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Welcome to the Greenwood.Net Curiosity Corner

Contrails and the Atomosphere

Jul 02, 2020

Curiosity Corner
Dr. Jerry D. Wilson,
Emeritus Professor of Physics
Lander University

Question: Why is it that sometimes jet airplanes leave vapor trails (the white trails of smoke), and other times they don’t? (Asked by a curious sky watcher.)

Reply: Well, I see you have your eye on the sky! First, the vapor trails, or “contrails” (short for condensation trails) aren’t smoke. As the latter name implies, they result from condensation. The main products from the combustion of jet fuel (hydrocarbon fuel) are carbon dioxide and water vapor. The exhaust contrails generally occur at 26,000 feet above sea level, where the temperature is around -40 degrees Fahrenheit. It’s also -40 degrees Celsius, so take your pick; this is the temperature at which Fahrenheit and Celsius readings are equal.

The cold temperature quickly cools the water vapor to the saturation point (where condensation occurs) and the vapor condenses into tiny water droplets, which quickly freeze into ice. As the plane continues on, a trail of these ice crystals are formed behind. However, when planes are flying at lower altitudes where it is not cold enough for condensation to occur, you won’t see them.

The atmosphere is an interesting place. It is divided into layers based on temperature. We live in the troposphere (from the Greek tropos, meaning “change”). The temperature decreases with altitude in the troposphere up to about a height of 10 miles, where it can be anywhere from -50 degrees to -60 degrees Fahrenheit. The atmospheric conditions of the lower part of the troposphere where we live are referred to collectively as “weather,” which indeed is subject to change.

Above the troposphere is a layer in which the temperature increases, decreases and then increases again with altitude: the stratosphere (from the Greek stratum, meaning “covering layer”). Together, the troposphere and the stratosphere account for 99.9% of the atmospheric mass. There is concern about high-speed jet planes that fly in the lower atmosphere. There is no “weather” in the stratosphere, and exhaust hydrocarbons are not washed back to the earth as in the troposphere, so it poses a pollution problem.

If the temperature in the stratosphere increases, then decreases, there must be a region of warming temperature... and there is! It’s called the ozone layer. At about 20 miles up, the ultraviolet (UV) rays from the sun and the oxygen concentration are just right for the disassociation of the oxygen (O2) into the ozone (O3), which forms a relatively warm ozone layer. This ozone layer in the stratosphere acts as an umbrella that shields life on Earth from harmful UV radiation by absorbing most of the short wavelengths. The portion of UV that does get through the ozone layer is that which burns and tans our skin in the summer (which in excess may cause skin cancer). Were it not for the ozone layer absorption, we would be badly burned and find the sunlight intolerable.

So now you can understand why chlorofluorocarbons (CFCs) that were used as refrigerant gases were banned some years ago. The CFC gases would escape (think of how many refrigerators are in operation) and drift upward to the ozone layer, where it would combine with the ozone and destroy it. You may be somewhat familiar with the gas. Ozone may be formed around electrical sparking discharges and can be detected by its distinct, pungent smell from which it derives its name: the Greek ozein, meaning “to smell.”

There, you’ve had your lesson on the atmosphere today.

C.P.S. (Curious Postscript): “If you think practicing what you preach is rough, try preaching what you practice.” –Bowen Baxter

Curious about something? Send your questions to Dr. Jerry D. Wilson, College of Science and Mathematics, Lander University, Greenwood, SC 29649, or email jerry@curiosity-corner.net. Selected questions will appear in the Curiosity Corner. For Curiosity Corner background, go to www.curiosity-corner.net.

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