Over the past few years, I have been asked numerous times about those magnificent wispy-white trails that jets leave behind as they cross the sky. I think its generally human nature to look up at the sky and wonder about why things are the way they are. This makes people naturally curious about this phenomenon.

Unfortunately, these trails known as contrails, have been given a bad reputation recently. Primarily due to lack of understanding, some people have theorized that the contrails left by jets as they zip across the sky are actually chemicals being dispersed to poison or cause harm to the general public. Thus, the innocent contrail has been given the more malicious name: chemtrail.

Yikes!

As a pilot of a large jet aircraft, I not only see contrails frequently, but fly an aircraft which produces them. When I was flying small Cessnas, I felt I had a pretty good understanding of how and why contrails formed. It was not until I made the transition to jets however, that things really clicked for me. After seeing a couple of jets zip by me emitting those beautiful white streaks in different weather conditions, the picture really came together.

I decided to write this post for two reasons. The first and primary reason is to help educate you about contrails and how they form. The second reason is to hopefully dispel the chemtrail myth through a better understanding of contrails.

Enjoy!

The Myths

Before going into the science of contrails, I thought it would be best to first lay out the chemtrail conspiracy theory. My hope is that as you read through the facts that follow, you can debunk the myth for yourself… USING THE POWER OF SCIENCE!

I took some time and searched the internet for information on the alleged chemtrail, and generalize the belief as best as I could below:

The belief: Chemtrails are the visible spray of chemicals dispersed by high flying jet aircraft. The reason for the dispersal of these chemicals seems to range from dispersing metallic material to help the government in aerial mapping, to poisons dispersed for population control.

Theorists believe that normal contrails exist, but claim that the traits below differentiate contrails from chemtrails:

• Trails that dissipate are normal contrails, while trails that pervade or hang around for a while are chemtrails.
• Trails that are stable are contrails, while trails that turn on and off are chemtrails.
• Trails in normal lines are contrails, while trails in “X” patters or grids are chemtrails

Now for the Facts

What are contrails?

Contrails, or condensation trails, are trails of small ice crystals that form in the cold upper atmosphere from the water vapor in the exhaust of aircraft engines.

How do they from?

Contrail formation is finicky and can be a little tricky to understand if you aren’t familiar with the upper atmosphere. This is likely why the contrail myth is so prominent. But don’t worry! After reading further, you will be a contrail master.

Contrails are not an alien topic if you look at them for what they really are. If you have ever seen your breath on a cold humid day, or if you have ever opened the freezer and seen fog produced, you have experienced first hand, the fundamental production force of contrails.

You may ask, well… I don’t think a jet aircraft breathes, and I certainly don’t think that someone left the icebox door opened on the jet before takeoff. Well… you would be correct, but the forces which produce the fog or cloudiness in both of these cases are the exact same forces which produce contrails.

First, a bit about jet engines and the combustion of fuels

Most people are familiar with some of the bi-products of the combustion of fossil fuels. I’m sure you are aware that when a car burns a fuel like gasoline, it spits out carbon dioxide and carbon monoxide, along with some unburnt fuel and some other smaller hydrocarbons. What few people realize however, is that one of the main bi-products of the combustion of fuel is water. Yes water!
In fact, If you look closely at the exhaust pipe of the car in front of you next time you sit in traffic, you will likely see small drips of water coming out of the exhaust. On a cold winter morning, you can even see the water in the car’s exhaust condensing once it reaches the cool air as it exits the vehicle’s exhaust pipe.

So, you may ask, exactly how much water does a jet engine (like the ones on my beloved CRJ700) expel as it flies through the air?

Luckily, I was able to recall enough of my old chemistry classes to give you a pretty accurate answer (and the answer may surprise you!).

Jet fuel is actually a mixture of a few different hydrocarbons. To keep things simple, I assumed that it was made up only of its primary component, dodecane.

If complete combustion occurs, dodecane combines with oxygen in the atmosphere to produce carbon dioxide and… you guessed it, water. The reaction looks like this:

dodecane + O2 → CO2 + H2O + Heat (to move the plane)

Or in a more nerdy fashion:

2 C12H26 + 37 O2 → 24 CO2 + 26 H2O

If you do the math, you will find that one pound of burnt jet fuel creates approximately 1.38 pounds of water.

This means if the relatively tiny jet engines on the CRJ burn 3200 pounds of fuel per hour in cruise, the plane expels a whopping 4400 pounds of water! Now thats a lot of water.

The point? Aircraft generate a lot of water in flight, and all of that water has to go somewhere.

From jet engine exhaust to contrails

So how does the water in the exhaust of a jet form a contrail?

As you just saw, jet engines add a lot of water to the air. One special property of air in the atmosphere is it has the ability to hold or retain water in the form of water vapor. Water vapor is an invisible, gaseous form of water.

If you have ever stepped outside in the summer on a hot humid day in Florida, you have experienced water vapor first hand. Although you can’t see the water vapor in the humid air that surrounds you, you can feel its effects.

The air in the atmosphere can’t hold an infinite amount of water vapor. You can think of air kind of as a college student trying to carry a stack of books out of the library. If the student only has to take a light load of books home then they can easily carry them. But if they try to take a stack of 45 books home, eventually they will get off balance and drop (condense ha) them all over the floor.

This same effect occurs in the atmosphere. If too much water vapor is present in the air, the air expels the vapor out of its gaseous state and causes it to condense. This is what causes clouds, fog, and yes, even contrails.

The ratio of the amount of water in the air to the amount of water it can hold is called relative humidity.

The amount of water air can hold is directly proportional to the temperature and pressure of the air. This means that warm air at low altitudes can hold a lot more water vapor than cold air at higher altitudes.

To reference our studious college student again, think of warm high pressure air as a ultra-buff student, and cold low pressure air as a scrawny student. The buff kid can hold a lot more books than the scrawny one.

At low altitudes, as a jet climbs to its cursing altitude, it expels water into the atmosphere. At these low altitudes, the air is a lot like our buff college student. Although the CRJ throws books at the student to carry, the strong buff student can handle the extra weight. Thus no contrail is formed. As the jet reaches high altitude, the air pressure decreases and temperatures drop significantly. Now, the air more closely resembles our scrawny student. When the CRJ piles on the books, the student capitulates and a contrail is formed.

This is why Contrails typically form at altitudes above 30,000’ and at temperatures below -40 C. These extreme conditions allow the water in the exhaust of the jet to easily saturate the air. When water vapor condenses at these extreme temperatures, it instantly freezes into the tiny ice crystals you see as the contrail.

Why contrails are not always visible

Even though the air at high altitudes is exceptionally bad at holding water vapor (like the scrawny kid), it is often more than capable of absorbing the water vapor expelled by the engines of jets. This means that in order for contrails to be formed, the relative humidity at altitude needs to be high, usually above 80% for the trails to pervade.

Because the relative humidity at altitude is so variable, even over short distances, contrails can form over one area of the sky and disappear over the next. This is not unlike clouds forming in one place but not another.

So I’m sure your thinking, Man! I really want to see some contrails now, but how am I supposed to know if the relative humidity aloft is conducive to their formation?

I hear ya!

NASA has actually built a web tool which helps you predict if contrails will form on a particular day at your location! See the following link below. A bit of a warning, it does require JAVA.

http://cloudsgate2.larc.nasa.gov/cgi-bin/site/showdoc?docid=33&cmd=forecast

Why some contrails disappear almost instantly behind the jet while others pervade and cover large areas

I’m sure you have noticed that there is a lot of inconsistency in contrails. Some last all day and spread out, while others disappear almost instantly. This is all a result of the relative humidity of the air.
Let’s visit our scrawny student again. Say our puny cold air, low pressure student can carry 10 books without dropping them. If the student is already carrying eight books (80% relative humidity) and the CRJ gives him four, the pile spills. The student then picks up the 10 he can actually carry and leaves the two behind in the form of a contrail.

Now imagine another scrawny student sees this calamity and runs over to help him. already carrying eight books himself, this student grabs the two left on the floor and they both walk away.

This is akin to a contrail that disappears behind a jet. The ice crystals eventually sublime and are absorbed by the air.

But what would happen if the same student was carrying nine books? If the CRJ gave him four, the student would spill three books on the floor. This time, his buddy would only be able to pick up one book, and two would be left. It would now take a small army of scrawny students to clean up the mess.

This is akin to a contrail that pervades behind a jet. On days with high relative humidity, it takes a long time for the ice crystals to sublime and be absorbed into the air. If the air is completely saturated (100% relative humidity) the ice crystals will remain until atmospheric conditions change. The ice crystals in pervasive contrails usually become spread out by the upper level winds and can form thin sheets.

Explaining the chemtrail myth with the facts

So let’s review!

• Why do some contrails pervade while others disappear shortly after the jet produced them? – On days with greater relative humidity aloft, the ice crystals take longer to sublime than on days with less relative humidity. In fact, if the air aloft is near saturation, contrails can linger nearly all day and the tiny ice crystals can be blown into thin sheets. You will also notice a correlation between upper level clouds and pervasive contrails. On clear blue sky days, contrails tend to disappear quickly. On days with more upper level clouds, (showing upper level moisture is present) contrails will tend to be more pervasive.
• Why do some contrails seem to turn on and off as a jet flies while others are stable? – It all goes back to relative humidity again. The relative humidity in the atmosphere can vary from place to place or can be relatively constant. This is why clouds can form in spotted layers one day and form sheets another. If a contrail appears to turn on and off behind a jet, it’s not because the pilot is turning the chemicals on and off, but rather that the plane is just flying through air that is more or less saturated.

• Why do contrails appear in “X” patterns or grids? – This one has less to do with atmospheric science, and more to do with the way jet traffic is organized. To keep it simple, here’s an excerpt of a chart of upper level airways:

How many X’s do you see =) ?

On a Personal Note

Thanks again for reading! I hoped this gave you a better understanding of contrails and why they do some of the weird things they do. The reality is that people who firmly believe the chemtrail conspiracy theory will continue to do so. My biggest issue with the chemtrail myth is that it leads people down a road of confusion and gives them a negative view of the wonderful world of aviation. Hopefully this post can help steer a few people away from that path.

As a side note, and in a more humorous light, there’s one more thing I have to add after working in the airlines for about a year. The airline world is so chaotic, it hard enough to get people to put the right amount of cokes on the plane and to have the ice box filled… or have rampers waiting to marshall us in (no offense to rampers). I just couldn’t imagine them being able to organize guys to fill up the chemtrail tank after every flight!