Hot and humid nights drove another summer of record heat across the U.S.
This was one of the hottest summers—if not the hottest summer—ever recorded for large portions of the United States, another data point on the undeniable trend toward warmer days and even warmer nights as our climate steadily changes.
Meteorological summer began on June 1 and ended on August 31. This clean division of months more closely tracks temperatures than dividing the year up based on astronomical solstices and equinoxes.
The overall pattern we saw this summer featured persistent ridges of high pressure over the western and eastern ends of the U.S., with troughing in the center of the country.
Ridges foster sinking air, which warms up as it compresses on descent toward the ground. Troughs force air to rise, leading to unsettled weather and generally below-seasonal temperatures.
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| Source: IEM |
We can see this divide in a map showing how much average daily temperatures departed from normal. A day’s average temperature is the daytime high and nighttime low averaged together. Troughing in the center of the country led to somewhat seasonable temperatures, while folks west of the Rockies and east of the Appalachians dealt with warmer-than-normal conditions.
What really stands out is the desert southwest.
Folks in Phoenix, Arizona, just lived through their hottest summer on record. The daily average temperature there was a whopping 98.9°F this summer, which beat the previous record (set last year!) by nearly two degrees.
This summer’s daytime highs in Phoenix were absolutely blazing hot—as of this post, the city is in the midst of a 100-day streak of daytime highs at 100°F or hotter, exceeding the previous record 100-degree streak by nearly three weeks.
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| Source: IEM |
But it was the nighttime lows that really stood out.
Every single day this summer saw a warmer-than-normal low temperature, with those departures often coming in 10 or more degrees above normal. That’s no small thing when midsummer nights don’t cool off much in the desert. The city’s low temperature in July only dipped below 88°F five times, and it remained 90°F or warmer at night 19 days throughout the month.
Here are the Summer (Jun-Aug) 2024 A) temperature rank, B) temperature departure from normal, C) precipitation rank, and D) precipitation percentage of normal. pic.twitter.com/8lUdLBQ1j7
— Brian Brettschneider (@Climatologist49) September 2, 2024
While not as unbelievably hot, the story of nighttime lows driving near-record summertime temperatures is constant up and down the East Coast.
Take a look at interior New England, where Caribou, Maine, just saw its warmest summer since records began there back in 1939. Repeated intrusions of muggy air in late June, mid-July, and in early August made low temperatures above 65°F a common sight in this small community near the Canadian border. For reference, Caribou’s average low on July 16—summer’s midpoint—is about 57°F.
This summer’s noteworthy heat and humidity follows the pattern you’d expect to see as the climate changes.
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| Source: Climate Central |
As I wrote back in July: “Climate change sets a new baseline for extreme heat throughout the United States and around the world. As the entire frame of reference moves toward a hotter climate, warm temperature extremes are far more likely that cold temperature extremes. Excessive heat will come in hotter than what we grew used to just one or two generations ago.”
A warmer atmosphere can also hold more moisture. This increase in humidity makes it harder for nights to cool off as much as they used to, which is a significant factor driving warmer-than-normal summer low temperatures.
