Look, Up In the Sky!
Over Houston early this past Saturday evening, contrails (aka vapor trails) filled the nearly cloudless sky. Peculiar, as the long, white traces were not the usual straight lines from horizon to horizon but were lazy ovals and broad s-curves — back and forth, ’round and ’round. To make matters worse, all this was happening on the 9th anniversary of 9/11. Local television stations reported their phone lines were lighting up with callers concerned about the “unknown aircraft” and its pattern in the sky.
Turns out the source of the trails was a NASA WB-57, a high-altitude research aircraft on a “sniffing” mission — atmospheric sampling — high over Houston. The flights are regular occurrences, but mostly invisible to those on the ground. What was different this time? Contrails.
Contrail is a compound word formed by “condensation” and, drum roll! — “trail.” A contrail is formed when condensation in jet engine exhaust (the main products of hydrocarbon fuel combustion are carbon dioxide and water vapor) freezes into ice crystals which then trace the aircraft’s flight. Depending on atmospheric conditions, contrails may linger for hours or merely seconds. These ice crystals have similar characteristics to cirrus clouds.
A lesser known phenomenon is the distrail. Another compound word, a dissipation trail is formed when an aircraft passes through a cloud and the warmer temperatures of the exhaust gasses allow the visible water vapor (the cloud) to dissipate. The distrail can appear to be a tunnel through the cloud.
Contrail and distrail forecasting is part of every military aviation weather meteorologist’s skill set. Certainly you wouldn’t want your airborne attack announced in advance by streaming contrails behind your stealth aircraft! Atmospheric conditions that enhance contrail (or distrail) formation can be identified and then military planners can avoid those conditions (usually as easy as selecting a different altitude) to maintain the element of surprise.
Military forecasters are not the only ones with an eye to the sky (sorry). Climate forecasters study condensation trails, as well. Consider: A contrail is, in effect, a small cloud. Put enough small clouds in the sky and you have the ability to effect climate by lowering the amount of incoming solar radiation and thus lowering the ambient temperatures of the lower atmosphere. Interesting, as this sort of “global warming” is entirely man-made. To what degree remains a question, but climate scientists had a unique opportunity to study contrail-free skies over the United States in the three days following the 9/11 terrorist attacks when all aircraft were grounded. No airplanes, no contrails. According to the studies, local diurnal temperatures were about 1 degree Celsius higher than immediately before.