In landscape painting values are used to convey the time of day (bright noonday, dusk, night, etc.) and weather conditions (sunny and clear, cloudy, foggy, etc.) and are critical in setting the mood. This is done by properly relating the values of the four main planes in the landscape: sky, ground plane, hills and mountains, trees.
The sky is nearly always the lightest plane in the landscape because it is the source of light. The ground plane is usually the next lightest because of its orientation to the sky. It is flat and therefore receives more direct light. Hills and mountains are usually the next lightest with trees and bushes being the darkest value. Trees and bushes are vertical elements and therefore receive less light making them generally darker. So in order to convey the sense of light present at different times of day or different weather conditions these value relationships must be established precisely. For instance, at mid-day when the sun is directly overhead, the ground plane is much lighter than the trees whereas in late afternoon or morning, the sun is low in the sky shining on the ground at a less direct angle. So at sunset the trees are still usually darker than the ground plane but are much closer in value than at mid-day.
