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Chasing thin white lines across the sky

• John Lorson: The Rail Trail Naturalist
• December 21, 2021
• 1235

The dog paw in the center of my chest was my first clue I’d blown it. If our goofy mutt had already been awake so long he felt the need to restart my heart as I lay motionless in bed, then my planned meeting with the stars was already a lost cause. Frankie wakes up well before sunrise every day and fully expects his scoop of kibble and jog around the block before I head off to work. That schedule puts me in the saddle right around sunrise and gives me at least a glimmer of daylight on one leg of my daily journey to work and home again.

But on this particular morning, I’d hoped to cash in on optimal viewing conditions for the peak of the Geminids, an event which is often the strongest meteor shower of the year. By the time the dog and I were afoot at opposite ends of the leash, the growing glow of predawn was rapidly washing away any opportunity to catch the late night rally.

Not one to stew in disappointment, especially on a crystal clear morning, I looked to the sky nevertheless and found and took interest in the patchwork of narrow white lines glowing brightly above the eastern horizon.

The high, vaporous streaks or contrails are a phenomenon known only to people who have lived beyond the early 20th century — the period in which powered human flight has existed. On this particular morning, I tried to imagine what would have gone through the mind of a person a mere two centuries ago had they looked up to see a thin white line slowly tearing through the blue canopy of sky. What spawned awe and wonder for one man might have easily spelled fear and doom for another. Fortunately for us, there is nothing to fear from the condensation cloud left behind a high-flying jet aircraft.

The term “contrail” is a truncated form of the phrase “condensation trail.” Some folks float the term “chem trail,” but this is largely a misnomer. While there are indeed chemicals present in the exhaust of jet aircraft, they are no more pernicious nor more visible at ground level than the exhaust of your lawnmower or automobile. Jet fuel is hydrocarbon based just as gasoline and diesel fuel, and during combustion, which takes place at extremely high temperatures, a good bit of water vapor is released along with carbon dioxide.

When that water vapor encounters the cold of the atmosphere where commercial aircraft typically fly — an altitude of 30,000-36,000 feet — it quickly condenses into droplets or even ice crystals in temperatures of -40 F and lower.

Humidity already present in the air plays a role in contrail formation and duration as well. If a contrail seems to linger thick and heavy long after a jet has passed, then there’s a good bit of humidity present in the upper atmosphere. That humidity (water vapor) may eventually work its way into stormy weather. Conversely, if the contrail is short and dissipates quickly, then fair weather is likely in the low-humidity conditions.

It’s been a fun pastime of mine since childhood to look at the direction of a contrail in the sky and try to figure out where that particular aircraft might be headed. I also like to watch jets that seem to be on convergent paths and predict where they might eventually intersect. Fortunately for all, the altitude of every aircraft in the sky is carefully monitored, and even when the lines appear to be on a collision course, you can rest assured they are likely thousands of feet apart in altitude.

The sky always has something interesting to offer. Never pass up the chance to look up and wonder.

Remember, if you have comments on this column or questions about the natural world, write The Rail Trail Naturalist, P.O. Box 170, Fredericksburg, OH 44627, or email jlorson@alonovus.com. You also can follow along on Instagram @railtrailnaturalist.