But if it's hot and dry, it's a little difficult to work with. It's tough to apply and you'll get an irregular appearance. In high heat, the solvents in the paint may evaporate faster than what is desirable, and may not form a good film. The problem is exacerbated by especially dry climates, with humidity levels of less than 30 percent. Consider the color of the substrate as you're painting.
Dark colors hold heat better than do light colors.
If you're painting over a dark surface, it's possible that the surface temperature is significantly warmer than the ambient temperature. Remember that metal surfaces in the direct sun might become inordinately hot. Paint them first thing in the morning, before they've had a chance to heat up, or cool them off before proceeding. If you're using a latex paint, you can cool an overly hot surface by sprayiing cold water over it. Latex paints aren't harmed by painting over a damp surface.