Operation Blue Skies: Using AI to avoid contrails

Contrails are a common sight in the sky as aircraft passing overhead produce hot engine exhaust fumes which causes water vapor to condense, forming contrails. However contrails can have a negative impact on the environment. In an effort to reduce the climate impact of contrails a UK based consortium are working together to work towards reducing the climate impact of these contrails. In this blog post I will look at the recent announcement that NATS and Google, along with others, will launch Operation Blue Skies, the world's first airspace-scale trial which aims to reduce the climate impact of contrails. Read on to find out more.

A UK-based consortium are working to reduce the climate impact of contrails

About Operation Blue Skies

A UK-based consortium made up of Google, the UK Department of Transport, the Met Office, NATS, Contrails.org, Imperial College London, and the University of Cambridge, has launched Operation Blue Skies, backed by the UK government as the world's first oceanic airspace-scale trial with the aim of reducing the climate impact of aviation contrails.

Operation Blue Skies is a £5 million, 30-month long program which is partly funded by £2.65 million from the UK Government's Department of Transport. The scheme includes two operational trials conducted during the winter months in NATS controlled Shanwick oceanic airspace, in the eastern half of the North Atlantic corridor. The program will evaluate the operational feasibility and environmental benefits of adjusting the altitude of commercial flights to avoid the atmospheric conditions that result in the formation of persistent warming contrails.

Shanwick airspace, where the trial is taking place, accounts for around 5% of global contrail warming

Atmospheric modelling shows that the Shanwick airspace accounts for approximately 5% of global contrail warming and a significant amount of warming is concentrated in the winter between the months of November and February. Operation Blue Skies is designed to target this specific time period by using AI models to identify when and where interventions to flight paths would have the greatest potential to reduce contrail-related climate impact.

The trials will focus on minor altitude adjustments within standard flight operations coordinated through standard air traffic control communication channels. Trial maneuvers will primarily take place during winter testing periods in 2026-2027 and 2027-2028, with around 20 - 40 test days per winter. However, testing will be carried out during periods of lower air traffic.

This trial will sit alongside the UK government's investment in other aviation decarbonization projects, including £219 million to boost homegrown green fuel production to cut emissions from flying.

The consortium brings together both technical and scientific expertise across different operational areas and each consortium member with have a role to play.

Google UK is developing AI models to predict and avoid warming contrails, while using machine learning and satellite imagery to track and verify the impact of the trials. Google UK is contributing on a pro-bono basis, contributing £1.4 million in-kind through AI research expertise, engineering time, and high performance computing infrastructure in order to advance open climate science.

NATS is responsible for trial airspace operations, ATC coordination, conducting safety assessments and air traffic controller training.

Contrails.org will lead forecast assessment and integration, design trial parameters and support data analysis. Contrails.org is working with the consortium partners to connect the scientific and operational pieces of the trials including: assessing and integrating forecasts, shaping trial parameters and supporting data analysis so Operation Blue Skies can test whether or not contrail avoidance is safe, measurable and practical across oceanic airspace.

Imperial College London and the University of Cambridge will evaluate trial outcomes and provide independent verification of climate impact.

The Met Office will develop a new UK contrail forecast capability and ensure meteorological accuracy throughout the trial.

About Contrails

Contrails are short for condensation trails. They are line shaped clouds that sometimes form behind jet airplanes at cruise altitudes. Contrails are generally more common and persistent in the sky during periods when there is a high moisture content in the atmosphere. Contrails are composed mainly of water in the form of ice crystals. They do not pose a direct health risk to humans, nor do they affect the cloudiness of the earth's atmosphere.

Contrails do not pose a direct health risk to humans

Contrails are produced by an aircraft's hot engine exhaust interacting with cold humid air several miles above the surface of the earth. If the atmosphere is cold and humid enough, the engine exhaust will cause the water vapor to condense onto fine particles forming water droplets. The water droplets freeze into ice crystals, creating a visible cloud. Since air temperatures at high altitudes are very cold and fine particles are naturally found in the atmosphere, only the addition of water vapor is needed for condensation to take place and a contrail to form. Just like how you can sometimes see your breath on a cold day, contrail visibility and persistence depends on the temperature and humidity of the air. Contrails are more likely to form and persist when the atmosphere is cold and humid. Contrails will not form if the air at the aircraft's altitude is too warm or too dry.

Depending on the temperature and the amount of moisture in the air, contrails may evaporate within minutes if the humidity is low, or last for hours if the humidity is high. Persistent contrails can increase the cloudiness of the atmosphere and if they are around for long enough, can form aviation-induced cirrus clouds.

The FAA states that contrails can generally be classified into three types: short-lived contrails, persistent non-spreading contrails, and persistent spreading contrail cirrus.

Short-lived contrails disappear within seconds to minutes of being formed under low humidity conditions. Newly formed ice particles evaporate quickly as exhaust gases are completely mixed into the surrounding atmosphere.

A persistent non-spreading contrail forms when the air is extremely humid and they will stay in the sky long after the aircraft is out of sight. It can last for a few minutes or longer than a day. The contrail can maintain its shape of a thin line for many hours if it is formed in a region of the atmosphere with low windshear.

Persistent contrails form in extremely humid regions of the atmosphere and can last for hours-to-days. Newly formed ice particles continue to grow by taking water from the surrounding atmosphere. The resulting line-shaped contrail extends beyond the aircraft. If contrails persist for long enough periods of time in a region of the atmosphere with high windshear, they can spread across the sky due to the prevailing winds at the level at which they formed.

Some contrails last only minutes while others can last hours-to-days

Recently formed persistent contrails can be easily identified from the ground and satellites as being formed from aviation activity. However, if contrails persist long enough to widen and spread, it can be difficult to distinguish them from naturally occurring cirrus clouds. This means that sometimes natural cirrus clouds can be mistaken for contrails when viewed from satellite imagery when they have formed patterns such as parallel lines. This may be a difficulty for Operation Blue Skies as they will have to determine if something that looks like a contrail is a contrail or a naturally occurring cirrus cloud in a parallel line pattern.

Aircraft exhausts produce similar combustion related emissions as cars. Airplane engine exhaust fumes are composed of: 71% carbon dioxide and 28% water vapor. The exhaust also contains less than 1% of other combustion products such as sulfur oxides, nitrogen oxides, carbon monoxide, unburned hydrocarbons and particulate matter (soot particles).

The impact of contrails on global warming is something that scientists have been aware of for a long time so it is great to see something being done about it. It will be interesting to see what these trials will uncover and if this could be the future of aviation, making small altitude adjustments to avoid contrails forming. While it will likely be some time before we hear the results from Operation Blue Skies, I will definitely be keeping an eye out for other developments in this area of aviation.

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Date Published: 31 August 2026