A brother and sister eat lunch in the shade in front of the fountain at Fort Lowell Park, 2998 N. Craycroft Road. It was a rare respite from the midday heat in a very hot August, which through Sunday had an average temperature of 92.3 degrees.

 Rebecca Sasnett / Arizona Daily Star

August was Tucson’s hottest month ever recorded, with sizzling temperatures breaking the previous record β€” set in July.

In addition, the summer season from June 1 through Aug. 31 was Tucson’s hottest in all 125 years of weather records, breaking the previous summer heat record set in 1994.

A monsoon gone AWOL on top of an already overheated long-term climate combined to produce the extreme heat, authorities said.

β€œIt was awful,” National Weather Service meteorologist Carl Serniglia said of August’s heat. β€œWe had four days in August of 110 degrees or hotter and 26 days of 100 or better.”

The heat wave finally ended when monsoon rains arrived on a large scale Friday.

However:

  • Through Sunday, August’s average daily temperature, as recorded at Tucson International Airport, was 92.3 degrees. That’s nearly a full degree warmer than July’s average daily temperature of 91.5 degrees, the weather service said. The previous record-high August temperature was 90.2 degrees, set in 1994.

The final August average temperature will be cooler once Monday’s below-average reading is factored in. But one day’s temperature won’t be enough to put August’s reading below July’s, Serniglia said.

  • August’s average daily high temperature in Tucson was 105.7 degrees, compared to a normal average high of 97.4 degrees. The average low was 78.9, more than five degrees above the normal August low temperature of 73.3 degrees.

Many times in past hot summers, weather service officials and other experts have pinned the majority of blame for high temperatures on the urban heat island effect. That phenomenon occurs when buildings and pavement built in a city absorb the sun’s heat in the daytime and radiate it out at night, dramatically scaling up nighttime lows.

But that wasn’t the case this summer. Not only was the temperature increase larger during the day than at night in Tucson, the extreme heat was seen across Southeast Arizona, in urban and rural areas alike, Serniglia said.

β€œIt was widespread, 6 to 8 degrees above normal,” he said. β€œThe high temperatures were just oppressive. We didn’t have the clouds and showers to hold them down.”

Typically, the clouds and showers form summer storms, which usually cool things down at least slightly. But this summer, Tucson and its environs sat under a strong high-pressure ridge, centered over the Four Corners area, that blocked the monsoon and refused to leave until this past weekend.

β€œWe are not having the rain events and the tremendous cloudiness to cool us off in the daytime,” noted Gregg Garfin, deputy director of the University of Arizona’s Institute for the Environment.

This year’s high pressure ridge didn’t migrate far enough north for any length of time to allow for a sustained flow of air that typically transports monsoonal moisture into Arizona, said Kenneth Drozd, another weather service meteorologist.

That means a greater fraction of solar energy goes directly into heating the air, rather than evaporating water from the soil β€” making temperatures quite a bit hotter than they would be otherwise, said Daniel Swain, a UCLA climate scientist.

"Certainly it stands to reason that if we do not get the moisture, we have less cloud cover, less rain, and hotter temperatures," Drozd said.

Scientists have not directly attributed this heat wave to the much broader trend of rising temperatures caused by the continued emissions of heat-trapping greenhouse gases into the atmosphere.

But Garfin, Arizona State University climate scientist Ashley Bancroft and UCLA’s Swain agreed global climate change is making extreme heat waves more common.

β€œIn general around the world, as well as specifically in Arizona and the Southwest, it’s clear that climate change is increasing the frequency and intensity of heatwaves,” Swain said.

In fact, the human fingerprint upon heat waves is actually most distinguishable for those carrying historically unprecedented heat, such as what struck Arizona and the broader West this summer, he said.

Phoenix also had its hottest month on record in August and its hottest summer ever recorded.

Phoenix’s average August temperature was 99.1 degrees, compared to a previous record for the month of 96.7 degrees set in 2015. Its summer-long average temperature was 96.7 degrees, compared to a previous record of 95.1 degrees set in 2015.

While the monsoon’s failure and the heat island effect were also important factors, β€œclimate change is making it progressively easier to achieve this kind of extreme, record-breaking heat,” Swain said.

The United Nation’s Intergovernmental Panel on Climate Change has concluded it’s very likely that human influence has contributed to observed changes in frequency and intensity of temperature extremes on the global scale since the mid-20th century.

Nationally, the frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has increased since the mid-1960s, the federal government has said.

While the Dust Bowl era of the 1930s remains the peak period for extreme heat, the number of high temperature records set since the late 1990s far exceeds the number of low temperature records.

In Arizona, the number of extremely hot days did not noticeably increase in the period of 2005 to 2014, the National Atmospheric and Oceanic Administration has said.

The state has seen an upward trend in both average daily maximum and minimum summer temperatures, with the highest values for each occurring since the year 2000, the federal agency said.

An August, 2020, heatwave continued into a second week in Tucson and throughout Arizona. Tucson broke a record with a high temperature of 111-degrees. Most days bounce around from 106-109, with little monsoon rain to cool the afternoons. Video by Josh Galemore / Arizona Daily Star

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Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter@tonydavis987.