Condensation Trails

Contrails are line-shaped clouds formed by condensed water vapor from aircraft at high altitudes.  

Condensation trails, known as contrails, are line-shaped condensation that occur when dry and/or cold air meets water vapor from jet engines at high altitudes (Kärcher 2018, Environmental Protection Agency (EPA) 2016). Contrails are primarily made up of water in the form of ice crystals and have no impact on human or animal health (New Hampshire Department of Environmental Services (NHDES) 2024, EPA 2000). Contrails are often mistaken for chemtrails, or chemical trails, a disproven theory that high-altitude aircraft are dispersing chemicals over large areas of the public or atmosphere (NHDES 2024). 

Aircraft engines release water vapor, carbon dioxide, small amounts of nitrogen oxides, hydrocarbons, carbon monoxide, sulfur gases, soot, and metal particles during flight (EPA 2016, EPA 2000). Only water vapor is essential for contrail formation.  

Persistent contrails can spread and influence cloudiness and climate. 

Contrail persistence depends on atmospheric conditions and high altitude (EPA 2016). For instance, if humidity is low, a contrail quickly evaporates after formation; but if humidity is high, the contrail persists and grows by absorbing moisture. The World Meteorological Organization (WMO) named aircraft contrails that persist for 10 minutes or more Cirrus homogenitus (WMO 2017a). These persistent contrails can spread and grow into cloud-like formations due to strong winds (WMO 2017b, Figure 1). 

Cirrus homogenitus affects the cloudiness of the atmosphere, which impacts atmospheric temperature and overall climate (EPA 2016, EPA 2000). All cirrus clouds contribute to climate change, regardless of whether they form naturally or from contrails. Regional studies have found that 80% of contrail-related climate change was caused by 2%-12% of flights (Teoh et al. 2022, Teoh et al. 2020). Current research is focused on identifying methods for reducing contrail formation, including new fuel formulations and low-emitting engine technologies (Contrails Research Roadmap 2025). However, because contrail formation is largely dependent on atmospheric conditions, these interventions are limited.   

Figure 1. Persistent contrails, Cirrus homogenitus. Image of over southern England, United Kingdom, photo by George Anderson (WMO 2017a). Water vapor from aircraft engines condenses to form contrails. High atmospheric humidity enables these contrail ice particles to persist and form into cirrus clouds.

 Aircraft emissions and visible trails can result from a variety of activities. 

Aircraft emissions are regulated by the EPA using global standards developed in collaboration with the international civil aviation organization (ICAO) committee on aviation environmental protection (CAEP) of the United Nations (UN) (EPA 2024). 

Chemicals may be intentionally released from aircraft for a variety of purposes, including (EPA 2016):  

• Aerial spraying to control the spread of disease-carrying insects and pests (mosquitoes, sand fleas, locusts, etc.).  

• Fire suppression using flame-retardant chemicals to control wildfires. 

• Cloud seeding where chemicals are released to encourage water crystals to form and grow in an effort to suppress hail or enhance rain (U.S. Government Accountability Office (GAO) 2024). 

• In-flight emergency fuel release to reduce landing weight for safety reasons. This is rare, and at low altitudes may form a mist that can reach the surface, which appears as a visible, short-lived smoke trail.  

In addition, aircraft generate other effects, including (EPA 2016):  

• Wingtip contrails which occur when a wing or winglet of airplane creates air bubbles in very humid conditions. Wingtip contrails are caused by wind surface, resulting in unique vapors trails that are not from exhaust gases. 

• Black smoke caused by older engines or military fuel blends containing additives to control ice formation and corrosion. Newer aircraft have no visible exhaust plume. 

• Chaff, which is used by military aircraft as a decoy to confuse heat and radar seeking missiles. It is made of silica fibers coated with aluminum. 

• Flares, which are used by military aircraft as decoys to defeat missiles’ tracking abilities, or as ground illumination for target identification. They are made up of magnesium which burns white. 

References 

Federal Aviation Administration (2018) federal aviation administration written re-evaluation, adoption of environmental impact statement and record of decision. LINK

Kärcher B (2018) Formation and radiative forcing of contrail cirrus. Nature Communications, 9:1824. https://doi.org/10.1038/s41467-018-04068-0 

New Hampshire Department of Environmental Services (NHDES) (2024) Chemtrails and Contrails. Environmental fact sheet.  https://www.des.nh.gov/sites/g/files/ehbemt341/files/documents/ard-69.pdf 

Teoh R, Schumann U, Gryspeerdt E, Shapiro M, Molloy J et al. (2022) Aviation contrail climate effects in the North Atlantic from 2016 to 2021, Atmospheric Chemistry and Physics, 22:10919–10935, https://doi.org/10.5194/acp-22-10919-2022   

Teoh R, Schumann U, Majumdar A, Stettler MEJ (2020) Mitigating the Climate Forcing of Aircraft Contrails by Small-Scale Diversions and Technology Adoption. Environmental Science & Technology, 54:2941-2950.  https://doi.org/10.1021/acs.est.9b05608   

United States Environmental Protection Agency (EPA) (2024) EPA Collaboration on International Air Pollution Standards for Aircraft. https://www.epa.gov/regulations-emissions-vehicles-and-engines/epa-collaboration-international-air-pollution-standards 

United States Environmental Protection Agency (EPA) (2016) Contrail facts. https://www.epa.gov/sites/default/files/2016-10/documents/afd-051013-001.pdf 

United States Environmental Protection Agency (EPA) (2000) Aircraft Contrails Factsheet. https://www.faa.gov/sites/faa.gov/files/regulations_policies/policy_guidance/envir_policy/contrails.pdf 

United States Government Accountability Office (GAO) (2024) Cloud Seeding Technology. Report to Congressional Requester. https://www.gao.gov/assets/gao-25-107328.pdf 

World Meteorological Organization (WMO) (2017a) Aircraft condensation trails. International Cloud Atlas. https://cloudatlas.wmo.int/en/aircraft-condensation-trails.html 

World Meteorological Organization (WMO) (2017b) Homomutatus. International Cloud Atlas. https://cloudatlas.wmo.int/en/homomutatus.html