Plane tails

People will often give me suggestions on what to write. Sometimes the suggestions are solicited and sometimes they are not. About a year ago, my wife, Cory, was brainstorming with me about what to write. We were in the car looking around and she wondered about jet contrails, those long white lines that stretch across the sky in the wake of a jet plane.

A year had passed and I had not delved into this topic for fear I would not have enough content. That explanation was never quite accepted by Cory who has not let me forget about not looking into her idea. I explained that sometimes I need to fully process an idea myself before breaking into the mysteries of science, a notion that has offered Cory little solace.

Thankfully, I was struck by inspiration earlier this month when I happened to be outside as the sun was setting. It was one of those beautiful sunsets we have in Helena and I started thinking about clouds, colors, and, yes, jet contrails!

It turns out contrails are very similar to clouds. They are formed out of similar “ingredients” and the process, while completely different, has the same result.

Contrails are formed when water vapor and other small particles from the exhaust of the plane condenses and freezes along with the surrounding air. Once this happens, the conditions through which the plane is flying come into play, because as Cory pointed out, sometimes we see contrails behind planes and sometimes we do not.

To see the contrails, the conditions need to meet specific criteria. This criteria consist of cold temperatures (minus 40 degrees) and enough moisture in the atmosphere to support the contrail.

While the water vapor to form the contrail is in part from the plane, it is also in part from the air around the plane, the very same air from which clouds are made. The air we breathe contains water in the form of water vapor. Throughout the day, air warms up and rises, and as it rises it expands and cools. When it gets cold enough, the water vapor freezes to tiny dust particles similar to the way the water vapor freezes to the exhaust particles of the plane. When there are billions of these particles together they become a cloud or a contrail.

In comparing clouds and contrails, I have noticed they are often the same colors, mainly white and gray.

Clouds and contrails appear white because they are reflecting the light of the sun; which are all the colors in a rainbow. White is all we see because we see the exact same amount of every color. On the occasion when clouds and contrails appear gray it is likely because there are other clouds or contrails casting shadows. They might also be so dense light cannot fully penetrate them to scatter the light equally.

There you have it Cory, science! Sorry it took me a little over a year to get to it! I hope it answers your questions; it has been so long, I really can’t remember what they were.