White Lines in the Sky

The white lines behind airplanes are called contrails. I honestly didn't know that. Photo from science-edu.larc.nasa.gov

So, every once in a while I learn something that seems so obvious in retrospect but that I was so completely unaware of that I have to shake my head and wonder what the hell else I've been entirely oblivious to my entire life.

Ever since I was a kid I've wondered why some airplanes leave special white lines behind them while others don't. When riding in an airplane I would wonder if ours was leaving a trail, but somehow I suspected I never was--I had this notion that the airplanes leaving behind the white lines are pretty uncommon and you'd somehow know if you were in one. Like, maybe they're way bigger than normal planes and the pilot would include a sentence or two in the pre-flight announcements alerting you. Something along the lines of "Ladies and gentlemen, folks on the ground will have the pleasure of seeing our path marked out behind us today. We mixed in one of those white fuel-colorings so they'll be seeing the lines".

Somewhere along the way I also developed a low-level of consciousness about "contrails"--something about the government and conspiracy theories and maybe the white lines were involved.

At long last, my consciousness has leveled up! I read a mind-blowing chapter of Gavin Pretor-Pinney's "The Cloudspotters Guide" on contrails and now I get it.

Maybe this is old news to you, but here's what's up with the white lines behind airplanes (all of this information comes from Pretor-Pinney's book):

1. They're called "contrails". Okay, I think I finally got this one.
2. The word "contrail" is a portmanteau (combination) of "condensation" and "trail". Which explains a lot because...
3. They're just condensation. That's right. They're just made of liquid water droplets--like all clouds (Gaseous water vapor condenses into liquid water drops).
4. There are no special airplanes. There are just special atmospheric conditions.
5. I repeat, there are no special airplanes. There are just special atmospheric conditions. By extension, there is no such thing as white fuel-coloring. Disappointing, I know.
6. I srsly cannot believe number 4/5 never occurred to me before. It's like being told that space and time are relative and only the speed of light is constant. Only more obvious. Just completely flipped my worldview on its head.
7. If an airplane (yes, any airplane) is flying through a region of air that is dry, no contrails. If an airplane flies through a region of air that is humid (meaning the air contains a lot of water), yes contrails. Why? Because the exhaust of airplanes contains both water vapor and tiny particles that seed cloud formation. If there's already enough water in the air (or if the addition of water from the airplane's exhaust is enough) for clouds to form, all it needs are seeds (known as "cloud condensation nuclei") to condense onto and form a cloud!
8. Temperature matters too (in addition to how much water is in the air) because warmer air holds more water vapor than cold air. That is, if you took some humid air and did nothing else but cooled its temperature, the water molecules would lose their energy of motion and clump together to form liquid droplets--aka cloud.
9. You can see how the composition of air varies across space if there are breaks within contrails!
10. Contrails are the only cloud type that is human-made.
11. Contrails can be indicators of weather because they tell you about the composition of the upper troposphere (the troposphere is the lowest portion of the atmosphere). If there are persistent contrails it can be an indication that the upper troposphere is moist and rising, which occurs a day or so before a low pressure (storm) system comes in.
12. Contrails are being researched as a possible contributor to global warming. It appears that the addition of human-made clouds from airplanes traps a significant amount of heat.
13. Airplanes aren't the only things that create contrails! Rockets and meteors create them too!

There's also way more to learn about contrails! Like the fact that they can spread out in a matter of hours to become miles wide, and that they can sport unexpected-looking features if the air is turbulent.

Anyhow, that's your little intro to the white lines in the sky!