The sky is a vaulted arch of diffused light encompassing the landscape. It illuminates our scene evenly as a blueish light-source on a clear day (or a gray light-source on an overcast day). If you go outside and look up, you will see the zenith of the sky vault which can get as dark as value 7. If you could follow the sky vault to the horizon, it will gradually lighten to value 9.5. In practice, your landscape will encompass only a small window of the sky vault. The picture zenith (below) is decided by your composition, and the range of value in the sky vault of your actual painting may be as little as 8th value at the top to 9th value above the tree line.
Another way of visualizing this concept below. The sky arching over the landscape with values from the horizon at 9.5 to 8 creates ambient light for the shadows. The sun adds more light creating distinct light and shade differences. A good generality to remember is that outdoors there is an approximate 2 value difference between light and shade compared to indoor lighting that has an approximate 5 value difference.
© John Ennis 2011
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