One of the possible ways of handling unexpected and varying lighting conditions is to use a white balance lens cap to average the light temperature. Haje over at Photocritic makes a very valid point about white balance and white balance averaging lens caps.
Basically he says that if direct sun is hitting the lens cap while your subject is in the shade your reading is going to be off. Makes perfect sense to me.
Here is a typical situation Haje describes where this method will not work.
“You are in the sun, but your subjects are in the shade. Your white-balancing cap will measure the light that’s hitting your camera (direct sunlight – or around 5,500 Kelvin or so). However, your subject will be in the shade (around 7,000 Kelvin or thereabouts).
The outcome is utterly predictable: Your white balance is going to be miles off.“#
Solution is pretty simple, either use your camera to do white balance or make sure that the light hitting the white balance lens cap is the same that hits your subject.
[Why white-balance lens caps don’t make sense | Photocritic]