Phoenix is best in the cooler months, October-April, when warm, sunny afternoons (mid-60s to mid-80s F/high teens to high 20s C) give way to nights that cool off considerably. Occasional rain showers and thunderstorms occur during winter, but snow is unknown in these parts. There are long stretches with little rain, with the average rainfall total at about 8 in/20 cm a year.
If you visit in summer, be prepared for heat—very high heat: Temperatures above 100 F/38 C are common June-September, though the low humidity makes it a bit more tolerable. Summer winds are dry, and are not as cooling as most visitors are accustomed to. Things get worse mid-July to mid-August as the usually dry air turns muggy. Be wary of sudden dust storms and downpours in summer.