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Dry ice helps clear the fog at airport

We have had a lot of freezing fog this winter, and it looks like there's a lot more to come. I have read the airport is "seeding" the fog to allow aircraft to land. I am glad this process allows planes to land safely, but I'm concerned about the possible effects on our air quality (and our lungs). What chemicals are used? Do the chemicals rain back down on us in the surrounding thick fog? Would you please explain how the process works?

— Jill K., Eagle Point

For the longest time, we here at SYA were convinced you had to pray to the fog goddess every time you wanted to land in Medford, as the airport's temperamental weather is legendary among travelers. After speaking with Kim Stearns, public information officer for the Medford airport, we now know better.

Turns out, they've got this nifty new, remote-controlled balloon contraption called CASPER, or Cable Attached System Providing Effective Release, which they launch whenever they need to clear the fog. They sent it up for the first time in mid-December last year to try it out under real-world conditions.

"The first day we sent it up, it took 20 minutes to clear the fog," said Stearns. "It was very effective."

The balloon floats above the inversion layer and disperses ground-up dry ice into the air, which freezes the water molecules in the layer of fog and makes them fall to the ground like snow, clearing the sky so that aircraft can land.

Stearns pointed out that in order for the process to work, the inversion layer where the ice is dispersed has to be below freezing. The weather can be deceptive, she explained, because sometimes it will be below freezing on the ground but still 35 or 36 degrees up in the air. To catch the right meteorological conditions, the airport usually can't start seeding until after 6 a.m.

Stearns explained that the dry ice they use for seeding comes from Medford Ice and is a nontoxic, water-based composition of frozen carbon-dioxide that is found in abundance in earth's atmosphere.

According to Stearns, the airport had been using aircraft to disperse the dry ice up until the end of 2009, but they often encountered problems because the hole the pilots would make in the fog would be swallowed back up by the time they made their approach, and would be unable to land.

"It was more of a safety issue," she said, explaining why the airport began searching for alternative means of clearing the fog. "We didn't want to jeopardize our pilots or their assistants."

CASPER was the solution the airport came up with.

"As far as we know, this is the only device of this type," she said, explaining that Medford was the first airport to use a balloon instead of airplanes to seed fog.

Stearns said the new system has gained a lot of attention after the December launch. The Medford airport has received several calls from other airports that routinely suffer delays because of fog, asking how the system works so they can use it at their own airports.

Send questions to "Since You Asked," Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by email to youasked@mailtribune.com. We're sorry, but the volume of questions received prevents us from answering all of them.