Why Do Planes Leave Trails?

Aircraft · 5 min read · Aug 24, 2022
why do planes leave trails

You have probably been in a cold weather environment and noticed that when you or the people around you speak, it seems like smoke is coming out of your mouths. Well, the principle of planes leaving white trails in the sky is basically the same.

However, there is more to planes’ white trails than just the effect of air temperature. Also, the exhaust gas from jet engines contains is than humid air which is why the vapor trails they leave do not dissipate as the water vapor from your breath.

If you want to know exactly what happens with a jet engine exhaust while flying at high altitudes, you should keep reading as we give you the details.

Condensation trails

Jet engine exhaust in the cold air of the sky leaves trails known as condensation or contrails. Contrail formation results from the jet exhaust gases transforming into water vapor when in contact with the surrounding air because, at high altitudes, the atmospheric conditions involve lower vapor pressure and a significant temperature difference with the exhaust gas.

Now, what makes jet contrails particular is their components. Let’s take a look at some of them.

Water vapor

Water vapor is the most obvious component. What normally happens is that the water vapor condenses and freezes because of the difference between the ambient air and exhaust temperature at such a high altitude. This helps the contrail formation.

Carbon dioxide

Part of the exhaust forming the vapor trails is the carbon dioxide generated from burning fossil fuels. Recent advancements in engine development have resulted in a reduction of the carbon dioxide generated.

Soot and metal particles

Soot is usually seen as black smoke coming out from older engine designs during takeoff and climb. This happens because, as described by Airplane Academy, “for older engine designs, in particular, to keep the internal engine components cool during takeoff and climb, excess fuel is used during the takeoff phase of flight, leading to an overly rich fuel mixture.

This rich fuel mixture does not burn as completely or efficiently, resulting in soot forming in the exhaust plume.”

However, the soot present in vapor trails is not as thick as to be visible, nor are the metal particles. Yet, while not visible, the soot and particles present in the air “provide condensation sites for the water vapor”, explains Jenn Stroud Rossmann, an assistant professor of engineering at Harvey Mudd College.

This makes it easier for the vapor trails to keep their shape and last longer depending on certain conditions, which takes us to the next point. Don’t mistake this type of contrails created by acrobatic pilots using smoke oil.

A flying airplane leaving a trail in the sky.

Surrounding air temperature and weather conditions

At a high altitude, atmospheric conditions are different from what we experience on the ground, including cold temperatures and much lower vapor pressure. These conditions have a direct impact on the vapor trails, which is why we can find different types of contrails.

The main difference between one type of contrail and the other is how long they stay visible and whether they stay in a long straight line or start spreading out.

Here are the three types of contrails.

Short-lived contrails

A short-lived contrail looks like the airplane’s tail. We can understand from the name that these trails only last a few minutes before they disappear, something that happens nearly as fast as they are made.

When a short-lived contrail forms, it means that the air is somewhat moist with only a small amount of water vapor available. Any ice particles created quickly return back to a vapor state.

Given their nature, short-lived contrails can be used as a sign of fair weather.

Persistent (non-spreading) contrails

These contrails form much longer white lines than the previous ones, and they also remain visible for a long time even after the plane cannot be seen. For a persistent contrail to form, the air that the plane travels through must be humid with a large amount of vapor available to create it.

Persistent spreading contrails

These are similar to non-spreading trails, but they spread out over a larger distance because of turbulence or other weather conditions. Persistent spreading contrails may even look like natural cirrus clouds over time, so they are sometimes called manmade clouds.

It is believed that persistent spreading contrails can be interpreted as a sign of potential stormy weather.

Two airplanes leaving trails in the sky.

Why do some planes leave trails and some don’t?

As it was described above, contrail formation depends on different factors, but mainly on the altitude and weather conditions. Therefore, it is more likely for turbofan and turbojet-powered aircraft flying at cruising altitude to leave contrails. This includes the likes of airliners, business jets, and military aircraft that usually cruise at the high altitudes where contrail formation is most likely to occur.

On the contrary, turboprop-powered aircraft that use jet engines to drive a propeller typically fly at lower altitudes, and they are less likely to leave contrails. Also, they extract power from the exhaust gasses to drive the propeller, and those gases pass through exhaust stacks to the outside of the aircraft.

“This combination of power extraction, exhaust stacks, lower altitudes, and warmer temperatures reduces the exhaust gas to outside air temperature gradient sufficiently to generally reduce the likelihood that contrails will form,” states Allen Herbert, a certified flight instructor and lifelong aviation enthusiast.

Why do plane wings leave trails?

The trails left by plane wings are known as contrail cousins because they are very similar although they differ in the science behind their formation.

Wings, especially at their tips, generate a turbulent airflow called a vortex. What happens is that, depending on the weather, the pressure of the vortex at the end of the wingtip drops enough to form the trail. These trails become visible when the conditions allow water drops to form inside the vortex, but they usually evaporate very quickly which is why these trails normally last a few seconds.

Vapor trails and the environment

Recent studies have suggested that the ice crystals contained in the white smoke left by turbojet engines contribute to global warming due to the greenhouse effect. According to an article published by the BBC, “Contrails, which heighten the effect of global warming, may account for more than half (57%) of the entire climate impact of aviation.”

Fortunately, a lot of research has come behind this to find solutions and with great results. Dr. Marc Stettler, transport and environment lecturer at Imperial College London, says changing the altitude of fewer than 2% of flights could potentially reduce contrail-linked climate change by a staggering 59%. “Tweaking the flight elevation by just a thousand feet can stop some contrails from forming,” he explains.

Also, Prof Christiane Voigt, head of the cloud physics department at Mainz University, Germany, and her team carried out trials with the German Aerospace Centre (DLR) to measure and mitigate the impact of contrails. She said, “Our results have been really positive. We were able to predict and avoid around 80% of the contrails with little cost, very few flights would have to be deviated to win a “large climate impact”.”

An Emirates aircraft flying with a contrail.

Final thoughts on contrails

Contrails create a very peculiar view of the sky and remind us of how amazing aviation can be. However, knowing that the white trail airplanes leave affect the environment in a negative way changes the perspective.

Fortunately, newer engines are being developed with higher efficiency, thus creating the possibility to apply solutions like referring traffic to routes that do not require higher altitudes where air conditions prompt contrail formation.

Moreover, new engine types that use alternative fuels are also under research and development with the intention to get rid of the fossil fuels the engine burns. This could be a huge progress for airplanes to climb high and fly at lower temperatures without leaving any trail at all.

In short, controlling the plane’s altitude to avoid generating white smoke could be a short-term solution, but better long-term solutions may be achieved by improving the technology of the plane’s engine.

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Jet pilot @NASA

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