Hottest month for a US city: Phoenix sets national heat wave record

A historically intense and prolonged heat wave has spent weeks baking the South and Southwest, bringing dangerous triple-digit temperatures to 70 million Americans. Phoenix, at the epicenter, just recorded its hottest month on record — and the hottest month ever observed in an American city.

Phoenix’s average temperature for July was a breezy 102.7 degrees, factoring in an average daytime high of 114.7 degrees and a nighttime low of 90.8.

The previous highest monthly average temperature in a US city was 102.2 degrees, located in Lake Havasu City, July 1996, according to the Arizona State Climate Office. Phoenix beat that mark by 0.5 degrees, a significant margin for such an already high record.

The July average of 102.7 degrees in Phoenix surpassed readings ever observed at any weather station nationwide, except for the inhospitable Death Valley, California, which is considered to be the hottest location in the world.

It’s not just hot. Climate anomalies are emerging across the globe.

While the heat in Phoenix eased just enough on Monday to end his record-shattering streak of 31 straight days at or above 110, the warm weather is expected to recharge later this week. Excessively hot watches is in effect from Friday morning to Sunday for the metro area with a population of 5 million. High temperatures of 111 to 116 degrees are expected each day, and the National Weather Service warns that the “extreme heat risk” will heighten the threat of “heat cramps and heat exhaustion … (which, without intervention, can lead to heat stroke).”

Phoenix’s record hot month, by the numbers

The number of records set in Phoenix during July, and the margins by which many of the old records were surpassed, is staggering. Here’s a rundown of Phoenix’s exceptional month (and beyond); Note that the bookkeeping is from August 1895.

  • Hottest month ever: With an average temperature of 102.7 degrees, Phoenix obliterated the previous record of 99.1, set in August 2020, by a whopping margin of 3.6 degrees. The month was 7.2 degrees warmer than the average for July.
  • Longest streak of 110-plus-degree highs: 31 days, from 30 June to 30 July. (Monday’s high was 108 degrees, ending the streak.) The previous longest streak was 18 days, set in June 1974.
    • Twelve days set daily record high temperatures, including July 19, 20, and 25, when the temperature reached 119 degrees.
  • Most 115-degree days in a month: July had 17 days that hit 115 degrees or more. The previous record was seven days in August 2020.
  • Longest streak of overnight low temperatures at or above 90 degrees: Phoenix did not drop below 90 degrees between July 10th and July 26th. The 16-day stretch sailed past the previous record of seven days in 2020.
  • Warmest overnight low on record: The morning low on July 19 was 97 degrees at Sky Harbor International Airport. The normal low night there in July is 84.5 degrees.
    • It was one of five nights in July where Phoenix didn’t drop below 95 degrees. Before this year, the city had only had six such nights, and never more than two in a year.
    • The average nighttime low for the month was 90.8 degrees, meaning nights ran about 6.3 degrees warmer than average.
    • There were 16 nights that brought or broke temperature records.
  • Highest daily average temperature on July 19: With a high of 119 degrees and a low of 97 — making for an overall average daily temperature of 108 degrees — July 19 was Phoenix’s hottest calendar day on record.

What caused such extreme heat?

There were probably three factors that combined to propel Phoenix into such uncharted territory for the month. It was probably an overlap of natural and anthropogenic factors, exemplifying the effects human action can have on the atmosphere and highlighting the associated consequences.

Among the contributing factors:

  • Natural variation. Some months, due to the inherent randomness of the weather, are naturally warmer than average. In this case, the overall weather pattern, which contained a stagnant ridge of high pressure, colloquially known as a “heat dome”, was the dominant weather maker. In addition to bringing warm, dry, sinking air that broke up the cloud cover, the heat dome staved off the southwest monsoon — a seasonal wind shift that brings in moisture. It further cut down on the amount of cloud cover, allowing unfettered sunshine to bake the earth and warding off showers and thunderstorms. Dry air is also easier to heat to higher temperatures.
  • Man-made climate change. The frequency, intensity and duration of heat events are demonstrably increasing due to increasing concentrations of heat-trapping greenhouse gases in the atmosphere, as a result of human activities. Since records began in the mid-1890s, Phoenix has warmed 7.9 degrees during July.
  • The urban heat island effect. It’s no secret that sidewalks, cement, sidewalks, and concrete are warmer surfaces than dirt, grass, or the canopy of a forest. This leads to the capture of heat. This is why cities are often much warmer than the surrounding rural areas, especially at night. It has a significant influence on rising temperatures. In 1920, Phoenix occupied five square kilometers. In 2010, 519 square kilometers – and it continues to grow. It undoubtedly plays a significant role here in addition to the contribution of greenhouse gases.

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