Is global warming adding to the Valley’s UV risks? – Daily Independent

Even with all the clouds covering the Valley at the end of July, the forecast UV index was still 8 out of 10.

The UV index scale — the National Weather Service’s means of determining the level of danger from ultraviolet sun rays in a given place — tells Valley residents how much potential skin or other bodily damage they’re risking on a given day.

While the clouds of monsoon season and other times in the winter mitigate UV risk somewhat, the high pressure that is usually present in the Valley keeps warm, dry, clear conditions most of the time.

Randy Ceverny, a president’s professor of geographical sciences at Arizona State University, said prolonged high pressure tends to keep those clear, sunny conditions in place, as most residents can attest is the main everyday condition for the aptly-named Valley of the Sun. Phoenix, according to the NWS, has an average of 299 sunny days per year. The sun shines 85% of the time.

“When you get prolonged high pressure, you won’t see the clouds that provide some protection from UV radiation,” Ceverny said. “Humidity helps, too, and we don’t have much of that in the Valley.”

Normal sea-level pressure is 29.92 inches and it’s 29.67 in the Valley. A Youngtown weather station’s records include a high of 30.543 in 2011 and a low of 29.214 in 2010 — both recorded during winter months.


While climatologists agree the Valley is becoming hotter and drier, it isn’t clear if that means UV rays are putting more Arizonans at risk for diseases such as skin cancer.
Climatologists agree the Valley’s temperature will increase by 3 to 5 degrees by 2050, along with about 3 million more people living here, not all agree on its habitability.

David Hondula, an Arizona State University professor who directs Phoenix’s new Office of Heat Response and Mitigation, said his concerns involve causes and effects, for sure, but are more centered on what that means to humans in the Valley now.

“It’s not immediately clear to me what the connection would be between higher air temperatures and increased UV exposure, but it’s not a topic I think about much day to day,” Hondula said. “It might be the case that some of the lower-concentration greenhouse gases are the same that decrease atmospheric UV protection, but I’d need to do some more research.”

Barometric pressure data is far down the list of weather numbers readily available to the general public. While temperatures, relative humidity, precipitation and wind speeds and gusts are often published, high and low pressure extremes and trends are much tougher to find.

Humidity is not much of a factor, Ceverny said, with UV rays in the Valley because even on the wettest, muggiest days, it’s still only at about 60%. That’s nothing compared to the 80% or 90% often found on seaboards and coastlines, where a marine layer can develop.

“It dampens the air, which helps a little, but that doesn’t happen often enough to help us,” said Ceverny, who recently needed to have melanoma removed from his arm.
Extremely thick smoke from fires might help guard against UV rays, he said, but burning down forests and other growth causes far more ecological and climatological problems than it would solve.


There aren’t many studies on how pressure affects UV, especially in the Southwest U.S. A 2019 study published by Science Magazine showed how vapor pressure deficits reduces global vegetation growth.

“Previous studies reported that increased VPD explained 82% of the warm season drought stress in the Southwestern United States, which correlated to changes of forest productivity and mortality,” the study noted. “In addition, enhanced VPD limits tree growth even before soil moisture begins to be limiting.”

The Arizona Department of Environmental Quality, according to spokesperson Caroline Oppleman, doesn’t keep records of UV indexes. Arizona State climatologist Erinanne Saffel said she’s not aware of any studies correlating any research on global warming and the UV index.

Saffel did explain how the UV index is calculated. It isn’t simple.

“It’s interesting to learn what goes into the UV index calculation,” she said.

The NWS’s description of the calculation can be found here


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