Flight
Lines
Why contrails hang around.
By Mariana
Gosnell
In John Ford’s last western, Cheyenne Autumn, long threads of white
cross the sky over army tents in
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
The first recorded sighting of a contrail likely occurred in southern
The finger-pointing problem has yet to be solved. In the early 1990s,
after the
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.”
There are those who consider contrails to be downright sinister:
noxious chemicals sprayed from aircraft to sicken populations and alter weather
patterns, according to conspiracy theorists. The claims seem to rest on the
notion that thin, short-lived contrails may consist of ice crystals, but the
thicker, long-lived ones are not. In reality, the expanded lines are merely
contrails that have evolved.
When