But the inversions don't stay momentarily and then move on. They hang around, trapping smoke and exhaust in the valleys that cradle the region's largest populations.
Boise. Salt Lake City. Elko, Nev. Missoula. Albuquerque. Spokane. They're all places where smog builds up, visibility goes down and a burning throat and cough are the norm.
He has studied the phenomenon for years and sees a way to reduce the gunk in the air as the problem worsens with growth. In 1990, the population of Ada and Canyon counties in the Boise valley was 296,000. It was an estimated 485,000 by mid-2002.
Dawson is in the middle of a two-year study of the valley's inversions, financed by the federal government. When a stagnant system sets up, as it did in late November and early December 2002, he measures temperatures and winds to correlate how the weather affects the air quality.
He has also been working with Timberline High School students, who are measuring air pollution levels and trying to set up a network of schools across the valley to do the same.
"We're educating the stakeholders," chemistry instructor Neil Greeley said. "In five, 10, 15 years, these kids are going to be living here and hearing about light rail transportation, car traffic. They're going to be hearing the issues."
Dawson said the Boise valley has the perfect geological features for prolonged winter inversions. It's ringed by the Boise Front to the east, Owyhee Mountains to the south and Oregon's Blue Mountains to the west.
"The valley acts like a dam with the mountains around it," he said.
But certain meteorological events must coincide to create the inversions.
First, he said, are the winter high pressure systems that regularly set up over the Great Basin region. The denser cold air sinks into the valleys, and the high pressure acts as a cap. It traps the wood smoke being pumped from homes and exhaust fumes from automobiles. Visibility drops and temperatures plunge.
Only above the cap - up a thousand feet or so - is the sky blue again and temperatures relatively balmy.
The situation can last for days or weeks until another weather system finally drives out the high.
In the November 2002 inversion:
n A high-pressure ridge was present and kicked off the stagnant period. The pollution started accumulating by Thanksgiving, worsened by the fact that more people stay home during the holidays and increase wood smoke emissions.
n By Dec. 4, pollution hit its highest level in 10 years. Air quality was unhealthy for the elderly and those with lung ailments. Wood burning and open burning were banned, and people were urged to limit driving.
"If you put human beings in an ambient environment like an inversion, everybody is affected," said Dale Stephenson, Boise State's environmental health director who is working with Dawson. "You're putting agents in the lungs that have to be dealt with by the pulmonary system to clean them out."
n On Dec. 7, fog shut down the Boise airport. Dawson said the mist can actually improve the air quality near the surface by forming around the particulates and dropping them to the ground.
n Two days later, the high ridge was finally driven east and the valley cleared.
Dawson is also working with electrical and computer engineering professor Joe Hartman, who is perfecting sensors to gauge air pollution around the Boise Airport for the Federal Aviation Administration. And Boise State is beefing up its computer capability to handle complex weather modeling.
Dawson looks ahead to when that technology will let officials know in advance that inversions are setting up.
"If we can use these models to get a forecast, we can maybe look a day or two in advance and let managers, local officials know we have a threat coming up, a serious inversion that's going to get stronger," he said. "That's our goal and our hope."
Boise's stagnant conditions could someday force motorists to take public transportation during severe periods and impose limits on industries releasing pollutants into the air. Controls on wood burning are already in effect.
Stephenson said Boise's inversions are hard on the elderly with pulmonary problems and on children whose pulmonary systems are developing.
Unlike international cities such as Mexico City where pollution takes a backseat to higher priorities like food and shelter, Stephenson said the American "standard of living allows us to be proactive to attack air quality. We're fortunate the United States makes it a priority to do it. Only rich countries can do that."
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