Contrails Explained: Why They Form and What They Mean

Contrails, the wispy white streaks that sometimes form behind aircraft, have long been a subject of interest – sparking debates about their environmental impact, their role in climate change, and even fuelling conspiracy theories.

While there is consensus that contrails have an adverse effect on the environment, scientific understanding still needs to improve to calculate their exact warming impact, and there is less agreement as to how much and over what timescales such effects occur. These uncertainties have led to differing views as to whether we know enough to act now. What we do know, however, is that doing nothing is the wrong thing. We need to act to reduce uncertainties. What NATS is doing is urgently pursuing research into trials that will give us more confidence that any future action to avoid warming contrails will support the all important limiting of global average surface temperature to 1.5 degrees in line with the Paris Accord.

Contrails are created when an aircraft passes through areas of very cold and humid air called Ice Super Saturated Regions (ISSRs), which can be quite large laterally, sometimes hundreds of kilometres across, but are relatively shallow in depth. In these conditions, the water vapour from the engine and the surrounding atmosphere condenses on soot particles in the exhaust creating ice crystals. While in many cases, these ice crystals disappear quite quickly, under certain conditions the contrails they create can linger and expand over a longer period of time, spreading to form cirrus clouds. It is these persistent contrails that we are most concerned about.

Contrails have attracted increasing interest as scientific consensus is that their overall warming effect, particularly at night due to the formed cirrus clouds trapping warmth in the Earth’s atmosphere, essentially insulates the planet.

Not all flights create contrails, and some contrails can actually have a cooling effect. During the daytime they can reflect the solar radiation back out to space, especially if the contrail cirrus forms over a darker surface area such as an ocean or forest. However, if a contrail forms over an area of high reflectivity like ice caps, snow or low cloud, their overall effect is still warming.

One of the key challenges is predicting in advance exactly where these persistent contrails will form. NATS is currently working on CICONIA, a SESAR Joint Undertaking project led by Airbus, and involving Air France, Swiss and easyJet and other European partners. Alongside work on improving the met forecasting and climate understanding, NATS and the airlines are trialling alterations to selected aircraft trajectories to avoid the areas where it is predicted that warming contrails will form. NATS’ main focus is on contrail mitigation over the North Atlantic, although we are also considering domestic operations and how it affects the Air Traffic Control operation. Alterations will be mostly vertical and by only one or two flight levels, but this slight change could be the difference between creating a persistent contrail and avoiding one.

Ideally, the predicted changes will occur in the flight planning phase, so the airline can plan and fuel for this updated trajectory, but a degree of tactical change is also being investigated, particularly for long haul flights. By flying such a climate optimised trajectory, airlines may burn a small amount more fuel, but modelling has shown that the climate impact of contrail avoidance vastly outweighs any slight increases in CO₂ emissions.

During 2025, we’ll be supporting several trial activities and simulations in CICONIA, with the aim of better understanding how ATC will best be able to facilitate climate optimised flight trajectories in the future.

The SESAR 3 CICONIA project is funded by the EU under the Horizon Europe programme, [grant number 101114613]. UK participants receive funding from UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding guarantee [grant numbers 10091271 (NATS) and 10073316 (University of Manchester)].