Why contrails hang around.
- By Mariana Gosnell
- Air & Space magazine, July 2007
In John Ford’s last western, Cheyenne Autumn, long threads of white cross the sky over army tents in Monument Valley, along the Utah-Arizona border. In the 1993 film Gettysburg, a brilliant white sliver hovers in the clear sky above the head of a Union officer. In Zulu, a dramatized account of the 1879 assault by thousands of Zulu tribesmen on a small British garrison in Natal, South Africa, whitish bands sometimes hang in the sky beyond the hills.
Nitpickers who enjoy finding movie bloopers point out that these man-made cirrus clouds are condensation trails, or contrails, which didn’t exist before the 20th century. They are created by airplanes flying at high altitudes, where the air is below –38 degrees Fahrenheit. Exhaust from airplane engines contains water vapor as well as other gases and particles of soot and metal. When the exhaust is expelled into and mixes with the cold air, the water vapor condenses into droplets, which instantly freeze into tiny ice crystals. What you see from the ground is a dense white stream of ice crystals behind an airplane.
Sometimes an airplane leaves no contrail at all, or an extremely short one—an indication that the air at cruise altitude is probably dry. There must be enough water molecules in the air to condense and freeze—in other words, the relative humidity must be 100 percent or greater. In dry air, any ice crystals that form would immediately evaporate.
Even if the air is moist enough, it might not be cold enough. At typical contrail-friendly altitudes, between about 28,000 and 40,000 feet, temperatures run from about –36 to –76 degrees. If the airplane leaves a long trail, you can assume that the air is not only cold but humid, allowing the ice crystals to persist. If the contrail stops, then starts up again, creating a broken line, chances are the airplane flew through a dry patch.
Immediately behind the airplane, between the tail and the head of the contrail, is a 100-foot stretch of clear air, representing the short time it took for ice to form from the mixture of hot exhaust and cold ambient air. You might see four white lines at first, or two, since each engine produces its own contrail, but before long they merge into one line. The line is likely to have what Patrick Minnis, senior research scientist at NASA’s Langley Research Center in Virginia and an expert on contrails, calls “structure”—striations or “puff balls”—produced by the spinning of the exhaust. “If the puff balls are close together,” Minnis says, “you might not notice them, but they’re almost always there.”
A lot of other things can happen to a contrail once it’s formed. Winds can move it along, widen it, fray its edges. If contrails grow large enough, crystals will fall into a drier layer below, where they evaporate, or fall into a saturated layer, where they may split and trigger the formation of more crystals. If there is wind shear, the crystals in the lower layers move at a different speed than their cousins above. “Typically, this will end with a contrail spreading horizontally and vertically,” Minnis says. “There have even been reports of crystals making it all the way to the ground.”
It’s not only jets that make contrails; piston aircraft do too. So do rockets. So, apparently, do birds. “I have heard of wild geese leaving vapor trails high over the Canadian Rockies,” Guy Murchie wrote in his book Song of the Sky. A goose exhaling warm, moist air into –38-degree air could produce a contrail, Minnis allows, although “it would certainly be a small one.”
The first recorded sighting of a contrail likely occurred in southern Tirol in the Italian Alps in 1915 when somebody named Ettenreich spotted “the condensation of a cumulus stripe from the exhaust gases of an aircraft”; the stripe stayed around for a while. It wasn’t until World War II that anyone took interest. In a single combat area, hundreds of aircraft sometimes generated so many contrails that pilots couldn’t see to keep in formation or find a target. “We were, in effect, clouding the sky over Germany,” wrote 34th Bomb Group member Hal Province to Veritas News Service reporter Jay Reynolds in 1999. Contrails could be used as cover for an attack: “Four Me-262s came in hidden by the contrails and hit four of us,” Richrad Scroxton wrote in a 1983 account now posted on the 100th Bomb Group Web site. Even more troublesome, contrails gave away aircraft positions. “We were easy for them to spot, as our contrails were heavy that day,” another bomber crewman noted, “pointing like fingers in the sky toward our squadron,” Mike Banta wrote in 1997 in an account of his B-17’s last mission, now posted on the 91st Bomb Group Web site.
The finger-pointing problem has yet to be solved. In the early 1990s, after the U.S. military developed the B-2 stealth bomber, it again became interested in contrails. Steve Weaver, a senior meteorologist at Wright-Patterson Air Force Base, Ohio, points out: “They spent all this money to develop a billion-dollar bomber that’s invisible to radar, but you can see its contrail with your naked eye.” The original B-2 design included a tank outboard of the main landing gear that would store a chemical to mix with the exhaust and suppress contrail formation. The Internet is a rich source of speculation as to what went wrong with that plan, but in the end, Ophir, an optical sensor manufacturer in Littleton, Colorado, saved the day. Its Pilot Alert System uses lidar (light detection and ranging) to differentiate contrails from clouds and tell the pilot to change his altitude when his aircraft is “conning.”
Erik Mathieson, a former Air Force pilot who today flies an Airbus A330, appreciates contrails. “They tell you if the airplane ahead of you at a similar altitude is getting a smooth ride—the line doesn’t undulate or dissipate rapidly—in which case you can expect a smooth ride too,” he says. “If there are several aircraft on closely spaced parallel tracks, contrails can let you know which altitudes are choppy and help you decide whether to climb or descend.”