Tired of this humid heat wave? Expect more of them

Photo of sunset over ocean
The recent heat wave this August was both hot and humid
(Gary Robbins / The San Diego Union-Tribune )

Researcher says muggy weather the result of changing climate in the Pacific Ocean


The heat wave that has smothered San Diego County for nearly two weeks may be waning, but it shows the wave of the future, when hot, muggy weather will be more common, says Alexander Gershunov, a climate scientist with the Scripps Institution of Oceanography.

“Heat waves are one of the extreme weather events that are most directly influenced by global warming,” Gershunov said. “Their activity has been increasing all over the globe.

“Specifically, in California, they’re not only becoming more intense and longer-lasting, but they’re also changing their flavor, becoming more humid.”

The recent heat wave sent mercury soaring to triple digit temperatures in dozens of San Diego communities. Besides the heat were high relative humidity levels of 70 percent to 80 percent, said Samantha Connolly, a meteorologist with the National Weather Service.

Relative humidity represents the ratio of the current humidity to the highest possible amount of water the air can hold. The higher it is, the wetter the atmosphere. In the summer, it’s usually 20 percent to 30 percent in San Diego, Connolly said. Damp air doesn’t allow much evaporation, so it feels hotter than it is, making it harder for people to cool off.

“That’s what we were seeing across most valley areas,” Connolly said. “We’re seeing temperatures in the 90s, but it feels like 100.”

The wetter air retains heat, so temperatures climb during the day and persist at night, allowing little relief. Nighttime lows, usually in the 60s during the summer in San Diego, didn’t drop much below 70 degrees Fahrenheit over the past week, Connolly said.

“The humid heat is more oppressing during the day, and it doesn’t cool off at night, so you don’t get the respite from the heat at night,” Gershunov said. “It starts off warmer the next day, and after one or two or three of these cycles of oppressive heat during the day, and hot, muggy nights, especially people with health vulnerabilities, begin to get sick, and some people die.”

The high humidity in the recent weather pattern comes from air flow from a portion of the Pacific Ocean west of Baja California that’s warming faster than global oceans on average, he said.

“A lot of the time in these big summer time heat waves, the air is brought in from the south, from that part of the ocean that’s warming a lot,” he said.

Climate change is expected to amplify heat waves in the future, as land and water temperatures increase, he said. Scientists are measuring those effects already, he said; over the last 22 years, San Diego broke heat records 89 times, but surpassed cold records only once.

“The warming associated with climate change is projected to accelerate in the future,” he said. “In terms of heat waves, it’s like having heat waves on steroids. So the background climate warming makes heat waves hotter, and in California more humid, as well.”

It’s not only uncomfortable, but also dangerous, he said. The normal human response to heat is sweat, which allows evaporative cooling through the skin. With humid air, sweat doesn’t evaporate off, and people can overheat.

That’s particularly hazardous to older or ill people, he said. And the COVID-19 pandemic makes it even harder to handle, since space at cooling centers may be limited by social distancing, and people facing economic hardship may not be able to afford to turn on air conditioning at home. It requires a new approach to reducing heat risk, he said, perhaps including reducing electricity rates during heat waves to make life-saving cooling more affordable.

“We probably need to rethink our mitigation and intervention strategies,” he said. “There’s got to be some plan to mitigate the impact of these humid heat waves.”