In total, the low temperature stayed above 90 degrees in Phoenix 16 times during July. The count of exceptionally hot nights obliterated the previous July record of 11 set in 2006, and has beaten the count for the most such hot nights in Phoenix for any year on record, with plenty of summer heat still to come.
Overall, both low and high temperatures during July ran the highest on record for the city, where weather records date to the late 1800s.
With an excessive heat warning in effect through Monday, and temperatures expected to top 110 into the middle of next week, the city keeps adding more days to the count of 90-degree or higher low temperatures.
The heat is in large part due to a strong area of high pressure, also referred to as a heat dome, parked over the region for much of the month.
As the hottest big city in the country, Phoenix averages about 175 days at or above 90 degrees and 110 days at or above 100, according to the National Oceanic and Atmospheric Administration (NOAA). You might crack a sweat just thinking about it.
But it’s a dry heat. And while that might not matter as much as people might think when it’s 110 during the day, such an arid atmosphere does tend to allow temperatures to drop off at night. Until recently.
Ninety-degree lows were almost unheard of in Phoenix before the 1970s, but then the city started seeing about one to two a year in the 1980s and ’90s. Since 2000, such occurrences have skyrocketed in frequency.
In the early 2000s the 30-year average was still about two, and yet in 2020 it’s about seven. To some extent, this may be related to population growth and the urban heat island effect, which keeps cities hotter than surrounding areas. But that does not explain the full extent of these trends.
Amber Sullins knows Phoenix. Born and raised there, Sullins is now the chief meteorologist at ABC 15 News in the city. When it comes to the 90-degree low temperature threshold, she stresses that it’s more than a trivial statistic.
Sullins said heat is the No. 1 weather-related killer in the region. This is a problem that can be quickly exacerbated when there is no relief at night and for an extended period of time, such as has been the case this year.
“It’s a very important metric since it means little to no overnight relief from the heat here in the city,” she wrote in an email.
That’s a point Paul Iñiguez made, as well. He’s the science and operations officer at National Weather Service forecast office in Phoenix.
“Just last year, according to the Maricopa County Department of Public Health, there were 197 heat-related deaths in the Phoenix area,” he said via email. He also stated that in the metro area over time, “we see daily high temperatures increasing, [and] low temperatures even more so.”
In 2020, there are extra concerns surrounding the coronavirus pandemic. It has been ravaging parts of the state recently, making the punishing heat tougher to deal with.
“Places where people typically go to escape the heat (like libraries, community centers, etc ...) have been closed or are limiting access to ensure social distancing,” Sullins said.
Although the hot nights might be the big story, it’s not the only noteworthy aspect to the heat of July 2020.
Days with highs of 110 or greater are also headed toward potentially tying or beating the annual record. Through Friday, there had been 28 such days, with more to come based on forecasts. The number to beat is 33 from 2011.
“The amount of 110-degree-plus days fluctuates a lot year-to-year based on our summer monsoon, but overall we are averaging about nine more 110-degree days a year than we did back in the 1970s,” Sullins said. She also pointed out that the 110-degree days are coming earlier than they used to, leading to an expansion of the 110-degree season in the city.
High pressure has dominated the region
One big reason for the heat may be that the monsoon has been, at least until recently, kept at bay due to strong and persistent high pressure over the region.
“There certainly has been a lack of showers and thunderstorms across the region, which we would typically expect during the summer,” Iñiguez said.
As indicator of the suppressed thunderstorm activity, he pointed to lightning data showing that this year is close to the record low in Arizona. Still, Iñiguez is hesitant to point to a specific cause for the latest bout of intense heat. “We will have to wait for a full look after the summer is done,” he wrote.
Regardless, high heat is expected to continue through the next week, and probably beyond. Longer-term outlooks from the NWS suggest that below average precipitation and above normal temperatures may persist in the region for months to come.
While it has always been hot in Phoenix, there is little doubt summers are getting hotter, as they are in much of the rest of the country.
Sullins stated that she’s “definitely seen the changes over my lifetime.” She continued, “I see it in the data as a meteorologist and I feel it as a Phoenix-native who knows how it used to be.”
According to an analysis from Climate Central, a science research and journalism organization, the average low temperature in Phoenix has increased by 5.5 degrees since 1970.
A combination of human-caused climate change and an expanding urban heat island are both to blame, though not necessarily equally. One of the clearest, most robust conclusions of climate change studies is that heat waves are becoming far more likely to occur, as well as more severe and longer-lasting. Recent research has even shown that there have been heat records elsewhere that would have been virtually impossible without the more than 1.8 degrees (1 Celsius) of global warming since the preindustrial era.
Iñiguez noted that rural stations, while seeing temperature rises, are not seeing them as fast as Phoenix. He also said that, “despite the ‘why,’ the reality is temperatures are warming drastically in a city of millions of people, and it is producing a significant health and economic impact.”
“There are very clear signals that our heat is getting worse,” he said.