20230802 extreme heat
Extreme heat hitting the Phoenix area has been damaging, cactuses, and other desert plants. Image Credit: AFP

A historically intense and long-duration heat wave has spent weeks baking the South and Southwest, bringing dangerous triple-digit temperatures to 70 million Americans. Phoenix, at the epicenter, just logged its hottest month on record - and the hottest month ever observed in a U.S. city.

Phoenix's average temperature for July was a blistering 102.7 degrees, taking into account average daytime high of 114.7 degrees and overnight low of 90.8.

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

The 102.7-degree July average in Phoenix surpassed readings ever observed at any weather stations nationwide, except for the inhospitable Death Valley, Calif., which is considered the hottest location in the world.

While the heat in Phoenix eased just enough on Monday to end its record-shattering streak of 31 straight days at or above 110, the hot weather is forecast to recharge later this week. Excessive heat watches are in effect from Friday morning through Sunday for the metro area of 5 million residents. High temperatures of 111 to 116 degrees are expected each day, and the National Weather Service is warning that the "major heat risk" will bolster 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 breakdown of Phoenix's exceptional month (and beyond); note that bookkeeping there dates to August 1895.

- Hottest month on record: With an average temperature of 102.7 degrees, Phoenix obliterated the previous record of 99.1, which was set in August 2020, by the enormous margin of 3.6 degrees. The month was 7.2 degrees warmer than average for July.

- Longest streak of 110-plus-degree highs: 31 days, from June 30 to July 30. (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 made it to 119 degrees.

- Most 115-degree days in a month: July had 17 days that hit 115 degrees or greater. The previous record was seven days in August 2020.

- Longest streak of overnight lows at or above 90 degrees: Phoenix didn't fall below 90 degrees between July 10 and July 26. That 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 overnight low there in July is 84.5 degrees.

* That was one of five nights in July when Phoenix failed to fall below 95 degrees. Before this year, the city had only ever had six such nights, and never more than two in a year.

* The average overnight low for the month was 90.8 degrees, meaning nights ran about 6.3 degrees hotter than average.

* There were 16 nights that tied or broke temperature records.

- Highest daily average temperature on July 19: With a high of 119 degrees and low of 97 - making an overall average daily temperature of 108 degrees - July 19 became Phoenix's hottest calendar day every recorded.

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 human-caused factors, exemplifying the effects human action can have on the atmosphere and highlighting the associated repercussions.

Among the contributing factors:

• Natural variability. Some months are, due to the inherent randomness of weather, naturally hotter than average. In this case, the overarching weather pattern, which featured a stagnant ridge of high pressure colloquially known as a "heat dome," was the dominant weather maker. In addition to bringing hot, dry, sinking air that squashed cloud cover, the heat dome fended off the Southwest monsoon - a seasonal wind shift that brings in moisture. That further cut back on the amount of cloud cover, allowing uninhibited sunshine to bake the ground, and staved off showers and thunderstorms. Drier air is also easier to heat to higher temperatures.

• Human-caused climate change. The frequency, intensity and duration of heat events is demonstrably increasing because of rising concentrations of heat-trapping greenhouse gases in the atmosphere, resulting from human activities. Since record-keeping began in the mid-1890s, Phoenix has warmed 7.9 degrees during July.

• The urban heat island effect. It's no secret that pavement, cement, sidewalks and concrete are hotter surfaces than dirt, grasses or the canopy of a forest. That leads to the trapping of heat. It's why cities are often notably hotter than surrounding rural communities, particularly at night. That has a marked impact on raising temperatures. In 1920, Phoenix occupied five square miles. In 2010, 519 square miles - and it's continuing to grow. That's undoubtedly playing a significant role here in addition to the contribution from greenhouse gases.