The first big heat wave of 2025 has arrived, coinciding with the official start of summer. More than 40 million people in the U.S. were under heat alerts Friday.

As the Bay Area’s first heat wave of 2025 began to crank up the thermometer on Thursday, the Bay Area Air District responded in a familiar way by issuing the season’s first Spare the Air alert.

A heat wave is moving in, with over 100 million people from the Midwest to the Northeast on alert for life-threatening heat this weekend and into next week.