September 21, 2026
Weather | September 21, 2026 | 2 min read
If this September has felt more like July, you're not imagining it. A new NASA-led study finds that extreme heat seasons have grown longer across about half of the world's land, and in the western United States, dangerous heat is hanging on further into fall. That matters for more than comfort. Hot, still days are also prime conditions for poor air quality, and a longer hot season means more weeks with the windows shut and the AC running.
The study, published in AGU Advances on September 10, 2026, was led by Catherine Ivanovich of NASA's Goddard Institute for Space Studies. Researchers compared 1980 to 1989 with 2015 to 2024 and tracked two kinds of extreme heat: dry heat, meaning the hottest 5% of daily temperatures, and humid heat, measured by wet-bulb globe temperature. Extreme heat seasons expanded across just over half of global land for dry heat and just under half for humid heat. (Columbia Climate School)
"There are very clear asymmetries in how extreme heat seasons are expanding in different parts of the world," Ivanovich said.
The western U.S. is seeing heat push later into fall. Phoenix offers a clear example: in 2024, the city set 21 daily temperature records in a row from late September to mid-October. The researchers note that late-season heat in the West now overlaps with peak wildfire season, while in the Southeast it overlaps with hurricane season.
Heat and air pollution tend to show up together. According to the EPA, ground-level ozone forms when pollutants from cars, power plants, and other sources react in sunlight, and it's most likely to reach unhealthy levels on hot, sunny days. Heat waves often bring stagnant air that keeps pollution close to the ground, and hotter, drier conditions can fuel wildfires that add fine particles (PM2.5) to the mix.
Timing makes it harder. The study notes that heat arriving outside the usual season is tougher on the body, and that cooling centers and heat alerts are still largely built around a summer calendar.
More hot weeks usually mean more time indoors with the windows closed and your HVAC system recirculating the same air. Every time the system runs, that air passes through your filter, which captures dust, pollen, and fine particles along the way. More runtime means the filter fills up faster.
One honest limit: HVAC filters capture particles, not gases, so they don't remove ozone. On high-ozone days, keeping windows closed during the afternoon and early evening, when ozone is typically worst, is the better move.
Match your filter schedule to the season you actually get
Many homeowners change filters by the calendar and assume they can coast once Labor Day passes. With heat hanging on into fall, a filter that has been working hard since June may already be overdue. A clogged filter restricts airflow, and the U.S. Department of Energy says replacing a dirty filter with a clean one can lower an air conditioner's energy use by 5% to 15%.
Pull your filter out and take a look. If it's gray and coated in dust, it's time. As a general rule, standard pleated filters need changing about every 90 days, and sooner when your system runs nonstop, while Filterbuy's 4-inch filters last 6 to 12 months. If you live in a wildfire-prone part of the West, where fall heat and smoke now overlap, consider stepping up to a MERV 13 filter.
Check your filter before fall heat hits