Even on days with a clear blue sky, the skies aren’t completely empty. We’re all used to seeing those great white streaks across the sky behind airplanes.
Cliff Mass says he gets a fair number of questions about these contrails, many from conspiracy-minded people who believe they are part of a nefarious government plot.
“There’s a bunch of people who believe there’s a conspiracy going on, and they don’t call them contrails; they call them chemtrails,” he said.
So what’s the story behind chemtrails?
“The belief is that these lines we see in the sky, they believe, are caused by chemicals emitted by a secret program run by the U.S. government, and it’s just not true,” said Mass. “Contrails are something we understand really quite well.”
Then what causes these contrails? Jet planes flying at high altitudes where the atmosphere is cool, says Mass.
“So if you have an engine that’s burning fuel, it puts out a certain amount of water vapor. That’s a natural byproduct of combustion,” he said. “When that water vapor goes into the cold air, it tends to condense first into water droplets, then rapidly freeze into ice crystals. So you get a line of these water vapors, and that causes a contrail.”
The reason that happens, says Mass, is that cold air can’t hold as much water vapor as warm air.
“So if you put any water vapor in there, it tends to dense out right away, and you get a line of these ice crystals and that produces the contrails,” he said.
So it’s no wonder these contrails look like clouds, because, essentially, these are clouds, says Mass.
“And sometimes they can last for minutes, and sometimes even hours. If you have a lot of planes flying, sometimes you can have a sky that’s criss-crossed with contrails, basically filling the sky with ice crystals,” he said.
The latest study suggests these contrails could contribute to climate change.
“The impact is for warming, strangely enough,” said Mass. “The clouds work in two ways: one, the ice crystals reflect some of the solar radiation from the sun—that cools.
“But they also absorb and readmit infrared radiation from the surface and the clouds below, and that tends to warm sort of like a blanket.
And it turns out the warming is more important than the cooling, so these clouds actually contribute to the warming.”
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The weekly KPLU feature "Weather with Cliff Mass" airs every Friday at 9 a.m. immediately following BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KPLU Environment Reporter Bellamy Pailthorp. Cliff Mass is a University of Washington Professor of Atmospheric Sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to a podcast of “Weather with Cliff Mass”shows.
