Guidance

Aircraft contrails

Updated 2 August 2022

Science and technical

What are contrails?

Contrails form when the warm moist exhaust fumes from an aircraft mixes with the cold ambient air producing ice crystal clouds. The nuclei of some of the ice crystals in a contrail will contain minute products of combustion, but they are essentially ice.

Why can trails come from the aircraft tail and wings and not from the engines?

More than 2 contrails can sometimes be visible even behind a 2 engine jet aircraft. However, there are sources of contrails other than the engines, such as the changes in pressure as air flows around the wing edges. Due to the laws of science there is a relationship between the pressure, volume and temperature of a gas.

As the air flows over a vehicle, or aircraft, it undergoes a number of pressure changes which can result in it being cooler in parts. As the air cools, it is less able to hold moisture, so it condenses out as a vapour which is observed as a contrail. Visible condensation from the vortices caused by the wing tips of aircraft is a common phenomenon.

Why do contrails seem to form grid like patterns in the sky?

Aircraft often follow similar routes separated by altitude, time, lateral distance and direction and that is why grid like patterns can be seen in the sky.

Why do some contrails disappear almost instantly whilst other are long lasting?

This is dependent on the atmospheric conditions surrounding the aircraft. If the air around the plane is very dry, the ice crystals will turn back into water vapour and disappear within a few minutes.

However - humid air causes the ice crystals to remain and may even spread out, leaving a wider and longer lasting contrail that may be indistinguishable from naturally occurring cirrus clouds.

Why do some aircraft leave contrails and others don’t, even when they seem to be flying at the same height?

Often, aircraft appear to be at the same level with one causing a contrail and the other not. However, the regions of humid air that cause the contrails are known to be wide but shallow. A difference in flight level of 1,000 feet is enough for one aircraft to cause a contrail and the other not.

In addition, contrails of more efficient engines with cooler exhaust gases can form at lower altitudes than those of less efficient engines.

Images from a German scientific paper showed an Airbus A340 (maiden flight in 1991) leaving contrails, and a Boeing 707 (maiden flight in 1957) not leaving contrails.

Both were flying at 33,000 feet, but the newer engines of the A340 produced more water vapour so made contrails. See Influence of propulsion efficiency on contrail formation (PDF, 267KB).

A number of aircraft seen leaving contrails do not appear on any web tracking services. Why is this?

The government is not responsible for these services, therefore specific questions should be directed to the operator of the relevant flight tracking service. However, if an aircraft is not visible on such a service, it is likely that it is not equipped with compatible equipment or is out of coverage for the service.

Effects on climate and health

What exhaust gases do aircraft emit?

As well as water vapour (the primary exhaust emission), jet engines emit carbon dioxide, small amounts of un-burnt hydrocarbons, oxides of nitrogen, soot particles (sometimes termed non-volatile particulate matter, or nvPM) and carbon monoxide. Such emissions are regulated by the International Civil Aviation Organization (ICAO).

According to some reports ‘chemtrails’ contain barium or compounds of aluminium and silicon (often called aluminiosilicates)?

We have no credible evidence of the release of chemtrails. Aluminosilicates are common in clay soils and a wide range of other minerals, therefore measurements in air and soils are dominated by the ground level sources of aluminosilicates.

The latest assessment of emissions from aircraft can be found in the most recent National atmospheric emissions inventory report.

Are there any ill health effects caused by contrails?

There is no evidence that contrails cause health problems. At the altitude that contrails are produced, the oxides of nitrogen and particulate matter which can cause respiratory problems become dispersed.

Do contrails impact on climate change?

Persistent contrail cirrus, formed by aircraft, can affect the reflection of solar radiation and therefore have an impact on the Earth’s climate.

The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018 suggests that the present climate impact of contrail cirrus is larger than that from historical emissions of greenhouse gases emitted by aircraft.

However, this research also notes that significant uncertainties remain in understanding the full impact of contrail cirrus due to the complexity of the physics of its formation and how it may cause climate change.

Whilst the impacts of contrail cirrus and other non-CO2 impacts of aviation remain uncertain, the European Aviation Safety Agency (EASA) recently published an initial analysis of possible policy measures to mitigate these impacts.

For any of these policy measures to be effective, a robust way of comparing aviation’s non-CO2 climate effects to that of CO2 would need to be decided upon to ensure aviation’s total climate impact was reduced.

There is no scientifically correct way to do this. The government continues to keep non-CO2 emissions from aviation under review and will reassess its policy position as more evidence becomes available.

Geo-engineering

The term geo-engineering covers a range of technologies that aim to counteract human-caused climate change by deliberate large-scale intervention in the Earth’s natural systems. These include:

• afforestation which the UK is pursuing
• direct air capture and storage
• marine fertilisation
• solar radiation management (SRM), including injection of aerosols into the stratosphere – we do not use SRM or have plans to do so

Read the UK government’s view on greenhouse gas removal technologies and solar radiation management.

Weather modification

We are aware that other countries have used weather modification techniques, completed by aircraft, known as cloud seeding with the usual intent to increase rain or snow.

We do not use cloud seeding or have plans to do so.