For a while there, it felt like it might rain forever, or at least long enough to deliver a much-needed blow against the current drought.
Well, those welcome, wet days brought on by a series of Pineapple Express storm systems have pretty much faded into the weather almanac, pushed aside by a familiar bully — the dreaded thermal inversion layer.
The atmospheric phenomenon often triggers a Spare the Air alert, a no-burn rule that prohibits the use of fireplaces, wood stoves, pellet stoves, outdoor fire pits or other wood-burning devices across the Bay Area.
Wednesday will mark the 12th such ban called by the Bay Area Air Quality Management District since Spare the Air season started in November. The season runs through May, depending on the weather conditions.
But much more than an seasonal annoyance, each thermal inversion and consequent Spare the Air alert is like a red flag that heralds drought-filled days to come.
Last year, the air quality district called an unprecedented 30 alerts during the winter pollution season, double the number from the previous record set in 2011-12. By this time last year, the air quality district had already called 27 alerts.
Most of the alerts during the 2013-2014 winter season took place in December. In contrast, there were only six alerts called this past December, a month that saw back-to-back storms from the Pacific.
“These kinds of dry, calm conditions with the inversion are the recipe for more Spare the Air days,” said Tom Flannigan, a spokesman for the air quality district.
While recent Spare the Air statistics show a rough correlation between no-burn days and the drought, Flannigan said rainstorms are not the only atmospheric cleansing mechanism. A strong weather system that produces heavy winds will also clean the air.
“Some years might have dry weather, but strong winds,” he said, adding that the weather forecast for the near future calls for more thermal inversions. “We’ve got a dry spell ahead of us,” he said.
The forecast through Monday will be mostly clear and dry, due to the current ridge of high pressure that’s sitting over the region, said Steve Anderson, a forecaster for the National Weather Service in Monterey. That pressure system, which sits above us at about 10,000 feet, traps cool, stagnant air at ground-level and lower altitudes, Anderson said.
“The air is not really having a chance to mix out,” Anderson said. “The air goes up and down and around normally, but when there’s high pressure, it’s like a lid.”
The inversion layer is behind what in recent years has become a Bay Area irony: you can’t use your fireplace when you need it the most. To be fair, Bay Area residents whose only source of heat is a fireplace or wood-burning stove are exempt from the no-burn rules.
Restaurants with wood-burning ovens are also exempt because their emissions are already heavily regulated by the air quality district and other state agencies, Flannigan said.
Also, the vast majority of airborne particulate matter — the chief culprit of air pollution in winter (as opposed to greenhouse gases in the summer) — is caused by residential fireplaces. There are 1.4 million fireplaces in the Bay Area, compared to several hundred wood-burning restaurant ovens, Flannigan said.
The Spare the Air ban went into effect during the 2006-2007 winter season, though compliance with the no-burn rule was voluntary until the 2008-2009 season, when violations were met with fines.