I would also venture to guess that clouds are hard to come by in Serengeti! At this point in my photographic explorations, I'm only using an in camera light meter. I understand that when I get a hand held it is going to improve my accuracy.

If I'm understanding correctly, light is going to affect the contrast of a scene because it will cast harder shadows, creating darker areas (possibly black). Back to the example of snow though, If I am shooting a shooting a scene (which i often do) that has a frozen pond with a light snow cover with a background on a hillside, with an overcast sky, it would still seem to me to be contrasty, due to the amount of shades the separate the white snow and the dark hillside?