The Santa Cruz Mountains have famously perfect sleeping weather, with summertime temperatures that at night typically drop into crisp and restorative low 60s.
But it is one of the oddities of this record heat wave that even our most historically chilly communities – those at the highest elevations, even near the coast — are sharing in our region’s sweat-soaked insomnia.
Nature’s air-conditioner is on the fritz.
“It was oppressive,” said Sanjay Khandelwal, where temperatures at his rural home — perched 1,100 feet above Los Gatos, cooled only by towering trees — dropped to 83.1 degrees at 4:40 a.m. Then it began warming again.
High overnight temperatures contribute to daytime misery, said meteorologist Jan Null of Golden Gate Weather Forecasting and an adjunct professor in the Department of Meteorology and Climate Science at San Jose State University.
“When you’re starting the next day a couple of degrees warmer than the previous day,” Null said, you’re getting a head start on the heat.
Meteorologists point to a phenomenon called a “thermal belt,” which creates high nighttime temperatures within a narrow altitude that ranges from 1,000 to 3,000 feet. That includes the upper hills in the San Francisco Bay Area and the coastal mountains, as well as the lower parts of the Sierra Nevada foothills.
Residents report an “inversion,” where temperatures rise, not fall, with elevation gain.
Hiking in the Marin Headlands on Tuesday morning, Travis Gohr’s Garmin thermometer reported 108 degrees. Then, “descending the hill, you could feel the instant temperature change at around 200-feet elevation, as the temp dropped around 20 degrees,” he said.
In the Palo Alto Hills, 1,700 feet in elevation, “I couldn’t sleep, so I left,” said Katherine Greene. At the crest of the Santa Cruz Mountains, 2,400 feet above sea level, Denise Maggioncalda suffered through nighttime temperatures that never dropped below 93 degrees.
“During this heat wave, the middle elevation regions have been extraordinary at night,” said Daniel Swain, a climate scientist with the UC Los Angeles Institute of the Environment and Sustainability. “At certain elevations, there’s almost no temperature recovery overnight.”
It’s part of a larger global problem linked to climate change, say scientists. On average, nights are warming faster than days across most of the United States, according to the 2018 National Climate Assessment Report.
In general, minimum temperatures are rising faster over time than maximum temperatures, according to a study published by the Royal Meteorological Society.
Nighttime heat is dangerous because it reduces our ability to cool down from the day’s heat. Continued heat puts more physiological strain on the body, especially for elders, children and pregnant women, according to Kristie Ebi, a professor at the Center for Health and the Global Environment at the University of Washington.
Even near the coast, California’s celebrated fog layer is nowhere to be found.
Out at the Point Reyes Lighthouse, which sits at a famously cold and foggy bluff that juts 10 miles into the sea, tourists marveled at the setting dressed in T-shirts and shorts.
The scene was toasty and picture-perfect, with calm seas, according to a National Park Service volunteer, whose car dashboard thermometer read a balmy 79 degrees.
Even the Santa Cruz area was handed an “Excessive Heat Warning” for Tuesday.
Why is the coast so warm? In the summer, the Pacific Ocean’s cold water is pumped from the depths to the surface – and when hot air rises over land, there’s a vacuum for this cool air to rush in.
But the upwelling is less reliable in the autumn, said Swain. “So we’re seeing all-time record high temperatures in September, not in the middle of summer. I think the late summer and early autumn months are really going to be the focus for coastal extremes.”
The foggy “marine layer” is unusually shallow — less than 200 feet deep, according to the National Weather Service.
Meanwhile, meteorologists are worried about the potential for dry lightning in Northern California this weekend. On Tuesday, there was a brief lightning strike west of Red Bluff. The town of Corning had drops of rain.
The moisture content of wildland vegetation, such as chamise, is expected to plummet, raising the risk of fire. “This heat will dry things out a lot,” said Craig Clements, a professor of meteorology at San José State University and Director of the Wildfire Interdisciplinary Research Center. “The heat will really be an issue for dead fuels.”
There are no immediate prospects for cooldown in many parts of Northern California. For reasons that are unclear, the high pressure dome over our region is slow to budge, according to meteorologists. Temperatures above 110 could be seen through Friday in many parts of the state.
“Have fun not sleeping tonight,” tweeted San Francisco’s anonymous Twitter personality Karl the Fog.