If you are a landscape painter, it is sometimes hard to observe the effects of light and shadow. We have a tendency to paint what we want to see, not what we actually do see. One way to notice the effects of light and shade is to watch time lapse videos of landscapes.
Watch this video and be amazed at how much more you notice about the ambient effects of sunlight and time of day. Speeding up the motion makes it much easier to see.
You can make your own time lapse sequences with a decent web cam and a programme called StopMotionPro. http://www.stopmotionpro.com I use it and recommend it.