Black and White
A very fascinating tip in art, that affects almost every observation in nature, is this:
When illustrating wires such as telephone wires, or high power lines, paint them as very light or white when they are in front of something, such as a bunch of trees, or buildings, and paint them as very dark, almost black, when they are in front of the sky.
Why? Well next time you are out and about, take a look and that is exactly what you will see, especially when the sunlight is on the same side of the wires you are on and not behind the wires. Observe how they go from near black in the sky to bright white across the trees.
If you observe very, very closely, you can see that the wires are both, white light from the sky hits the tops of the wire, and shadow caused by the wire on itself makes a black underside. In the sky you only notice this black because the white blends with the sky colors, and in front of dark objects the black blends in and you only notice the white. Paint it that way and your work improves because you are seeing better.
Now at sunrise or sunset, have you ever noticed how the orange is very bright on wires and lines? That is because the light is sideways to the wire (and to you as well), and so much more light is being reflected back to your eyes that it glows. Look carefully and you will see the wires turning several shades of orange as the wires swoop down and up. Often you can hardly see any shadow on the wire, or even highlight, just orange or yellow!
Cylinder-like tree trunks, light poles, fence posts,etc. have a white line on both edges. A large one on the sunny side, and a little one on the back. That is, because of the atmosphere, much of the natural light from the sun is bouncing off the air and lighting the back side of objects. All objects have this effect, but the round or cylinders are the most noticeable because you can see around the corner (so to speak).
Grapes and oranges and pretty much all round things have lighted edges, even on the back shadow sides. Sometimes it is so very faint, but it is there, as long as there is an atmosphere. Sometimes in the very cold climes, this effect is very pronounced because the frozen crystals in the air bounce more light, but sometimes because the cold is so very cold, the moisture “freezes out,” making clean dry air, and the shadows are much more dramatically dark.
So to depict back light, always place a little light along the back dark edge. I pay attention to the seasons and the time of day. It is surprising how very different the same objects can bounce light with small changes, not to mention big ones.