Expanding urban heat island fuels record warm nights in Phoenix

Phoenix matched its record for the warmest low temperature on Friday, when the overnight temperature fell only to 97 degrees. The record was first set on July 19, 2023.

Researchers at Arizona State University say the city’s expanding urban heat island is helping drive hotter nights. The effect is not limited to the area around Sky Harbor, which sits near the center of Phoenix’s urban heat island.

Development west and northwest of Phoenix has replaced agricultural fields and open desert with homes, buildings and roads. Those surfaces absorb and hold heat during the day.

Overnight winds from the west and northwest can carry that stored heat into Phoenix and parts of the East Valley. Researchers call this movement of heat thermal advection, and say it is contributing to rising nighttime temperatures.