From All Things Considered:
Phoenix actually suffers from two heat problems. One is a product of growth. Desert nights don't cool down they way they used to, because energy from the sun is trapped in roads and buildings, a phenomenon researchers call the "urban heat island effect."
As Phoenix grows, so does the problem, says Nancy Selover, the state climatologist.
"We keep thinking we'll probably see a night when we only get down to 100 as a minimum temperature, which is kind of shocking," Selover says.