A white Christmas is coming to the Valley. But only if you count the hovering smoky white-gray haze. It’s the result of climate conditions that are bringing unusually warm winter weather and some of the worst air pollution in years.
The forecast calls for a good chance of a record-tying high temperature of 78 on Christmas Day, and possibly a recordbreaking degree or two higher. The National Weather Service says the unseasonably toasty temps look like they’ll last at least until the middle of next week.
Almost certain to stay longer is the stagnant air that is trapping pollution over the Valley and raising the health risk for people with respiratory ailments.
The other Christmas season color will be brown — as in the brown cloud of pollution that typically accumulates above the Valley each winter.
This year, aided by a lack of rain and wind to clear the urban skies, the perennial winter air quality problem is just about as big and bad as ever.
"It’s like the brown cloud on steroids," said Steve Owens, director of the Arizona Department of Environmental Quality. "With this stagnant air, it is not dissipating like it normally would. It’s building on itself every day."
Since Nov. 1, the department has issued high-pollution advisories in the Valley on 13 days, nine of them since Dec. 1 — including an advisory in effect today.
That is more high-pollution advisories than were issued in the last four years combined from November to January, Owens said.
The predominant pollutant this time of year isn’t the gaseous, misty ozone that moderate winds can carry out of the Valley.
This year’s brown cloud is bulging with particulates, matter that comes from dust whipped up from dry soils and the "gunk," as Owens calls it, churned into the air by motor vehicles burning gasoline and diesel fuel.
The combination of dry and stagnant air and warm weather in the Southwest is a result of a complex interplay of hemispheric weather patterns affecting
almost the entire country, meteorologists said.
A "high-amplitude jet stream" carrying warm winds from the Pacific Ocean’s subtropical region has slung a ridge of high atmospheric pressure over most of the West, said Jan Curtis, climatologist for the National Water and Climate Center.
The result is temperatures of five to 15 degrees above normal, primarily in Arizona, New Mexico, Utah, Nevada, Colorado, Idaho, Montana and Wyoming.
The same jet stream patterns sending warm Pacific air into the West are pushing air from the frigid northwestern part of the hemisphere down into the eastern United States, causing colder than normal temperatures there, Curtis said.
The downside of the warmer winter weather brought into the Southwest by the subtropical winds is the pollution.
"Generally, whenever you’re under a ridge of high pressure, winds will be very light and you get air stagnation," Curtis said. "It’s bad for big cities because of the traffic and industry."
The high atmospheric pressure holds down the particulates emitted in vehicle engine exhaust. "It’s like putting a lid on the atmosphere. The pollutants get trapped," said Mike Bruce, spokesman for the weather service’s Valley office.
Bruce said there is no sign of rain in the forecast, so the pollution is likely to persist at least through the end of the month.
The conditions are prompting the Maricopa County Air Quality Department to step up monitoring of dust-control measures at construction sites and industrial sites, said spokeswoman Holly Ward.
The department also anticipates calling for more no-burn days, asking that residents refrain from using fireplaces and wood-burning stoves.
Under present climate conditions, "if you’re creating smoke, you’re going to be creating pollution," Ward said, "and because the air is stagnant that smoky air is going to be hanging around and people in your neighborhood are going to be breathing it.’‘
But state and county environmental officials said that with the usual drop in vehicle traffic and construction activity on weekends and holidays, there’s less of a chance of a high-pollution alert or a noburn day being called for Christmas Day.
Weather forecasters said the same atmospheric pressure patterns rarely persist for entire seasons, so a warm and dry December doesn’t mean winter won’t turn wet and cold.