What makes airplane contrails

Contrails don't form for every airplane. The atmosphere where the plane is flying needs to have low vapor pressure and low temperature. There are three types of contrails.

The short lived ones that only last a couple minutes after a plane passes. They shade us by reflecting incoming sunlight back into space. During the day, cooling compensates part of the warming. But at night, with no sunlight, only the warming effect operates. Red-eye flights are a red light for climate.

Research in the American South and Midwest has concluded that when contrails are around, they raise night-time temperatures sufficiently to reduce the day-night differences by 3 degrees C. So how does this play out globally? What does it mean for climate change?

The figure is for , the base year for the U. Federal Aviation Administration dataset used by the authors. It was double the 24 milliwatts from the CO2 that had accumulated in the atmosphere from a century of aviation and is a significant part of a total anthropogenic effect at the time of around 1, milliwatts.

Contrails and the cirrus clouds they create last for only a few hours at most. They do not accumulate and leave no lingering effect, whereas much of the CO2 emitted by aviation sticks around in the atmosphere for centuries. As more and more is emitted, it keeps building up — even if aviation were halted, the CO2 would remain for a long time.

So in the longer run, if we want to shut down global warming, there is no alternative to curbing CO2 emissions. But if the world wants a big short-term contribution from aircraft to keeping us below some specific temperature target, such as 1.

Within days it would take some of the heat out of the atmosphere. Can it be done? Researchers spoken to for this article offer three approaches, though they disagree about which has the most potential. One is to divert aircraft away from air where contrails are likely to form. This can be done vertically by changing altitude, or horizontally by detouring around the problem air. There is a potential trade-off.

In retrospective studies of real flights, Banavar Sridhar, a senior scientist at the NASA Ames Research Center in California, found that changing altitude — usually by flying lower where the air is warmer — could produce a reduction of 35 percent in contrails and contrail cirrus for an extra fuel burn of only 0.

Even so, if done right, there could be a net overall reduction in warming. Volker Grewe of the Institute of Atmospheric Physics looked at real transatlantic flights across the seasons and how their flight paths could have been altered to cut their combined warming effect from both contrails and CO2.

He found that there could have been a 10 percent reduction in warming for only a 1 percent increase in operating costs. Contrails over Lisbon, Portugal, in February Any sharp surface, such as the tip of a wing, can cause vortical flow in its wake if it is sufficiently large or the flow is sufficiently fast.

On occasion, these trailing vortices may interact with one another. In one well-known example of this fact, the Crow Instability causes the vortices to develop symmetric sinusoidal oscillations and eventually to merge and form vortex rings behind the jet.

This instability can be triggered by turbulence in the surrounding air or by local variation in air temperature or density, which may itself be the result of the stratification of the atmosphere. When the contrails are visible and strong, it is possible to see the white streaks become wavy and then leave rings floating high in the sky, like smoke rings from a giant cigar. Recent research has suggested that the ice clouds contained in contrails cause greenhouse effects and contribute to global warming as part of the insulating blanket of moisture and gases in the atmosphere.

Researchers in this area seized on the opportunity presented on September 11 and 12 over the U. The complete cessation of commercial air traffic offered a control sky without contrails for use in quantifying the environmental effects of contrails. Already a subscriber? Sign in. Thanks for reading Scientific American.