Phoenix shatters extreme heat records into fall

• This event wasn't limited to Phoenix, with much of the Southwest seeing record heat between mid-September and early to mid-October. But the Arizona capital saw some of the most jaw-dropping milestones.

By the numbers: The unprecedented fall heat wave catapulted the city to the top of the rankings for the number of days in a single year with high temperatures that reached or exceeded 110°F.

• On average, the city sees 21 such days per year, but in 2024 that figure stood at 70 days as of Oct. 16. This beat the 55 days set last year, during what was then the sprawling city's hottest summer.
• Phoenix-Sky Harbor International Airport set or tied a daily record high for 21 straight days, from Sept. 24th through Oct. 14th, according to Victor Murphy, who serves as the southern region climate service program manager with the National Weather Service.
• Prior to this year, Phoenix had never reached 110°F during the month of October, but this occurred four times during the month this year. The average last occurrence of such heat is on August 12, whereas the previous latest occurrence was on Sept. 19.
• "They hit 110F on October 7th. This is almost 2 months later than the avg. last occurrence, and nearly 3 week[s] later than the previous last occurrence," Murphy told Axios via email.

According to Maricopa County, where Phoenix is located, 389 heat-related deaths have been confirmed so far in 2024, with 292 deaths under investigation.

• The number of confirmed deaths is lower than at this point during 2023, but the number of cases under investigation is higher and a final comparison is not yet clear, the county stated on its heat surveillance website.

Stunning stat: The 21 straight days of record high temperatures in Phoenix beats what is thought to be the previous record string of daily high temperature records for a high quality weather reporting station.

• The old record occurred in Burlington, Iowa, during July 1936, Murphy said.
• "Burlington established a daily record high temp for 14 consecutive days, from July 4th to July 17th" of that year, he said.

The intrigue: The causes of the latest heat wave trace back to two key factors, in addition to the fact that climate change is tilting the odds in favor of more frequent, intense and longer-lasting heat extremes.

• These include a powerful and resilient area of high pressure that was parked for weeks across the Southwest.

• This led to widespread unusual warmth in multiple states.
• For example, the early fall heat led Palm Springs, Calif., to set a record with its greatest number of 110°F days in a single year, with 84 such days. This eclipsed the previous record of 66 days.

Also, Phoenix had been unusually dry during the summer and fall, Murphy said, which allowed more of the sun's energy to go into heating the air and surface rather than being taken up by evaporation from moist soils.

Context: Studies of specific extreme heat events, such as deadly Mediterranean heat waves this past summer, have clearly shown climate change is playing a crucial role in making such conditions possible.

• One study last year found record heat in the U.S., Europe and Asia in July would have been "virtually impossible" without climate change. Other research into specific heat waves have come to similar conclusions.

What they're saying: "80% of the top 5 driest summer and early fall time periods [in Phoenix] have been in the past 20 years" dating back to 1895, Murphy said. 

• "Only 20% of them occurred in the 110 years from 1896 to 2006. This falls hand in glove with the prevailing wisdom that the desert Southwest is getting hotter and drier," he said.