If you ever have looked up the sky on a sunny day, it’s very likely that you have noticed the white trails that airplanes leave in their wake as they fly by. So how come that airplanes release white smoke trails?
Airplanes release white smoke trails since their engines not only produce exhaust gases, but also moisture vapor. While the former doesn’t leave any traces visible to the human eye, the latter will condensate due to the low air pressure and cold temperatures around the altitudes most airplanes operate at. The lines that appear are generally referred to as “contrails”.
The white smoke trails that appear in the wake of an aircraft’s path is a topic that to some extent has been shrouded in myths and erroneous assumptions.
In this article, we’ll debunk these myths, and look closer at:
- The conditions under which white smoke trails emerge
- Black smoke trails: What are they and how are they different from white smoke trails?
- Chemtrails: Is it true that aircraft exhaust fumes contain harmful chemicals?
Let’s begin!
Table of Contents
How and Why Airplanes Release White smoke
Contrails basically are clouds that form when the water vapor from the exhaust fumes condensates and then freezes into small particles that are generally referred to as “aerosols”.
This has to do with the fact that outside temperatures at the altitude where many planes operate can be as cold as -60 ° Celsius ( 76 °Fahrenheit ). When the air rushes through the jet engine, it heats up to as high as 600 ° Celsius ( 1,100 °Fahrenheit ) and is immediately cooled down, which causes the water vapor to condensate and form small ice crystals.
These crystals will then, as any type of cloud, reflect the surrounding light which is why it becomes visible to us.
Another factor that’s involved in creating contrails, is that of the pressure differences that occur when an airplane moves through the air. This on its own can be enough to cool down the air so that it gets saturated and starts to condensate. This is why you sometimes see trails not only in the wake of the engines, but also forming on the wingtips or propeller tips.
The faster the airplane goes, the more likely it is that we’ll start to see smoke trails coming from the wingtips, or any other part of the airplane. This has to do with that the pressure differences grow with the speed of the airplane, and simply cause a stronger reaction.
This is referred to as a vortex smoke trail and will be explained separately in just a bit.
Do Propeller Aircraft Release Smoke Trails?
Some people believe that contrails only are produced by jet engines, but that’s not true.
Jet airplanes do produce contrails more often than propeller airplanes, which has to do with their higher speed, and that they generally operate at higher altitudes that tend to be colder. As we covered earlier, the higher the speed, the bigger the pressure differences get, which means a higher probability of contrails forming. In addition, the temperatures are colder the higher up you get, which means that it takes less humidity for the air to get saturated, which is when the water in the air will start to condensate.
So, in short, you could say that contrails come from the condensation of water vapor inside the internal combustion engines. Thus, it doesn’t matter if it’s a cylinder engine, turboprop, or turbojet.
Let’s now look at the other type of contrails.
Vortex smoke trails
Vortex smoke trails appear around the propeller of an aircraft, especially on those that have a high angle of attack, like older fighter airplanes. However, they tend to be very shortlived, since no additional vapor is added.
Vortex smoke trails may also appear on other parts of the airplanes, like in the image below, where we’ve got so-called wing tip vortices that can be seen if the air is fairly cold and humid.
Why Do Vortex Smoke Trails Appear?
As the airplane moves through the air, the differences in air pressure on the different parts of the airplane will cause some up the air to swirl up and turn into spirals.
As these spirals exceed a particular speed, which is determined by the current humidity and temperature, the centrifugal forces will expand the air inside the spiral, which will cause the temperature to drop. As a result, the water inside that air will condensate and cause visible trails.
The image below explains this phenomenon in a clear way.
Since no water vapor is added to the air like with regular contrails, these types of smoke trails tend to be much more short-lived.
Black Smoke Trails
In the earlier days of aviation when airplanes engines weren’t as refined as they’re nowadays, it was quite common to see black trails in the sky.
Black smoke trails are not caused by the condensation of water vapor, but the incomplete combustion of fuel. Thus, the excess fuel which is not burnt will be released through the exhaust system, which then forms a black smoke trail.
Today, airplane engines have become much more efficient, and consequently release much less unburnt fuel, which has turned black smoke trails into somewhat of a rarity.
How long is the gap between the engines and the white smoke?
The gap between the engines and the contrails will vary quite a bit depending on a couple of factors.
Here they are listed below:
- The speed of the airplane: If the airplane moves faster, it will spit out air at a higher speed, which will widen the gap.
- The speed of the exhaust: This is related to the first point
- The temperature of the surrounding air: If the temperature is lower, the exhaust gases will be cooled down more quickly.
- The temperature of the exhaust: A higher temperature obviously means that it takes longer for the air to cool down and condensate.
Another factor to keep in mind is the size of the airplane. If two airplanes where one is double the size of the other move at the same speed under the same conditions, the gap will seem bigger on the smaller airplane. It simply looks bigger relative to the airplane’s size.
So as you see, there are quite some factors that determine how big the gap will become.
How long do the white smoke trails stay in the atmosphere?
A contrail can stay in the atmosphere for anything from a second or less, to several hours. It all depends on the conditions at the current altitude of the airplane.
Here are the top factors that determine how long a contrail will remain:
- Wind: If there are strong winds the contrail will dissolve quite quickly.
- Humidity: If the air already is saturated with moisture, the contrail will hold for longer periods of time.
- Temperature: This relates to the previous point, since colder temperature means that the air can hold less moisture, and thus will be saturated more quickly.
- Sunlight: Strong sunlight will help the contrail to evaporate, making it less long-lived.
What about Chemtrails?
Chemtrails, which also is known as the chemtrail conspiracy theory, presents the idea that the trails left by airlines are not normal contrails but packed with additional substances.
These substances are thought to be released with the goal of exerting human population control and psychological manipulation, among other things. Some people will state that they cause health problems and respiratory illnesses as well.
The reasoning often goes that normal contrails won’t last for as long as some of the trails that can be spotted actually do. However, as a reader of this article, you know that the time it takes for contrails to dissipate varies greatly with the current atmospheric conditions.
As you might have guessed, this theory has been dismissed over and over again, and there is no evidence at all to suggest that chemtrails are anything else than normal contrails.
Ending words
Hopefully, this article has given some insights into what the trails you see so often on the sky actually are.
Here is a short conclusion of the content of this article:
- Contrails consist of water vapor that appears due to the internal combustion of fuel in the airplane engine.
- Contrails, or vortex smoke trails may also appear due to the differences in pressure that arise as the airplane moves through the air. However, these tend not to be as long-lived.
- Black smoke trails are the result of incomplete combustion of airplane fuel.
- Chemtrails is a conspiracy theory that has been disproven over and over again.
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