Contrails
Contrails, short for condensation trails, are narrow clouds of ice crystals that form in the wake of aircraft or rocket engines flying at high altitudes, typically above 8,530 meters. They occur when water vapor in exhaust gases condenses and freezes in cold, humid air, particularly when temperatures range from 4 °C to –60 °C and relative humidity exceeds 100 percent. Contrails become visible almost instantly as ice particles grow large enough to scatter light and can persist for hours or even spread into larger cirrus clouds, depending on atmospheric conditions.
The presence of contrails has implications for climate change, as they can cover around 0.1 percent of the Earth's surface, influencing temperatures by affecting the balance of incoming and outgoing radiation. Studies suggest that the increase in cirrus cloud cover due to contrails may contribute to global warming by trapping heat in the atmosphere. Notably, the phenomenon was highlighted following the grounding of US air traffic after the September 11, 2001 attacks, which revealed the significant cooling effects of contrails' absence. While some research indicates that night flights may disproportionately contribute to the greenhouse effect of contrails, the overall impact of increased air traffic on climate change remains a subject of ongoing study and debate.
Subject Terms
Contrails
Definition
Condensation trails, or contrails, are long, narrow cirrus clouds composed of ice crystals that form behind aircraft or rocket engines flying in the upper atmosphere. When fuels containing hydrogen, such as hydrocarbons, burn in air, the engine exhaust contains water vapor. The vapor in the hot exhaust rapidly condenses and freezes when it mixes with cold, humid atmospheric air, forming a trail of ice crystals. Contrails typically form where the air temperature is 4 °Celsius to –60 °Celsius and relative humidity exceeds 100 percent, with low wind turbulence. Favorable conditions typically occur above 8,530 meters. Contrails have been reported since 1915, but they became much more common as jet aircraft traffic increased. Geese flying in cold air have been seen to leave small contrails as they exhale moist air.
The engendered by contrails starts to form on microscopic dust particles. Contrails become visible approximately 0.1 second after leaving an engine, as the ice particles grow large enough to scatter sufficient light for them to be seen. Although contrails start as one exhaust behind each engine, they often merge into one wing-tip vortex from each wing tip. The lower pressure and temperature in the core of each tip vortex also helps accelerate condensation. Contrails move down in reaction to the aircraft’s lift and the higher density of ice. During daytime, ice crystals absorb sunlight. The warming air around them can convect the contrails several hundred meters up.
When the air is dry, the ice particles sublimate to vapor quickly, resulting in a short contrail. In humid air, contrails can persist for several hours or thousands of kilometers, given the speed of aircraft, and they grow into cirrus clouds as thick as 500 meters and several kilometers wide. Ice-particle size in such clouds is on the order of 200 micrometers, and their density is on the order of 1 to 50 particles per cubic centimeter of air. Many trails are over two hours old, and they continue to accumulate moisture from the air during that time. Indeed, little of such a cloud comes from the original jet exhaust. A contrail cloud may contain one thousand to ten thousand times the water released by the aircraft engine itself.
Significance for Climate Change
The cirrus cover due to contrails has been estimated to cover as much as 0.1 percent of Earth’s surface. The most famous experiment on contrails was conducted by the National Aeronautics and Space Administration (NASA) in the days following the terrorist attacks of September 11, 2001. US civil air traffic was grounded for three days. During those three days, the difference between daytime high and nighttime low temperatures over the continental United States increased by roughly 1 degree Celsius when compared to the thirty-year average. At the same time, the trails left by six military aircraft persisted and eventually covered over 19,700 square kilometers.
The increase in cirrus cloud cover due to contrails has been studied as an anthropogenic factor in global warming. Ice crystals absorb, scatter, and reflect radiant heat. Some studies indicate that contrail cloud cover inhibits outward radiation from Earth’s surface and lower atmosphere more than it reflects incoming solar radiation, contributing to the greenhouse effect. Some argue that long-wave is absorbed more by ice crystals than by air or water vapor, so that cirrus clouds have a net warming effect.
Other studies suggest that night flights, which constitute only 20 to 25 percent of air traffic, may contribute to 60 percent of contrails’ greenhouse effect. Published data project that a fivefold increase in air traffic would cause a net global warming effect due to contrails of 0.05° Celsius. Some argue that any detectable anthropogenic change is a cause for concern. Others point to a more severe localized effect due to the heavy traffic over industrialized nations in the temperate zone, where the air is moist and cold for a greater part of the year compared to equatorial regions.
Changing from hydrocarbon to hydrogen fuel will not reduce water vapor, but may reduce dust particles in the exhaust. A different aspect is the deposition of carbon dioxide and heat in the upper atmosphere. are believed to contribute 2 to 3 percent of all anthropogenic global warming. Contrails are highly amplified reminders of that problem.
Bibliography
Atlas, David, Zhien Wang, and David P. Duda. “Contrails to Cirrus: Morphology, Microphysics, and Radiative Properties.” Journal of the American Meteorological Society, January 2006, 5–19. Print.
Grewe, Volker, et al. “Reduction of the Air Traffic's Contribution to Climate Change: A REACT4C Case Study.” Atmospheric Environment 94 (2014): 616–625. Environment Complete. Web. 19 Mar. 2015.
Hoyle, C. R., B. P. Luo, and T. Peter. “The Origin of High Ice Crystal Number Densities in Cirrus Clouds.” Journal of the Atmospheric Sciences62, no. 7 (July 2005): 2568–2579. Print.
Jensen, E. J., et al. “Spreading and Growth of Contrails in a Sheared Environment.” Journal of Geophysical Research103, no. D24 (1998): 31,557–31,567. Print.
Penner, J. E., et al. “Aviation and the Global Atmosphere.” Intergovernmental Panel on Climate Change, Special Report. New York: Cambridge University Press, 1999. Print. Waldek, Stefanie. "Contrails: What Are They and How Do They Form?" Space.com, 27 Aug. 2023, www.space.com/what-are-contrails. Accessed 21 Dec. 2024.