Phoenix has relentlessly baked for months. In July, it clinched its hottest month on record. Then August topped that and became the new hottest month.

The back-to-back records propelled the country’s sixth-largest city to its hottest summer (June through August) on record by a wide margin. The summer finished 1.6 degrees above the previous high mark.

The scorching heat in Phoenix and other parts of the desert Southwest comes as several other parts of the Lower 48 also registered their hottest summers.

Phoenix feeling like fire

The heat milestones established in Phoenix during the summer are too many to list.

“By essentially any metric, this has been the city’s hottest summer on record,” Paul Iñiguez, science and operations officer at the National Weather Service office in Phoenix, wrote in an email.

Among the most significant records:

  • Hottest August and hottest month: August’s average temperature of 99.1 degrees, 0.8 degrees above the previous hottest August, represents the hottest month on record in Phoenix. The blistering average high of 110.7 is the hottest of any month as well. In addition, the 12 nights with low temperatures at or above 90 best the previous August record of eight such nights, set in 2011, for a new August record.
  • Hottest July and second-hottest month: The average temperature of 98.9 degrees sets a July record, boosted by an average high temperature of 109.8 degrees. With 16 instances of nighttime lows 90 degrees or greater, the most of any month, there was little relief.
  • Hottest summer: The summer started inauspiciously, with the average June temperature of 92 degrees ranking as the 21st hottest on record. Then came the back-to-back furnace-like conditions in July and August. The summer average temperature of 96.7 degrees demolishes the old summer mark of 95.1 degrees set in 2015 and 2013.
  • Most 90-degree nights: The temperature never dropped below 90 on 28 nights. The old record was 15 such instances, set in 2013 and 2003. There were two separate week-long streaks of nights not falling below 90.
  • Most 110-degree days: The 50 days with high temperatures at or above 110 degrees destroys the old record of 33 such days in 2011. In July and August, 66 percent of days were at or above that mark, with roughly a dozen high temperature records set in the process.
  • More excessive heat warnings: The Weather Service issued these warnings, the most severe heat alert, 43 times, vastly outpacing other years in the past decade-plus.

How this happened

A combination of a weather pattern conducive to heat, urbanization and long-term, human-caused climate change all played a role in Phoenix’s historically hot summer.

Phoenix’s temperatures have trended upward for many decades because of urban sprawl and the associated increase in heat-absorbing surfaces such as asphalt and concrete. Simultaneously, atmospheric concentrations of greenhouse gases in the atmosphere have increased, causing temperatures around the world, and in Arizona, to increase. The background warming — which is especially significant in Arizona compared with other states — dramatically increases the odds of heat extremes.

According to an analysis from Climate Central, a climate change research and communications nonprofit, the average low temperature in the city has sprinted upward by 5.5 degrees since 1970. Phoenix ranks fourth on its list of fastest-warming cities, with average temperatures climbing 4.3 degrees between 1970 and 2018, the group found.

This summer, the heat was intensified by a lack of clouds, rain and storm activity because of a weaker-than-usual summer monsoon, which typically runs from July into early fall. The Weather Service noted that 2020 is in the running for the lowest number of lightning strikes during the monsoon season on record in Phoenix.

Since July, an unusually strong and persistent area of high pressure, or heat dome, has sprawled over the West, setting records not only in Phoenix but also in Tucson and many other locations west of the continental divide.

Because of the heat and lack of monsoon rainfall, much of the Southwest is in the throes of a major and expanding drought. The dryness sets up a self-reinforcing feedback cycle in which a drier land surface makes it possible for air temperatures to heat up faster, with less solar radiation going toward evaporating water.

A silent killer

With increasingly hot summers, the risk of heat-related illnesses and deaths is rising. Heat is a silent but efficient killer. Over the past 10- and 30-year periods, it has killed more people than any other weather hazard in the country.

Heat also preys upon inequality, as a recent story from the Arizona Republic documents, with increases in heat-related deaths largely affecting homeless people and low-income families without access to air conditioning.

The numbers are troubling in Maricopa County, home to Phoenix. Following record numbers of heat deaths last year, 2020 is ahead of that pace.

“Year to date we have 40 confirmed heat deaths in Maricopa County, compared to 32 at this time last year,” David Hondula, a senior sustainability scientist at Arizona State University, wrote in an email.

The count of 40 deaths is preliminary. Mark Kear, a geographer at the University of Arizona, emphasized that mortality figures from this year are still being tabulated.

“The number of heat death cases under investigation in Maricopa is concerning,” he wrote in an email.

Kear noted the number of heat mortality cases under review is more than twice those reported this time last year, running at 272 now compared with 134 a year ago.

When the novel coronavirus and heat mix

In the spring, Kear and other researchers highlighted the potentially deadly threat from the overlapping crises of the novel coronavirus and Phoenix’s intense summertime heat. In an op-ed in the Arizona Republic, they expressed concern that vulnerable groups would stay home to avoid exposure to the virus, despite a lack of access to air conditioning.

One of the co-authors of that op-ed, Patricia Solís of Arizona State, said the ongoing economic fallout from the pandemic could result in the loss of housing or the inability to pay bills, including the critical electricity bill, meaning people could be without cooling even though they still need it.

The Weather Service’s Iñiguez noted that the city has had fewer available cooling centers because of the pandemic, plus a lack of informal cooling locations as a result of limited indoor dining options.

From early July through mid-August, when some of the hottest temperatures occurred, coronavirus cases spiked in Maricopa County and other parts of Arizona. According to a Washington Post database, the county is still reporting a seven-day average of 245 new cases per day, along with 25 deaths.

No significant relief from the heat is in the forecast, and excessive heat watches are already in effect for Labor Day weekend.