Phoenix, which had 128 days at or above 100 degrees Fahrenheit last year, is one of the hottest and fastest-warming cities in the U.S., and most American cities are expected to drastically heat up in the next decades with heat waves and triple-digit days. In the Valley of the Sun, work and play are shifting into the cooler hours. Neighborhoods are active at dawn and dusk when residents hike, jog and paddleboard. Last year, heat caused or contributed to the deaths of 182 people in Maricopa County, which includes Phoenix. Ariane Middel, a professor of urban climate at Arizona State University, says, “We are almost a living laboratory. We can test strategies and see different ways to keep adapting and mitigating. By the time it gets hot in other places, they can take what we have learned here.”