January 28, 2007
Airport Takes on Fog With CO2
By Christensen, Tyler
Fog grounded three flights at Missoula International Airport last week, and airport director Cris Jensen was delighted.
The Missoula airport purchased the "fog-seeding" system just before Thanksgiving in anticipation of another busy travel season dampened by fog-laden weather, Jensen said. Airport officials were looking for ways to sweep the skies, especially after the four-day Thanksgiving weekend in 2005 - when fog cost the airport a total 69 flights.
Every plane that can't take off or land in Missoula means lost revenue for the airport. Each flight brings a $4.50-perpassenger facility charge, landing fees based on aircraft weight and, of course, traffic to the airport's restaurant, gift shops and parking lot.
But until now, there wasn't much the airport could do about fog.
"We just sat here and hoped it would go away," Jensen said. "Fog is one of those things. Snow we can plow, but we can't plow fog."
The airport started rounding up ideas some 10 years ago, when employee Rob Foote began researching various fogseeding programs. Jensen witnessed one such program using silver iodide when he worked at Reno/Tahoe International Airport a number of years ago.
Some airports use liquid propane, but officials at Missoula's airport nixed that idea over concerns about flammability and environmental pollution, he said.
Missoula's fog-seeding system uses carbon dioxide in gas form, a naturally occurring substance in the environment.
The system isn't particularly fancy or complicated. Basically, steel cylinders containing carbon dioxide are hooked up to a machine that pumps about 2 gallons of gas each hour though a hose held aloft by a wooden pole.
Field maintenance crews drive the setup down the runways, letting the gas - cooled to roughly minus 100 degrees Celsius rise though the fog, causing the tiny water droplets to condense and fall to the ground as dry, powdery ice crystals.
The airport initially assembled two units, then built two more after seeing how well they worked, Jensen said. The defoggers themselves are relatively inexpensive, at about $200 each. The gas - the same kind used to power paint ball guns is similarly inexpensive.
"I'd say we've been extremely pleased," Jensen said. "The first morning we tried it we got a couple of flights in and another one out, and I figured it had basically paid for itself already."
However, the efficacy of the system depends on weather conditions, he said. It's not going to work all the time, so some flights will still be grounded occasionally.
"Some days we're going to have fog and we're not going to be able to do anything with it," Jensen said.
The system only works on "cold fog," or clouds of moisture colder than zero Celsius, explained Don Griffith, president of North American Weather Consultants Inc., which is working with the Missoula airport. His Sandy, Utah-based company primarily runs programs designed to increase snowfall and rain in mountainous areas.
"The principles we use in increasing snowfall and rainfall are the same ones we use in decreasing fog," Griffith said. "Fog, by the way, is just a cloud on the ground."
Warmer fogs are too stable to be affected by the de-fogger, he said. Fortunately, about 70 percent of Missoula's fogs are cold ones.
But the airport also needs enough wind to swirl the carbon dioxide through the fog; otherwise, the gas just hangs there, Griffith explained. So far, the Missoula Valley seems to be exhibiting enough air movement to make the system a success.
If it continues to be successful, his company will look at ways to automate the system so maintenance crews won't have to drive it around in a truck every time they want to clear the air.
Copyright The Missoulian Dec 13, 2006
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