Painters are like farmers: They love the sunshine unless they need rain, and they love the rain unless they need sunshine. Sunny, warm weather is great for evaporating moisture out of wood, but too much sun wreaks havoc with an in-progress paint job.
Here's the problem. When you're painting a large surface, like the exterior wall of a house, you have to work in sections, moving the ladder and drop cloths as you go. If the sun is glaring directly down on the wall, then the paint will dry faster than you can move your equipment. When you apply wet paint over dry paint, it ends up creating dark lap marks where the sections overlap [source: Benjamin Moore]. The result is uneven and splotchy color.
Ideally, you should time your paint job so that you are always actively painting on the shady side of the house. If that's not possible, try to paint on a cloudy day — but not a rainy one!