When you're planning a fantasy landscape painting, it often helps to do a quick atmospheric study. Here's one I did for the painting Waterfall City: Afternoon Light, from Dinotopia: Journey to Chandara.
If you make a 3-D schematic maquette, you can do the study while looking at the model. This study uses white and black charcoal on brown Kraft paper. The goal is to study the chiaroscuro: and to consider which things are light on dark and which things are dark on light.
An atmospheric study also gives you the chance to think about the mist and atmosphere. In a scene like this, not everything should be crisply lit. Some areas should be in shadow, and others half hidden behind veils of atmosphere.