gasses in the atmosphere scatter much more  blue from the direct rays of  the sun than the other wavelengths.  It scatters throughout the sky, reflecting the blue color back to you from all over.  The sun will appear warmer (yellower)  than white light because the blue is removed from the direct rays looking at the sun.  The atmosphere away from the sun will reflect mostly blue back to you that was scattered about - but depends on how much atmosphere the blue light has to travel through.  At the zenith, the distance light travels through the atmospere is less than the distance through the atmosphere at the horizon.  It just so happens that the atmospheric distance at the zenith above is just right to mostly reflect the scattered blue light back to your eyes.  At the horizon, the atmoshphere distance is greater.  The molecules in the atmosphere continue to re-scatter the blue light away again, making the sky at the horizon paler and warmer in relation to to the sky at the zenith. 

As the sun begins to set, the light must travel further through the atmosphere before it gets to you. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright. The color of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes.  The sky will also appear warmer at the horizon than during the middle of the day.