Clouds that are front lit have a brighter centre and darker edges (top image). Clouds that are back lit have a darker centre and brighter edges (second image). It may seem simple but many of us never take the time to enjoy this simple fact. The science of scattering requires that this is the case. Cloud illumination clearly reveals the location of the sun. You may not be able to see the sun, especially in a painting but there can be no doubt as to where it must be! These two images were taken seconds apart from the middle of Singleton Lake.
I use this fact all the time in CSI - Creative Scene Investigation. I also used it in the 50 Years of Our Flag Painting. It is a useful device to transition from scene to scene. In fact I used back lit cloud in every transition except one. The sole exception is the deformation zone cloud approaching the white quartzite of Killarney from the southwest.
The example I will highlight is that of a very thick layer of back lit stratocumulus. These particular clouds were water droplet clouds with only the edges allowing any sunlight through at all. The same dark cloud mass is transformed into rocks and thus also provides the terra firma for the light house and associated buildings.
This is a bold transition. As I was painting this, I was thinking of Lawrence Nickle's reference to the Group of Seven's treatment of clouds as "boulders in the sky". In this case, I needed this option and I hope my Friend Lawrence would approve.
By the way that is Tom Thomson's dove gray canoe at the portage for Ragged Falls.