Why do Planes Leave White Trails?

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On clear days we can sometimes see long white tracks crisscrossing the sky. These tracks are essentially just thin clouds, called contrails, made from high altitude aircraft flying over.

As an airline pilot for the last four years, I have the best seat on the plane. I get to watch other aircraft pass by ours up close. I love watching them zoom past, leaving contrails in their path.

This article will explain why and how this phenomenon occurs. Despite some conspiracy theorist’s claims, there is nothing scary or government-controlled about these contrails in the sky. You will understand how they are made, where they occur most, and if they impact the environment.

Why do airplanes leave tracks in the sky?

Why Do Airplanes Leave Tracks in the Sky?

The tracks, called contrails, made by airplanes are from hot engine exhaust mixing with the atmosphere. They are a unique type of ice-cloud that form at high altitudes in freezing temperatures.

Aircraft engines run at scorching temperatures. The exhaust gases include a small amount of water vapor in the form of tiny ice crystals. They become visible when released into the air behind the engine.

Since a contrail is composed of ice crystals, it is a type of condensation trail. It is similar to seeing your breath on a cold day. On the ground, we can see these trails as long streaks in the sky.

These contrails can last for hours or just a few minutes depending on the atmospheric conditions.

Contrails behind an airplane

What Does Contrails Mean?

The word “contrails” is a combination of condensation trails. Contrails was first used in the 1940s to describe the clouds observed forming behind aircraft.

That time frame coincides with the earliest jet-engine aircraft. They are also formed by high-altitude propeller-driven aircraft but are most common behind jet aircraft.

Contrails are visible crystallized ice droplets in the form of clouds. They occur at high altitudes, making it a cirrus cloud. Contrails are similar to naturally produced cirrus clouds.

Naturally made clouds can be composed of suspended liquid water, but contrails are composed only of crystallized ice droplets.

How Are Contrails Formed?

Extremely specific conditions must be met to produce contrails. Which is why they are only sometimes visible from the ground.

Inside of an aircraft engine outside air is mixed jet fuel. It is combusted at extreme temperatures inside the engine and creates thrust propelling the aircraft forward. If the humidity is high enough and the temperature cold enough in the air contrails will form. The exhaust air rapidly cools behind the engine water droplets are formed and freeze creating the contrail.

Researchers have identified the environmental conditions necessary for contrails to develop. Using this knowledge, the precise altitude contrails will form can be determined.

In WWII, the heavy bombers sometimes would create contrails. This was dangerous as enemy ground flak gunners could spot the planes approaching. They started studying when it occurred to keep the element of surprise.

Modern-day stealth aircraft can avoid radar detection, however, a contrail is clearly visible to anyone on the ground. Care must be taken by these fighter pilots and bombers to avoid enemy detection from a contrail.

What Are the Ingredients of Jet Fuel, and Are They Necessary for the Formation of Contrails?

Jet fuel consists primarily of kerosene (source). It has a high flash point and a low freezing point which makes it ideal for use in high altitude jet aircraft where temperatures are low.

Water is the only component that is necessary for the formation of contrails. In jet fuel, there can be small impurities and a few additives. Water in jet fuel would be dangerous. At high altitudes, the water would freeze blocking fuel lines. However, small impurities in the fuel, primarily sulfur, can contain microscopic water molecules.

These water molecules passing through the hot engine and exiting as exhaust can create contrails. That does not necessarily account for the contrails we see. Water is naturally in the atmosphere. When the conditions are right there is enough within the air to create contrails behind aircraft.

At What Altitude Do Planes Leave Contrails?

Contrails are most commonly found behind aircraft flying at a high altitude, above 8,000 meters or 26,000 feet. The air temperature at this high altitude is cold enough, nearly -40 degrees Celsius, for the formation of contrails.

On extra cold and humid days, it is possible to see contrails develop lower.

These airplane made clouds are considered cirrus. Cirrus clouds are defined as high altitude, wispy clouds, composed of ice crystals. Unlike puffy cumulus clouds which are found at lower altitudes and hold liquid water.

At low altitudes, the temperature is too low for ice crystals to be present. The water vapor can not freeze to form clouds unless the air temperature is around -35C or -34F.

What Are the Different Types of Contrails?

There are three types of contrails. Short-lived contrails, persistent non-spreading contrails, and persistent spreading contrails:

  1. Short-Lived Contrails. Sometimes when you see a jet pass overhead it has a short contrail that disappears quickly. This is considered a short-lived contrail. It occurs when the air is relatively dry and the water vapor sublimates into a gas within a few minutes.
  2. Persistent Non-Spreading Contrails. These contrails are formed when atmospheric conditions have high humidity and a stable air mass. They can linger behind a passing aircraft for a couple of hours. They stay uniform in shape and length.
  3. Persistent Spreading Contrails. This type of contrail is similar to the previous. Instead of the control remaining in a thin orderly cloud, it spreads about and becomes larger. Spreading is caused by turbulent airflow or an unstable air mass. They will look like regular cirrus clouds.

Each of these contrails is created from the same process. They are all suspended ice crystals created behind passing aircraft. The only difference is the atmospheric conditions which determine how long they last, and if they spread.

How Long Do Contrails Last in the Sky?

Contrails can last for just 30 seconds or for hours after an airplane has passed. This can create an interesting pattern to look at on the ground as planes cross overhead.

The humidity is the primary factor determining how long contrails will last. When humidity is high the clouds can linger for hours. If they are persistent-spreading contrails they can last for many hours as they form into contrail-induced cirrus clouds.

On average most contrails last for about one hour. Slowly the ice crystals in the contrail will sublimate back to water vapor.

Do Contrails Affect the Environment?

Since contrails are made up of water they do not cause a direct impact on the atmosphere or pollution. However, as an artificially created cloud, they can alter sunlight and could trap more heat in the lower atmosphere.

Despite many years of research, the formation of clouds is still not fully understood by scientists. Many studies are conducted to analyze persistent contrails. Persistent contrails lasting for hours can change the amount of sunlight that reaches the ground. Cirrus clouds also act as a blanket creating a greenhouse effect.

In busy air traffic areas, there are more days with persistent-spreading contrails. That means more cloud cover over typically metropolitan high-density areas. Cities over the United States, Canada, and Europe have the highest percentage of contrails.

Contrails over Nova Scotia (Canada)
Multiple contrails above Nova Scotia (Canada) – satellite photo by Nasa (public domain)

As air travel increases more contrail coverage can be expected. The full result of this increased coverage is not yet understood. There is a concern about increased global warming, but it can be difficult to isolate contrails as a singular source for climate change.

The good news is data is being rapidly collected. This will allow scientists to more accurately predict where contrails occur. This information can be utilized by airline flight planners, air traffic control, and pilots to mitigate the potential risks of contrails.

What Are Chemtrails?

As an airline pilot, there is an ongoing joke about chemtrails. Many of my friends and I will make jokes about the government telling us to do their secret work for them.

Some conspiracy theorists believe that the tracks you see in the sky are not from water vapor but from chemicals added to airplanes released over cities. This is complete nonsense, but this particular conspiracy theory has gained attention over the last decades.

I find it funny to think that the governments could tell thousands of pilots across the world to keep such a serious secret. In the cockpit, we often gossip and chat. Pilots are also known for being chatty at parties. There is no possible way a conspiracy of this size would stay a secret in the hands of a pilot.

Some of these conspiracy theorists have determined there must be a special switch in the cockpit that allows us to release the chemicals. I have been in hundreds of different cockpits. I have looked at thousands of switches, yet not a single one was for anything of the sort.

Another common claim is that these “chemtrails” are seen over busy cities. The simple reason is that more airplanes fly over busy metropolitan areas. Navigation systems still rely on ground stations that are primarily located near cities. That means more air traffic will fly over the same few routes. But, if you go far out into the country, you will still see airplanes and contrails.

So, Are Contrails Dangerous?

No, those long tracks in the sky are just clouds. They are made from airplanes, but they behave similarly to naturally occurring cirrus clouds.

They do not create any additional pollutants and are not a byproduct of the fuel. Only water vapor already present in the air.

They are not from the government performing secret experiments on the citizens either. They are from the hot exhaust gas mixing with cold, humid air and disappear as the ice crystals turn back into water vapor.

There is more research currently being conducted to determine if persistent-spreading contrails increase the greenhouse effect.

Final Words

Contrails are a natural byproduct of flying aircraft. They are created from hot exhaust rapidly freezing small water vapor into ice crystals. Depending on the humidity and temperature they can last for a few minutes, or a few hours.

Persistent spreading contrails are still being researched to determine if they are creating adverse environmental effects.

You can rest assured the governments of the world are not releasing chemicals into the air, that is a well-debunked conspiracy theory.

So relax, take a look up, and enjoy the impressive view of white paths across the sky!

Phil McCain

Phil McCain

Phil McCain is an experienced U.S. airline pilot. He has accumulated over 3500 hours of flight time across the United States, Canada, and Mexico. He is also a flight instructor, industry expert, and passionate traveler. He currently lives in Detroit, Michigan. Outside of the cockpit, Phil loves to read, cook, run, and explore the world around him. Here’s his website.