I heard this lesson from Kurt Huggins of Teethering Bulb when I attended his girlfriend Zelda’s workshop a few months ago. Unfortunately I do not remember where Kurt said he heard it from.

There is a rule of four values in landscape compositions and it is as follows:

The sky is always the lightest value, even at night time. After the sky comes the ground, it is the second lightest. Then comes incline planes, such as hills, and the darkest value will always be objects such as trees.
So from light to dark it is:

1. Sky
2. Ground
3. Incline Planes
4. Trees

Links:
Teethering Bulb Portfolio

Wicked Fairytale – Artwork by Kiri Østergaard Leonard