This lighting angle works well with landscapes, because it casts dramatic shadows and reveals forms and textures. If you want to photograph a beautiful sunset, you'll be pleased to know that exposure isn't critical. In fact, a number of exposure settings will work, depending on the mood you want to create. Less exposure will give you darker, richer, more dramatic effects. When you want to capture the afterglow of a sunset, try two to four stops more exposure than for the sunset.
Rainbows require no special exposure.
Just set your exposure as you normally would, unless you include a large expanse of relatively bright sky in the scene. In this case, base your exposure on the foreground. Avoid overexposure, which lightens the colors of the rainbow. Either use a telephoto lens or stay in your car and photograph through the window.