Thursday, February 07, 2013

Fairbanks, Alaska: Cool, moist weather caused contrail behind testing helicopter

Sam Harrel/News-Miner

A helicopter sits on the tarmac at the Fairbanks International Airport on Wednesday, Feb. 6, 2013. Two companies have been conducting cold weather testing for the past month and a half at the airport.

FAIRBANKS — A helicopter flying test runs above Fairbanks on Tuesday stirred up an unusual phenomenon — fog. 

 The helicopter belongs to a company conducting cold-weather tests out of Fairbanks International Airport.

It flew back and forth across the flatlands south of the University of Alaska Fairbanks campus Tuesday afternoon, said Jake Sirevaag, the visitor services supervisor at the UA Museum of the North on West Ridge.

“You could see a small trail behind the rotor of the helicopter,” Sirevaag said. “Then it started heavily condensing in a trail behind it. It left a dense wake.”

Rick Thoman, with the National Weather Service, said conditions for the phenomenon were set up by the presence of higher levels of moisture in the air, which was sometimes visible Tuesday as a thin fog.

As the moist air passed over the helicopter’s rotors, the pressure dropped. That drop cooled the air enough that the water vapor condensed, Thoman said. He didn’t see the helicopter wake Tuesday but saw the photograph in Wednesday’s Daily News-Miner.

“The rotors in effect cause the fog,” Thoman said.

That’s certainly what it looked like to Sirevaag. “It hung in the air for a very long time, spread out and became the fog,” he said.

Cool moist air flowing rapidly across a wing will cause the phenomenon, Thoman said.

“You see this actually not infrequently if you have a window seat and you’re landing or taking off at Sea-Tac,” Thoman said, referring to the notoriously damp climate in Seattle. “You would more or less never see this flying out of Phoenix.”

Even though temperatures were below freezing, the fog formed Tuesday was not frozen, Thoman said. Rather, it was composed of supercooled water droplets. “That’s the normal state of fog until you get well down into the minus 30s,” he said.

The helicopter’s abnormal shape also caught Sirevaag’s attention. It was “large and unusual,” he said.

Tim Hill, aviation supervisor at Alaska Aerofuel, said he has hosted helicopters from two companies that have conducted cold weather tests for the past month and a half at the airport. Hill said he couldn’t provide any more information at this point. Companies often avoid publicity while conducting such tests.

Melissa Osborn, chief of operations at the airport, said three helicopters have been involved in the testing this month and last.

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