How contrails form and evolve

An illustration of how contrails form, listing these steps: Jet engines emit water vapor as well as CO2 and other pollutants. Water droplets condense around some of these pollutants. Droplets freeze into ice particles that make up contrail clouds.

Contrails form in jet engine exhaust where heat, water vapor and black carbon (soot) mix with the ambient atmosphere.

Soot particles and other aerosols form cloud nuclei that condense exhaust water vapor into liquid droplets. The exhaust plume sinks down and away from the aircraft due to the wake vortex. The plume is diluted by ambient air, which cools the plume and freezes liquid droplets into ice particles.

Persistent contrails can last hours and grow to become indistinguishable from natural cirrus clouds.

How contrails contribute to global warming

An illustration of warm air being trapped into the atmosphere by the contrail of a plane with the following text: Some of the sun's energy gets reflected away from the Earth by contrail-induced cirrus clouds…but most of the sun's energy passes through, warming the Earth. At night, the earth radiates heat back out into space…but the contrail-induced cirrus clouds absorb some of this heat and act like a blanket around the Earth.

Ice clouds, including natural and contrail-induced cirrus clouds, are better at trapping outgoing longwave (thermal) radiation emitted by the Earth than reflecting incoming shortwave (solar) radiation from the sun. This means that, in general, cirrus clouds trap more heat in the Earth's atmosphere than they reflect. These clouds continue to absorb outgoing radiation at night, even when there is no solar radiation to reflect.

The contrail impact on incoming and outgoing radiation is defined as the radiative forcing. Contrail radiative forcing depends on time of day, season, cloud cover, and albedo (reflectivity) of the surface of the earth.

In aggregate, contrails are strongly warming, increasing the overall heat stored in the Earth's atmosphere.

How we can mitigate contrail impact

Our Focus

Intelligent route planning

Persistent contrails form in thin regions of the atmosphere (500 - 1000m thick in altitude) that are sufficiently cold and humid. Forecasts can predict where airplanes are likely to form warming contrails and enable pilots to avoid these regions.

Unlike fleet-wide adoption of DAC and SAF, this approach has a low cost and logistical footprint.

Sustainable aviation fuel

Sustainable aviation fuel (SAF) is biofuel produced from sustainable feedstocks used in place of traditional jet fuel. SAF has lower concentrations of aromatic compounds resulting in reduced soot emissions.

Studies of commercial flights using a blend of up to 50% SAF show a reduction of 50% to 70% in soot emissions. Contrails produced by planes burning a SAF blend show similar reductions in size and concentration of ice particles, decreasing their warming effect.

In 2019, only 0.1% of all aviation fuel was SAF, a number that is only forecast to increase to 2% by 2030.

New engine technology

The newest generation of jet engines burn fuel more cleanly and efficiently, resulting in a large reduction in soot emissions.

Studies estimate that fleet-wide adoption of new engines would reduce contrail warming impacts by as much as 70%.

These engines are currently being installed on new commercial airplanes, but will likely take over a decade to expand to the whole fleet.

Why focus on route planning first?

With just minimal adjustments to only 5% of flight paths, we can eliminate 80% of contrails-induced warming.

An illustration displaying the fact that only 5% of existing flights will need to be rerouted to affect climate change.

The best available data indicates a kind of “super-Pareto principle” at play, where tweaking only a few flight paths would eliminate almost all of contrails-induced warming. In practice, this means that just 1 in 20 flights would need to fly over, under, or around areas of the sky predicted to produce harmful contrails.

Better yet, properly implemented, these adjustments would be cheap: Our studies show a fleet-average cost of roughly $5.00 per flight, or less than $0.50 per tonne of CO2 equivalent warming avoided.

How the model works

Our analysis and data products, built in collaboration with our partners, will allow the industry to predict contrails just like we predict turbulence, icing, storms and other phenomena. And with everyone’s help, it will do so with increasing accuracy over time.

An illustration showing the steps in Reviate's rerouting model: 1. Forecast Input. 2. Modeling. 3. Flight Planning. 4. Verification.
  • Forecast Input Weather forecasts, satellite images, flight locations, and other data are fed into contrail forecast models
  • Modeling Models determine where harmful contrails are likely to occur and compare these predictions with observations
  • Flight Planning Flight planners calculate the fastest route with the lowest fuel consumption accounting for contrail impact in their flight plan
  • Verification Ground-, air-, and satellite observations verify contrail avoidance and feed back into forecasting models to improve accuracy

Observational data makes the model smarter

Ground-based observations inform real-time contrail avoidance and improve contrail forecast accuracy when they are fed back into the model. Credit: Ed Gryspeerdt

Southern United States seen from space. The red lines are contrails detected from the satellite imagery. Credit: Google Research