Contrails are the white lines that sometimes form behind high flying aircraft. They are actually a type of cloud. The cloud forms because jet exhaust contains quite a bit of water. If the humidity is high, then the contrails can persist for a long time, like clouds do.
When jet exhaust comes out of the engine, it’s superheated. So the water is in the form of vapor, steam, and hence it’s invisible. As it mixes with the surrounding (freezing) air it very quickly cools down, and at a certain point it will condense out into water droplets, and then freeze into ice. Because it takes a fraction of a second to do this, then there’s a gap between the engine and the contrail.
The causes of this gap are the same as the causes behind the gap you see when steam is coming out of a kettle under pressure:
The size of this gap varies quite a bit, based on various factors I’ll discuss below. Here’s some variations:
There are several variables that you need to account for in explaining these differences:
- The speed of the plane
- The speed of the exhaust
- The temperature of the surrounding air
- The temperature of the exhaust
- The size of the plane.
Now the size of the plane does not actually affect the size of the gap unless you are measuring that gap in “plane lengths” – which you really should not. A large plane does not automatically produce a larger gap, so it’s not a good unit of measurement. The plane length of an A380 is 238 feet, the plane length of an A320 is only 123 feet. So all other things being equal, the gap on the the smaller plane will look like it’s twice as long as the gap on the bigger plane, if you measure it in plane lengths.
So, consider speed. If the plane were moving at 500 knots, and it were simply letting some water vapor steam out the back, then that steam would be blown away from the plane at 500 mph, so the length of the gap would be determined by how long it takes the vapor to condense.
If the plane was not moving at all, and just shooting out the jet exhaust, then the exhaust would be blown back at that initial speed, and then quickly slow down, but there would still be a gap.
Combine those two things, you’ve got #1 the speed at which the exhaust contrail eventually moves away from the plane, and #2, the initial speed at which it moves away.
Now consider temperature. The vapor has to cool below the temperature at which water will condense. This cooling happens by the exhaust gases (temp #4) mixing with the surrounding air (#3). The hot exhaust mixes with the cold air, just like if you pour a cup of hot water into a cold bath. This mixing happens very rapidly due to the turbulence behind the plane.
So the length of the gap depends on how quickly this cooling happens. It will be quicker if the temperatures involved are low. Modern efficient engines have much cooler exhaust than older engines so will have shorter gaps. The higher you fly, the colder it gets and the shorter the gap gets
Causes of Short Contrail Gap
- Low speed plane
- Low power setting (low exhaust speed and cooler exhaust)
- High altitude (colder surrounding air)
Causes of Long Contrail Gap
- High speed plane
- High Power setting (high exhaust speed and hot exhaust)
- Low Altitude (warmer surrounding air)
Of these, probably the one that has the greatest effect is air temperature, which is generally determined by altitude. This is confirmed by Ulrich Schumann, in Atmospheric Physics: Background – Methods – Trends:
For threshold conditions, contrails become visible about one wing span behind the engines. For lower temperature, contrails can be seen forming already a few meters behind the engine.
Prop planes like this C-130 don’t fly as high as jet planes, so when they do create contrails, it’s generally going to be near the warmest temperature possible, and hence the gap will be longer. Prop planes may also entrain the exhaust gasses in vortices behind each engine, resulting in slower mixing with the surrounding air than the more forceful turbulence of a jet engine.
Planes can also make aerodynamic contrails from the wing surfaces or propellors. These are very different to the normal exhaust contrails. Since these are caused by a lowering of pressure, the contrail formation is nearly instant, as the air immediately reaches the correct temperature, so there’s no cooling time required, so no gap. The following photo shows a C-130 like above, but with aerodynamic contrails coming from the tips of the propellors. It also illustrates the vortices that form behind the individual engines, which will slow down the mixing of engine exhaust with the surrounding air, lengthening the gap seen above.
One more thing that can affect the apparent gap is how it is illuminated by the sun. Contrails dont just spring into existence as solid white clouds, they start out quite faint and transparent. If this region is lit by direct sunlight, then it’s more visible, and the gap will seem shorter. If it’s not lit by the sun – like if the plane is in the shadow of a cloud, then the gap will seem longer.
The following photo illustrates this. The contrail on the left of the photo is being shaded by the body of the plane. Even though both contrails are the same behind the plane, meaning they have the same short gap, the gap on the left contrail looks much longer.