Warmest, driest December on record capped Tucson's third-hottest year

Record-setting warmth enveloped Tucson in December, making it the third month out of the past four with record-high average temperatures.

Rainfall continued to avoid Tucson last month, giving us our driest December on record and maintaining a dry period that has now stretched nearly two months. No measurable rain has fallen at Tucson International Airport since Nov. 3.

For the year, Tucson’s annual average temperature was the third highest since weather record-keeping began here in 1895. A string of below-normal months from January through April kept us from breaking the city’s record for the hottest year set in 2017.

But unrelenting, punishing heat in the summer and fall and a freakishly warm December pushed us to near-record levels for the year.

This year’s near-record heat also continues a pattern of extremely hot weather that has now stretched for more than a decade here and has led many climate scientists to finger human-caused climate change as a principal cause.

All but one of the city’s 10 hottest years on record have occurred starting in 2014. The exception, in 1989, had the coolest average temperature of any of those 10 years, National Weather Service records show.

December’s average temperature of 59.5 degrees came in a “whopping” 6.5 degrees above normal for the month and 1.4 degrees above the previous December record high, set in 1981, the National Weather Service said.

The average high temperature of 75.5 was an even more lopsided 10 degrees above the normal December high temperature.

Phoenix also had a record-warm December and experienced its hottest year on record, the Weather Service said. Phoenix’s average December temperature was, like Tucson’s, 6.5 degrees above normal for the month.

And last month marked the first December since 1981 in which no rain fell at Tucson International Airport. The city’s northwest side managed to eke out less than five-hundredths of an inch of rainfall last month, “with very isolated areas” there receiving a tenth of an inch, the Weather Service said.

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The two-month dry spell is still a ways from setting a record, however. That belongs to the 117-day period of no rain that ended on Dec. 30, 1950, the weather service said.

December’s weather essentially continued the fall’s weather pattern of very unusually warm, dry weather, said Michael Crimmins, a University of Arizona climate scientist and professor at UA’s Department of Environmental Science.

Except for November, which was slightly cooler than normal, all of the past seven months through December experienced record-breaking or record-tying heat. In November, even though the average temperature came in 1.6 degrees below normal for the entire month, the month’s last 14 days all saw temperatures in the 70s and 80s.

“We had the fall weather pattern hang on and on. We had the monsoon kind of putter out early and we ran into a miserably dry September and a brutally hot October. We had a rigid high pressure mass. Then we had one day of rain in November, and I thought ‘man, this is it’,” Crimmins said.

“Right after, the ridge of high pressure tagging us in October came back. We couldn’t get a cloud in edgewise.”

Looking at the impacts of climate change on the past decade’s weather, “the data is pretty stark,” Crimmins said.

“Look at it over any time frame, monthly, seasonal, annual, they’re all coming up,” he said, referring to the region’s continually rising temperatures.

The rising temperatures are also producing more hot days that threaten peoples’ health, the nonprofit research group Climate Central told the Arizona Daily Star.

For all of 2024, Tucson experienced 63 days of what the group classifies as “dangerous heat days.” Those are days in which the average daily temperature was at least 88.5 degrees. Of those 63 days, Climate Central determined that 39 could be attributed to human-caused climate change.

Phoenix experienced 65 “dangerous heat” days in 2024, of which 35 could be attributed to climate change, the group said. Because Phoenix is hotter than Tucson, a “dangerous heat day” there has an average temperature of at least 96.

Climate Central bases its calculations of “dangerous heat days” on what it calls a minimum mortality temperature threshold. That’s the temperature at which documented cases of heat-related illness and death begin to show a statistically significant increase compared to the average, said Climate Central spokesman Peter Girard.