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Announcing Citizen Science Challenge 2: Monitoring and Studying Aircraft Contrails

Contrails like these are formed by high-flying aircraft. They can alter climate by changing the Earth's radiation balance. Forrest M. Mims III. Click image to enlarge.

Contrails are manmade clouds formed by high-flying aircraft. New data show that contrails can have an impact on regional climate.

The serious student and the citizen scientist can use various methods to monitor and study contrails. The purpose of this Citizen Scientist Challenge is to suggest various experimental methods for monitoring contrails and determining their possible environmental effects.

When an internal combustion or a jet engine burns fuel, the exhaust contains a good deal of water vapor. Clouds form only when the temperature of the air is at or below the dew point and when sufficient water vapor is present. If these conditions are met, the water vapor in a jet's exhaust condenses into a condensation trail or contrail behind the aircraft.

Contrails are a mist of water droplets when they first form. The water quickly freezes into tiny ice crystals. Cirrus clouds are also formed of ice crystals. So a contrail is a kind of manmade cirrus cloud. Jet aircraft typically cruise at altitudes between about 23,000 feet (7,620 meters) and 45,000 feet (12,192 meters). You can learn something about water vapor in the air at these altitudes by observing and photographing contrails.

This cirrus overcast covered most of the sky and was formed from multiple, persistent contrails. Forrest M. Mims III. Click image to enlarge.

The break in this contrail indicates drier air than the air in which the contrail is visible. Forrest M. Mims III. Click image to enlarge.

When a contrail persists all across the sky, the air at the altitude of the contrail is uniformly moist. Often contrails are broken into a series of dashes across the sky. The sections where the contrail disappears indicate regions where the air is dryer than the air where the contrail is visible.

Contrails that become wide and diffuse are often made that way by wind. Contrails that stay pencil thin mean that the air is either very still or the plane that formed them was either flying with or against the wind.

These contrails were photographed from an aircraft flying at an altitude of about 10 km. Note the cloud layer below the aircraft. Forrest M. Mims III. Click image to enlarge.

These are not contrails. They are cirrus clouds organized as waves. The clouds appear pink because they were photographed at sunset. Forrest M. Mims III. Click image to enlarge.

Environmental effects of contrails

This image by the MODIS instrument on NASA's Terra satellite shows dozens of contrails over the Southeastern United States on the morning of 29 January 2004. Image courtesy of NASA's Langley Research center. Click image to enlarge.

Contrails at twilight can be quite beautiful, especially when there is a colorful sunrise or sunset. But there is another side to contrails, for they can have a distinct and measurable impact on local climate.

I first observed this phenomenon in 1970 when I began working as a science and electronics writer in a metal storage shed converted into a tiny writing office and electronics workshop. This was in Albuquerque, New Mexico, where contrails often formed overhead on cold winter mornings and caused a very obvious cooling effect in my office when they blocked the Sun.

Only a few persistent contrails were enough to keep my electric space heater running overtime. Consider the potential cooling effect of a dense blanket of contrails over several States, as shown in the remarkable MODIS satellite image above.

A single contrail drifting past the Sun has only a very brief effect on the temperature in the shadow that drifts across the surface far below. This is nicely illustrated in the plot below, which shows direct solar irradiance measured by a Microtops Sun photometer as a thin contrail drifted across the face of the Sun. The contrail's shadow is indicated by the sharp dips in the blue (1020 nanometer) and brown (940 nm) traces on the graph.

Contrails that persist can closely simulate natural cirrus overcasts and potentially have a significant impact on the surface temperature. During one particularly widespread occurrence of persisting contrails over the Midwest United States on 17-18 April 1987, analysis of National Weather Service observations indicated that average maximum surface temperatures near the center of the contrail region were 2-4 degrees Celsius cooler then in surrounding locations just outside the contrail region. Decreases in average diurnal temperature range (DTR) near the center were even greater (Travis, 1996). At night persistent contrails can have an opposite effect on temperature by keeping the temperature at the surface warmer than when the sky is clear.

I have measured the effect of contrails on sunlight in Texas, Alaska and Switzerland. For example, consider the aerosol optical thickness (AOT) of a typical contrail over Fairbanks, Alaska. The maximum increase in AOT caused by the contrail over the background AOT of the blue sky on either side of the contrail was 0.15 at 376 nanometers (nm) in the ultraviolet-A, 0.17 at 540 nm in the green, and 0.16 at 680 nm in the red.

These data are very similar to the mean AOT of thin cirrus clouds at solar noon in South Texas on 12 days I studied. On those days, the mean increase in AOT over the background AOT on the nearest days with a clear sky was 0.20 at 540 nm and 0.15 at 680 nm (376 nm not measured). Thus the measured AOT of a contrail closely resembles that of thin cirrus (see Mims and Travis, 1996).

Further evidence of an important contrail effect on daytime solar irradiance is provided by ground observations of reduced sunlight caused by a contrail overcast near Lausanne, Switzerland, that I measured using a homemade solar radiometer (Mims, 2000).

Numerous contrails on this otherwise cloud-free day evolved into a nearly overcast sky by local noon. Global (full sky) and diffuse solar irradiance were measured at local noon, when the Sun was surrounded but unobscured by contrails, and 17 minutes later, when the Sun was blocked by a large cirrus patch evolved from an accumulation of contrails. The cirrus at the Sun reduced the global irradiance at 376 and 680 nm by, respectively, 3.2% and 4.5% (540 nm not measured; see Mims and Travis, 1997).

Unless new methods are found to power high-flying aircraft, contrails are here to stay. The networks of aircraft contrails in flyways over portions of Europe and North America are among the densest anywhere, and they are increasing. The United States alone has more than 2.000 air routes.

There are several ways to study contrails over your location. The simplest is to keep a "contrail calendar," a log of contrail conditions at your site. This may not seem that significant, but even NASA is interested in such data. See, for example, "Contrail Education," a NASA web site that discusses the importance of student contrail observations at http://asd-www.larc.nasa.gov/GLOBE/

Basic contrail logging can be supplemented by sky photographs, especially wide-angle images. It's important to keep a log of when photographs. Even better is to use a digital or film camera that date and time stamps each photograph.

Still another way to monitor contrails is to measure their effect on local temperature. This is best done by automatic home weather stations or data loggers (e.g., Onset Computer Corporation, www.onsetcomp.com) that store temperature at preset or programmable intervals. Of course you will need to devise a way of recording the presence of contrails at the Sun to make sense of the temperature data. A sky-looking digital camera is ideal. You should also consider learning how to find satellite data for your site, a topic to be covered in a future article in The Citizen Scientist.

Some areas have especially dense contrails because of increased air traffic. Maps of the numerous intersecting air routes across the United States, Europe and other regions will suggest various potential study sites, many of which occur at or near major population centers.

The Citizen Scientist will consider publishing the best investigations resulting from this Citizen Scientist Challenge. For your research to be seriously considered, you will need to explain how and what you measured or observed in an organized, well written report. Photographs and well done illustrations will be important.

Students, please let us know if you pursue the Contrail Challenge as a science fair project. If your project is well done, please send us your findings and a photo of your science project display so we can share your research with readers of The Citizen Scientist.

Mims, F. M. and D. J. Travis, Aircraft Contrails Reduce Solar Irradiance, EOS 78, 448-449, 1997.

Mims III, F. M., Solar Radiometer with Light-Emitting Diodes as Spectrally-Selective Detectors, Optics and Photonics News 11, 3-4, 2000 (also archived in Applied Optics).

Travis, D.J., Diurnal Temperature Range Modifications Induced by Jet Contrails, Preprints of the 13th Conference on Planned and Inadvertent Weather Modification, 110-11, Atlanta, Georgia, 1996.