Contrails - Research, comments and links

Contrails and Aviation-induced Cirrus Clouds

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Clouds Caused By Aircraft Exhaust May Warm The U.S. Climate

NASA scientists have found that cirrus clouds, formed by contrails from aircraft engine exhaust, are capable of increasing average surface temperatures enough to account for a warming trend in the United States that occurred between 1975 and 1994.

"This result shows the increased cirrus coverage, attributable to air traffic, could account for nearly all of the warming observed over the United States for nearly 20 years starting in 1975, but it is important to acknowledge contrails would add to and not replace any greenhouse gas effect," said Patrick Minnis, senior research scientist at NASA's Langley Research Center in Hampton, Va. The study was published April 15 in the Journal of Climate. "During the same period, warming occurred in many other areas where cirrus coverage decreased or remained steady," he added.

"This study demonstrates that human activity has a visible and significant impact on cloud cover and, therefore, on climate. It indicates that contrails should be included in climate change scenarios," Minnis said.

Minnis determined the observed one percent per decade increase in cirrus cloud cover over the United States is likely due to air traffic-induced contrails. Using published results from NASA's Goddard Institute for Space Studies (New York) general circulation model, Minnis and his colleagues estimated contrails and their resulting cirrus clouds would increase surface and lower atmospheric temperatures by 0.36 to 0.54 degrees Fahrenheit per decade. Weather service data reveal surface and lower atmospheric temperatures across North America rose by almost 0.5 degree Fahrenheit per decade between 1975 and 1994.

Minnis worked with colleagues Kirk Ayers, Rabi Palinkonda, and Dung Phan from Analytical Services and Materials, Inc., of Hampton, Va. They used 25 years of global surface observations of cirrus clouds, temperature and humidity records from the National Centers for Environmental Prediction (NCEP) reanalysis dataset. They confirmed the cirrus trends with 13 years of satellite data from NASA's International Satellite Cloud Climatology Project.

Both air traffic and cirrus coverage increased during the period of warming despite no changes in the NCEP humidity at jet cruise altitudes over the United States. By contrast, humidity at flight altitudes decreased over other land areas, such as Asia, and was accompanied by less cirrus coverage, except over Western Europe, where air traffic is very heavy.

Cirrus coverage also rose in the North Pacific and North Atlantic flight corridors. The trends in cirrus cover and warming over the United States were greatest during winter and spring, the same seasons when contrails are most frequent. These results, along with findings from earlier studies, led to the conclusion that contrails caused the increase in cirrus clouds.

"This study indicates that contrails already have substantial regional effects where air traffic is heavy, such as over the United States. As air travel continues growing in other areas, the impact could become globally significant," Minnis said.

Humidity is the amount of water vapor in the air and determines how long contrails remain in the atmosphere. Contrails that persist for an extended period of time are most likely to impact the climate.

Contrails form high in the atmosphere when the mixture of water vapor in the aircraft exhaust and the air condenses and freezes. Persisting contrails can spread into extensive cirrus clouds that tend to warm the Earth, because they reflect less sunlight than the amount of heat they trap. The balance between Earth's incoming sunlight and outgoing heat drives climate change.

NASA's Earth Science Enterprise funded this research. NASA's Earth Science Enterprise is dedicated to understanding the Earth as an integrated system and applying Earth System Science to improve prediction of climate, weather, and natural hazards using the unique vantage point of space.

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Langley Technical Report Server

Patrick Minnis, J. Kirk Ayers and Steven P. Weaver, Surface-Based Observations of Contrail Occurrence Frequency Over the U.S., April 1993--April 1994 , NASA RP-1404, December 1997, pp. 83, (4MB).

Format(s): Postscript, or PDF

Keywords: Contrails; Cirrus clouds; Surface observation; Subsonic assessment; Aircraft effects; Climate change

Abstract: Surface observers stationed at 19 U.S. Air Force Bases and Army Air Stations recorded the daytime occurence of contrails and cloud fraction on an hourly basis for the period April 1993 through April 1994. Each observation uses one of four main categories to report contrails as unobserved, non-persistent, and indeterminate. Additional classification includes the co-occurence of cirrus with each report. The data cover much of the continental U.S. including locations near major commercial air routes. The mean annual frequency of occurrence in unobstructed viewing conditions is 13 percent for these sites. Contrail occurrence varied substantially with location and season. Most contrails occurred during the winter months and least during the summer with a pronounced minimum during July. Although nocturnal observations are not available, it appears that the contrails have a dirunal variation that peaks during mid morning over most areas. Contrails were most often observed in areas near major commerical air corridors and least often over areas far removed from the heaviest air traffic. A significant correlation exists between mean contrail frequency and aircraft fuel usage above 7 km suggesting predictive potential for assessing future contrail effects on climate.
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Long tracks of exhaust left behind by commercial jets

July 27 —What happens to the long tracks of exhaust left behind by commercial jets? NASA scientists are discovering that some of those tracks, known as contrails, linger for hours and actually become clouds. NBC’s Robert Hager reports on the phenomenon and what it may mean for our global climate.

NASA RESEARCHER Patrick Minnis studied satellite photos of the airplane jetstreams, which occur only when the sky is moist, close to forming thin clouds. What he discovered may sound far-fetched — but the evidence is clear.
  For example, a distinctive oval-shaped contrail left behind by a test flight drifted across California for six hours, finally turning into a 60-mile-long cloud system.
  “We were very excited because it opened our eyes up to the fact that there’s possibly a lot of clouds up in the sky that were originally contrails,” Minnis said.
In another case, a figure-8 cloud created its own 60-mile swath of clouds over Texas and Louisiana over  a nine-hour period.
  NASA’s Bruce Anderson actually went aloft to follow in the turbulent wake of a contrail. He came up with a theory for the role of contrails in causing atmospheric moisture to condense into clouds.
“We do know that the sulfur in the fuel from aircraft generate aerosol particles, and those in turn can influence the formation of clouds,” Anderson said.
   Could jet-caused clouds be enough to affect climate?
   “The number of clear days over the U.S. has decreased in the last 30 years, and we suspect that much of that is due to an increase in cirrus clouds, which we suspect is probably due to an increase in air traffic,” Minnis said.
   And if such clouds trap heat, could they even contribute to global warming?
   That connection to the greenhouse effect is still murky. But there is evidence that contrail clouds can have a big impact on weather patterns. Last year, in the wake of Hurricane Nora, researchers got their most dramatic view yet: Moist air blanketed the nation’s midsection from Nebraska to Texas, and scores of contrails fused into one enormous cloud — stretching for more than 800 miles.
     Part of its Atmospheric Effects of Aviation Project, NASA has placed much of the
research data on the Web.

The Atmospheric Effects of Aviation Project

For the past 10 years NASA has held a conference on The Atmospheric Effects of Aviation Project (AEAP). Several hundred researchers from around the world attend annually. In 1997 Researchers from NASA Langley Research Center in Hampton, Virginia, presented evidence that contrails are contributing to global warming and causing local effects over areas with heavy air traffic. This was reported by Jim Scanlon a journalist in attendance at the conference. He also reports that Fred Singer held a session where he presented a session that argued that the steady increase in air traffic for the last 20 years was responsible for the nighttime warming detected over North America.


The Atmospheric Effects of Aviation Project (AEAP) consists of two major efforts to assess the effect of aircraft on the atmosphere. The Atmospheric Effects of Stratospheric Aircraft (AESA), sponsored by the High-Speed Research (HSR) Program at Langley Research Center, is a study of the potential effects of the operation of a projected future fleet of high speed civil transport aircraft (HSCTs). The Subsonic Assessment program (SASS) is a study of the effects of the present subsonic aircraft fleet and of projected future subsonic fleets, and is sponsored by the Advanced Subsonics Technology Program (AST) at Langley Research Center. Objective Develop scientific basis for assessment of atmospheric impact of subsonic and supersonic aviation, particularly commercial aircraft cruise emissions.


Spreading and Growth of Contrails in a Sheared Environment

Atmospheric Physics Branch

The evolution of persistent contrails has modeled over time-scales of 15-180 minutes using a large-body simulation model with detailed microphysics. Model results have been compared to satellite and in situ measurements of persistent contrails from the SUCCESS experiment. In particular, we simulated the evolution of the persistent contrail observed on May 12, 1996. In simulations with large ambient supersaturations and moderate wind shear, crystals with length > 200 microns are generated within 35 minutes by depositional growth. In situ measurements in the May 12 contrail case showed that these large crystals did in fact form. The large crystals fall rapidly and the contrail horizontal extent increases due to the wind shear (see Figure 1). Strong radiative heating (with rates up to 30 K/day) drives a local updraft and lofts the contrail core several hundred meters.


Safeguarding Our Atmosphere
Glenn Research Reduces Harmful Aircraft Emissions

Depletion of the Earth's natural ozone layer and climatic changes affect everyone. These problems are both global and national concerns. How and how much do aircraft emissions affect our environment? These are important issues facing the aircraft industry. The U.S. Clean Air Act regulations and environmentally conscious citizens demand reduced emissions. The International Civil Aviation Organization (ICAO) voices worldwide concern about air quality and climate change

Reducing aircraft emissions is key to the U.S. aircraft industry's remaining competitive in the global market. The Glenn Research Center, NASA's lead center for aeropropulsion, is developing quieter, more fuel-efficient engines with fewer harmful emissions.

There are several types of aircraft emissions. Each type has an effect on the environment. If the aircraft industry continues to grow as predicted, reducing these emissions is critical. NASA scientists and engineers are working to reduce aircraft emissions, but the problem is complex.


Contrails are Bad News