September 2025 Heatwave: By the Numbers

This September, much of the United States experienced what meteorologists are calling a record-breaking heatwave. From Chicago to New York, cities sweltered under unusually high temperatures, and the numbers reveal the full story. Across the contiguous U.S., the average temperature was about 3.1°F above the 30-year normal, making it one of the hottest Septembers on record. Chicago recorded 12 days above 90°F, the most ever in September, while New York City saw its warmest day of the month at 95°F. Nationwide, more than 1,200 daily high temperature records were broken, with the Midwest and Northeast particularly hard hit.

The effects of this heat went far beyond discomfort. Electricity demand surged as people relied on air conditioning later into the season, putting extra stress on local power grids. Crops approaching harvest in the Midwest faced heat stress, raising concerns about potential reductions in yield. Hospitals reported an increase in heat-related visits, especially among older adults and outdoor workers. Even transportation felt the impact. Heat can warp train tracks and soften road surfaces, causing delays in some areas and showing how extreme temperatures ripple through daily life.

This record-breaking September is not an isolated event. Looking back over the past 25 years, September temperatures have been steadily rising, and heatwaves are becoming both more frequent and more intense. Climate scientists note that global warming is the main driver of these extreme events. This year, El Niño conditions—when unusually warm ocean waters in the Pacific change weather patterns around the world—likely made the heat even more pronounced. The underlying trend of steadily increasing temperatures, however, is clear.

The takeaway is simple: numbers matter. Behind every headline about “unseasonable heat” are thousands of daily observations, spikes in energy use, and local impacts. Understanding these trends helps cities, farmers, and hospitals plan and adapt, and it shows that seasonal expectations are shifting. September 2025 sends a clear message: summer may be over on the calendar, but the heat is far from finished, and the data gives a glimpse of what future Septembers could bring.