Who can resist the temptation to gaze at a condensation trail when it appears in the sky? The long, thin shape is in such contrast to the normal curves and bumps we are used to seeing in clouds that these slender ribbons of white catch our attention. They also offer the challenge of spotting the aircraft that made them—usually a little ahead of the trail, and so high up as to be hard to see.
Aircraft condensation trails form because water is one of the by-products of the combustion of aviation fuel. The water is violently ejected from the aircraft engine as one of the hot gases comprising the exhaust. Initially the exhaust is too hot to allow the gaseous water to condense, but as it turbulently mixes with the surrounding air it cools rapidly.
Once the exhaust has moved behind the plane to a distance of about twice the plane’s wingspan it has been diluted by about 100 times its volume of surrounding air. As well as cooling the air, and so making cloud likely to form, the turbulent mixing also lowers the water content, making cloud less likely to form.
Whether or not a condensation trail appears depends on the extent of these opposing trends, and on the temperature and humidity of the air the plane is flying through. Generally the environmental air needs to be about minus -30° C or colder; otherwise the water from the exhaust will not produce a large enough change in the air’s relative humidity for contrails to form. Consequently, the plane needs to be flying above about seven kilometres.
The air also needs to be fairly humid. In fact, contrails are usually observed when the air at high altitude has enough humidity so that there are other high clouds in the sky.
Let’s look at the process in a little more detail.
Because of the shape of an aircraft’s wings, the air motion past the wing produces a lifting force that keeps the plane in the sky. In an equal and opposite reaction, a large amount of air is vigorously forced downward. As the exhaust gases are ejected from the engines they become part of this downwash of air behind the plane.
Downward-moving air is subject to higher surrounding pressures, and is consequently compressed, which raises its temperature, thereby making it harder for cloud to form.
On some occasions when an aircraft is flying just above a cloud layer, the downwash will cause the cloud to evaporate in a long line known as a distrail.
The air in the downwash, having been heated by the engine exhaust, is subject to an upward buoyancy force. The struggle between the downward push from the plane’s motion and the buoyancy force produces the regularly spaced lumps typically seen in contrails. Similarly, a distrail formed in a cloud sheet may be a line of dots, rather than a continuous line.
Once water from the aircraft exhaust has condensed to form the cloud droplets comprising the contrail, the droplets usually freeze quickly. The ice particles then tend to fall towards the earth, and can persist for some time, as ice changes back to water gas less readily than do liquid droplets.
Often the falling ice particles will encounter lighter winds at lower levels. This has the effect of changing the contrail into a large quasi-horizontal sheet of cloud, as the top edge moves swiftly across the sky while the bottom edge is slower moving.
In parts of the world with large numbers of aircraft overhead, such as the eastern United States and southern California, there are often many contrails present at the same time. Sometimes the contrails spread and join, forming an almost continuous layer of high cloud.
This observation has raised the concern that contrails may be having an impact on climate, either by reflecting incoming sunlight away from the earth and causing cooling, or by trapping the earth’s outgoing infra-red radiation and re-radiating it back down, and so promoting warming.
Aircraft are not alone in creating cloud trails. Satellite photos have revealed that ships can also cause condensation trails, although by a different mechanism. Whereas aircraft cause clouds by putting more water into the atmosphere, ships do so by adding small particles of soot.
The exhaust from a ship’s chimney probably contains more than 20,000 particles per cubic centimetre. These particles act as condensation nuclei, facilitating the formation of cloud droplets by attracting water molecules in the air, which stick to them, thus starting the condensation process.
A ship trail does not form in clear air, however. It results from the thickening of a layer of thin cloud, trapped 500 m or so above the sea surface, beneath an anticyclone.
Normally there is no shortage of condensation nuclei over the ocean. Breaking waves make small bubbles on the sea surface. As the bubbles break they flick tiny drops of seawater into the air. These droplets contain dimethyl sulphide (DMS), a chemical released by phytoplankton when it is nibbled by zooplankton.
When the droplets of seawater evaporate just above the sea surface, the DMS remains in the air, where it changes into particles that act as condensation nuclei. (It is DMS, by the way, which causes the characteristic smell we notice when we go to the beach.)
With all the wind we get around New Zealand, the sea is rough enough that we never lack for DMS condensation nuclei, so no ship trails are observed here.
The places where ship trails are found are under anticyclones in the eastern North Pacific and in the north and east of the North Atlantic Ocean. They are not often observed—on average, about once every six weeks. However, ship trails can be long-lived, lasting as much as 30 hours.
Ship trails will not form if the air has been over an industrial area recently, such as North America or Europe, or over a dusty area, such as the Sahara Desert, as it will have abundant condensation nuclei.
If, instead, the air has come off a clean area, such as the Greenland ice sheet, before moving into the anticyclone, it will be lacking in condensation nuclei. On such occasions, the exhaust from the ship’s funnel can cause a cloud hundreds of kilometres long.
Sometimes several ship trails cross at large angles, almost making a grid for a heavenly game of noughts and crosses!