More sweltering summer trend: West has hotter days, East has hot nights


Strange as the deadly heat wave that hit the Pacific Northwest has been, it is part of a pattern of uneven summer warming across the United States that has been going on for decades.

The West is toasted on hotter summer days while the East Coast is overwhelmed by hotter, stickier summer nights, according to an analysis of decades of summer weather data in the United States by the United States. Associated Press.

State-to-state average temperature trends from 1990 to 2020 show America’s summer heatwave increasing further in some of the places that have just been baked with extreme heat over the past week: California, Nevada, New Mexico, Arizona, Utah, Oregon and Colorado.

The West is the fastest warming region of the country in June, July and August, with an average rise of 3 degrees since 1990. The Northwest has warmed almost twice as much in the past 30 years than the Southeast.

That includes Portland, Oregon, which set a record 116 degrees, 3 degrees higher than temperatures on record in Oklahoma City or Dallas-Fort Worth.

While much of the root cause of last week’s extreme heat is an unusual but natural weather condition, scientists are seeing the footprint of man-made climate change, citing altered weather patterns that block heat to different places for longer periods.

“The ridiculous temperatures in the Pacific Northwest can on the one hand be considered a black swan event (ultra-rare), but on the other hand are quite consistent” with long-term trends, said meteorologist Judah Cohen of the private company Atmospheric and Environmental. Research. “So I’m not going to predict when the next time Portland hits 116, but I do think the hotter summers for the region as a whole are here to stay.”

Climate change alters and weakens the jet stream, narrow bands of wind that surround the Earth and flow from west to east. These changes allow the main high and low pressure weather patterns to stabilize. High pressure stagnates in the west more often in the summer, said Michael Mann, a climatologist at Pennsylvania State University. The high pressure brings hot, dry weather which, when blocked, can create what are known as thermal domes. The low pressure brings wet weather.

Another factor is rising water temperatures in the Pacific Ocean, which is also generating more so-called high pressure ridges in the west, said Gerald Meehl, a scientist with the National Center for Atmospheric Research who studies waves. heat.

These patterns appear so often that their effects can be seen in the long term data. The northwestern United States, western Canada and Siberia, which also just experienced a breathtaking heat wave, are among the fastest-warming land regions during the summer since 1990, a Cohen said.

The Midwest warms more slowly during the summer than the two coasts. That’s because blocked low-pressure areas often result in cooler air to the Great Lakes region, said Victor Gensini, climatologist at the University of Northern Illinois.

Water explains the big difference between western and eastern heat trends, the scientists said.

“In western states where drought has spread and intensified over the past decade, soil moisture has declined. Dry ground heats up faster than wet ground during the day because all solar energy is used to heat rather than evaporate moisture, ”said Jennifer Francis, climatologist at the Woodwell Climate Research Center. “Dry soil also cools faster at night. “

That’s part of why the West, which is getting drier over the decades and mired in a 20-year mega-drought, sees these insane triple-digit daytime temperatures.

The east is getting wetter and wetter over the decades, according to NOAA records, and the east coast experiences its biggest increase in warming at night. Overnight lows in New Jersey and Delaware have warmed by 3 degrees since 1990, the largest increases in the country.

Water vapor is a greenhouse gas, explained Francis, “So at night it retains more heat. “

North Carolina state climatologist Kathie Dello attributes the trends to human-caused warming. “There is no other explanation,” she said.

She added that while the extreme daytime peaks can be mind-blowing, the warmer nights can be dangerous as well. “Hot nights may not seem like a problem, but they pose a risk to the public health of people who do not have sufficient cooling,” she said.

And hiding from the heat is getting harder and harder: “All my places to go for a quick break were absurdly hot – Oregon, North Carolina, even upstate New York? Where is he left to go? Even Canada is not safe.


Read more articles on climate issues by The Associated Press at


Follow Seth Borenstein on Twitter at @borenbears.


The Associated Press’s Department of Health and Science receives support from the Department of Science Education at Howard Hughes Medical Institute. The AP is solely responsible for all content.


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