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The Science of Contrails

In recent weeks, there have been some people expressing concern over what they believe to be dangerous “chemtrails” in the sky. Chemtrails are claimed to be white trails of chemicals sprayed by aircraft as they pass overhead, either to somehow affect people on the ground, or, in a more recent variation of this idea, to control the weather – but what does science say?

Earth's atmosphere is made up 78% Nitrogen, 21% Oxygen 1% other gases including water vapour. Clouds form when the air pressure and temperature at specific altitudes are right for the water vapour content to condense. As more water vapour is incorporated in the cloud, a level is reached when the air is said to be saturated, at which point rain drops form and fall to the ground.
WWII bombers, 1945WWII bombers

WWII bombers

WWII bombers

All engines that burn hydrocarbon fuel produce an exhaust that contains a lot of water vapour. We can see this in cars if the air is very cold – each exhaust pipe produces a white plume of steam that disappears after a while. Obviously a commercial passenger jet burns lots more fuel – around 2.5 tons of fuel per hour for an A320. This produces ENOUGH water vapour, which can form a condensation trail or “contrail”. For the water vapour to turn into condensation (droplets) the surrounding temperature must be very cold. This is why it’s very rare to see condensation forming during landing or take-off. At a higher altitude however, temperatures can drop to well below zero – ideal for condensation. At around -40°C or lower, the water from the exhaust not only condenses but freezes instantly, making the resulting cloud last much longer in the sky. This mostly happens in the upper troposphere and lower stratosphere – between 8km and 12km altitude. Eventually the contrail dissipates, incorporating with the rest of the atmospheric water vapour. This additional vapour content is too minimal to cause any effect in the air resulting in cloud formation. An analogy can be made to throwing a bucket of water in the sea. While the water is 'in flight' it can be observed and when it becomes part of the sea, the sea does not change its behaviour as a body of water since it is much larger than the bucket.

Contrail formation goes beyond just temperature however. If the air is cold but there is a lot of moisture in the air, you will get an overcast sky. The contrails will be there, above the clouds, but won’t be visible from the ground. If the air is very dry, the contrails will evaporate more quickly, and if there is a strong wind at the plane’s altitude, the contrails will be blown away and disappear quickly. If the conditions are right however, all the flights at a particular altitude will produce a contrail, which will only move slowly with the air currents. Since commercial flights all tend to follow set flight paths, this will often produce parallel trails or even a grid on such days.

Contrails have been forming for as long as aircraft have been flying. There are many photos of WWII bombers and fighters leaving the same familiar trails of moisture we see today. Like today, sometimes they dissipate quickly and sometimes they last for a long time. Like ordinary clouds, they can look white, grey or even orange at sunset. Nowadays one can even use websites like to check which flights are flying overhead at a particular moment, and use an Appleman Chart to predict the probability of contrail formation if you know the temperature at the aircraft’s altitude.

Unfortunately, whenever there is a day with the perfect conditions for contrail formation, people are confused or alarmed by the unusually large number of contrails seen in the sky. In an age where anyone can publish anything on the internet, conspiracy theories inevitably appear and spread quickly. Conspiracy theorists claim that the contrails that last for a long time are actually “chemtrails” made up of various chemicals rather than water vapour. Initially the claim was that these were chemicals to control or affect people on the ground. Eventually this idea was largely discarded but a new idea gained popularity – that these are chemicals to control the weather, also known as geo-engineering. This was inspired by various discussions related to climate change, where scientists speculated about various methods of reversing global warming, some ideas more radical than others. Some people became convinced that these contrails are actually those ideas being put into practice.

There are indeed some methods of affecting the weather, and they have been used. The best known is “cloud seeding”, where a fine powder, usually silver iodide or dry ice, is dropped on a cloud to cause it to deposit its moisture as rain. This is sometimes used to prevent hail or fog at airports. Attempts to use similar method to reduce a storm’s potency were inconclusive. However the moisture content cannot be conjured out of nothing – it can only be performed on existing, thick clouds. Depositing such a powder in a clear sky would accomplish nothing.
Below: An Appleman Chart, which can be used to predict the formation of contrails
based on atmospheric conditions

Appleman Chart

An Appleman Chart, which can be used to predict the formation of contrails
based on atmospheric conditions

There have been many discussions and proposals about other methods of influencing the weather. Most of these have concentrated on reducing the destructive effects of hurricanes and similar storms. These are run through computer simulation to determine the effect, and so far no method has been found that will have a significant effect on a storm of that size, though research is ongoing. Other ideas are related to climate change, and have included proposals to increase cloud coverage in order to reflect more sunlight – although for a cloud to have any measurable effect it would have to be vastly larger than the thin contrail left behind an aircraft.

In short, there is no such thing as “chemtrails”. The trails seen in the sky, whether long-lasting or not, are formed of water condensation from the combustion of fuel in the jet engine. There are many valid concerns about the environment and mankind’s effect on it, but this is not one of them.