Arizona
There’s more to Phoenix hot-weather records than you may realize
The National Weather Service provides helpful information for extreme heat in Arizona, but you may need some help deciphering the data on the website page.
For example, under “Average First 100° F Day,” there are two sets of figures for the Phoenix area: “May 10 (POR)” and “May 2 (1991-2020).” So, which one do we go by?
According to The Republic’s environment and climate editor, Shaun McKinnon, those two sets of figures can be interpreted this way:
“May 10 is the ‘period of record,’ which means the average for 1896-2025. May 2 is based on the last 30 years (which is the time frame the National Weather Service uses to determine ‘normal’ temps. Every 10 years, the service slides the date ahead 10 years to keep it at 30). The difference between the two figures shows that 100-degree days are arriving on average eight days sooner than over the long term.”
Which means it’s starting to get hotter sooner in the year now.
The data also applies on the other side of what we Arizonans loosely call “the summer season”: The average last 110-degree day is Aug. 13 (period of record) and Aug. 26 (1991-2020), which anyone who has lived here long enough knows by experience means the extreme heat is lasting longer.
What you need to know about Phoenix-area heat
A Tucson native whose family spent summers in the White Mountains, Rebecca “Becca” Dyer lives in the Valley and is a copy editor/online producer for The Arizona Republic/azcentral.com. Send tips on Arizona history items to rebecca.dyer@arizonarepublic.com.