As Phoenix Heats Up, The Night Comes Alive

Dozens of hikers set out for the summit of Piestewa Peak on a July evening, their flashlights dancing in the dark. “You feeling O.K.?” Trevor Plautz, a park ranger, asked two women, one of whom had stumbled and was breathing hard. “You have enough water?” Both soon turned back, moving slowly down unlit rocky switchbacks. An owl chittered. “You definitely feel the heat, but the nights are better,” Mr. Plautz said. “A lot of people hike right now instead of during the day because it is a lot cooler.” Phoenix, which had 128 days at or above 100 degrees Fahrenheit last year, is one of the hottest and fastest-warming cities in the United States.