page_title: Environmental impact of aviation
section_title: Water vapor (H2O), and contrails
hierarchical_section_title: Environmental impact of aviation / Climate change / Mechanisms and cumulative effects of aviation on climate / Water vapor (H2O), and contrails
caption_attribution_description: English: Cirrus clouds on a very cold day.
context_page_description: The environmental impact of aviation occurs because aircraft engines emit heat, noise, particulates and gases which contribute to climate change and global dimming. Airplanes emit particles and gases such as carbon dioxide, water vapor, hydrocarbons, carbon monoxide, nitrogen oxides, sulfur oxides, lead, and black carbon which interact among themselves and with the atmosphere.
  Despite more fuel-efficient and less polluting turbofan and turboprop engines, the rapid growth of air travel contributes to an increase in total pollution attributable to aviation. From 1992 to 2005, passenger kilometers increased 5.2 percent per year. In the European Union, greenhouse gas emissions from aviation increased by 87 percent between 1990 and 2006.
  Comprehensive research shows that despite anticipated efficiency innovations to airframes, engines, aerodynamics and flight operations, there is no end in sight, even many decades out, to rapid growth in CO₂ emissions from air travel and air freight, due to projected continual growth in air travel.
context_section_description: One of the products of burning hydrocarbons with oxygen is water vapour, a greenhouse gas. Water vapour produced by aircraft engines at high altitude, under certain atmospheric conditions, condenses into droplets to form condensation trails, or "contrails". Contrails are visible line clouds that form in cold, humid atmospheres and are thought to have a global warming effect (though one less significant than either CO₂ emissions or NOₓ induced effects). Contrails are uncommon (though by no means rare) from lower-altitude aircraft, or from propeller-driven aircraft or rotorcraft.
  Cirrus clouds have been observed to develop after the persistent formation of contrails and have been found to have a global warming effect over-and-above that of contrail formation alone. There is a degree of scientific uncertainty about the contribution of contrail and cirrus cloud formation to global warming, and attempts to estimate aviation's overall climate change contribution do not tend to include its effects on cirrus cloud enhancement. However, a 2015 study found that artificial cloudiness caused by contrail "outbreaks" reduces the difference between daytime and nighttime temperatures. The former are decreased and the latter are increased, in comparison to temperatures the day before and the day after such outbreaks. On days with outbreaks the day versus night temperature difference was diminished by about 3.3 °C (6 °F) in the southern US and 2.8 °C (5 °F) in the US midwest.
caption: Cirrus cloud formation
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