Contrails, Clouds Made by Airplanes

You have probably seen white trails left by airplanes in the clear blue sky. They are called contrails, short for “condensation trails.” Contrails are created only under certain conditions and even dubbed as a hygrometer in the sky. In this post, we will look at how contrails are created as well as environmental issues!

What are contrails?

Contrails, aka condensation trails, are a type of line-shaped clouds produced by aircraft engine exhaust, typically in a clear and cold sky. Contrails are created at a high altitude where droplets do not easily evaporate but freeze. They usually occur at high altitudes, usually above 8,000 m where the air temperature can reach below –38°C.

Contrails are generated for two reasons. The first factor is aircraft engine. Contrails are formed when hot exhaust gas from the engine meets cold air, creating water vapor in the form of ice crystals. As these water vapors must be rapidly frozen, contrails are caused only when aircrafts take flight at high speed in high altitude.

The second is aircraft wings. If you take a close look at aircrafts’ wings, you will notice curvature on the upper part. The curvature makes a difference in air velocity between the upper and lower parts of the wings. When the divided air reunites at the end of the wings, air pressure and temperature drop instantly. This is how water vapor rapidly freezes to create contrails.

The duration of contrails is influenced by altitude, temperature, vapor, sunlight, air pressure, and wind. They occur more often and visibly during winter when it is cold and humid.

Long-lasting contrails as signs of rain?

Then, how long can contrails last in the sky? Contrails disappear soon as the air becomes dry.

Just like how your laundry dries well in dry weather, water vapor scatters in the air quickly under dry atmospheric conditions. However, if the air is wet, contrails can last for more than an hour.

In this regard, contrails serve a function as a hygrometer in the sky. If you can see contrails for a long time in the sky, it is highly likely that it will rain.

Contrails and global warming

Recently, there have been studies on the effect of the aviation industry on global warming, which point out contrails as a contributor to global warming. A research team at Manchester Metropolitan University revealed that two thirds of the effect of the aviation industry on global warming from 1940 to 2018 come from contrails and carbon dioxide generated during flights.

It is because contrails formed in the sky prevent solar radiation from going back to space. Eventually, contrails cause a vicious cycle that accelerates global warming. While many different solutions are suggested, the most cost-effective way is to adjust the altitude for airplanes.

Civil engineering and environmental engineering teams at Imperial College London also published that decreasing 610 m in flight altitude makes a difference in the adverse effect of contrails. Isn’t it surprising while contrails do not have pollutants, they can accelerate global warming?

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