Information on Contrails from Aircraft

This page provides information on contrails – the long, thin clouds that form behind aircraft at high altitudes – and how they form. This page also provides information on terms that are often inaccurately used as alleged synonyms of contrails, such as “chemtrails” and geoengineering. EPA has received many inquiries from the public on these topics. Our goal is to provide clear, science-based information while addressing common questions, concerns, and misconceptions.

• What are contrails?
• How do contrails form?
• How long do contrails last?
• Research on the environmental impacts of contrails
• What are “chemtrails”?
• What is intentionally sprayed from airplanes?
• Are contrails related to geoegineering?
• What is HAARP and is it related to contrails, geoengineering or weather modification?

What are contrails?

Contrails are line-shaped exhaust clouds or “condensation trails” that are visible behind jet aircraft. Aircraft engine exhaust is composed of water vapor, carbon dioxide (CO2), and other emissions that are commonly found when combusting fuel, such as sulfur oxides (SOx), hydrocarbons (HC), and particles (see the graphic below). Since aircraft typically fly at high altitudes where it is very cold, the very hot exhaust coming out of the jet engine reacts with the very cold air sometimes causing an exhaust cloud to form that you can see under certain atmospheric conditions. Jet aircraft form contrails under these atmospheric conditions for the same reason that you can see the exhaust from your vehicle or your own breath on a cold day.

Contrails are a normal effect of jet aircraft operations and have been since its earliest days of air travel. If you are seeing a lot of contrails in your area it is because there are a lot of jet aircraft flying overhead. 

How do contrails form?

For a contrail to form, the correct conditions must occur immediately behind a jet engine in the expanding engine exhaust plume. A contrail will form if the humidity becomes high enough (and the air temperature becomes low enough) for water vapor to condense on particles (often soot) in the exhaust and the surrounding air to form tiny water droplets, and then these droplets freeze at low temperatures into ice crystals.

Depending on the temperature and the amount of moisture in the air at the aircraft’s altitude, contrails evaporate quickly (if the humidity is low) or persist and grow (if the humidity is high). Conditions that are favorable to forming contrails occur frequently at high altitudes. When atmospheric conditions favorable to forming contrails are combined with high numbers of commercial and private jet aircraft flights, there can be many contrails in the sky at one time.

It is also important to note that the humidity and other atmospheric conditions at high altitudes may be very different from the conditions we experience on the ground. For example, there may be high humidity conditions at high altitudes, but low humidity conditions at ground level.

How Long Do Contrails Last?

Many contrails are short lived and form and evaporate in a matter of seconds-to-minutes due to low humidity conditions in the atmosphere where airplanes are flying. Persistent contrails result when airplanes fly through layers of high humidity conditions (known as super saturated air) in the atmosphere. They can last for a few minutes or longer than a day. Persistent contrails can interact with wind and clouds to form cirrus clouds, which are thin, wispy clouds that are common at high altitudes.

Contrails can also take different shapes and appearances depending on atmospheric conditions, wind movement, and aircraft flight patterns. These appearances can include straight lines, grid or cross patterns, twisted or spiral, curved, and fragmented contrails. These are all possible depending on a variety of weather conditions and flying movements.

Research on the environmental impacts of contrails

Current models indicate that persistent contrail clouds could have a small net warming effect. However, more research is needed to better understand and quantify the impact of persistent contrails on the climate. EPA is supporting research by the Federal Aviation Administration (FAA) and other federal agencies.

FAA and other federal agencies continue to research the formation and persistence of contrails. For more information, visit: FAA | Contrails.

What are “chemtrails”?

“Chemtrails” is a shortening of the term “chemical trails”. It is a term some people use to inaccurately claim that contrails resulting from routine air traffic are actually an intentional release of dangerous chemicals or biological agents at high altitudes for a variety of nefarious purposes, including population control, mind control, or attempts to geoengineer Earth or modify the weather.

It should be noted that chemicals are sometimes intentionally sprayed from aircraft for legitimate purposes like firefighting or farming, but these are well-documented occurrences, regulated, and used for specific legitimate purposes. Releases of this type are from low-flying propeller aircraft, not high-altitude jets. For more information, see the next section below.

EPA proactively works with other federal agencies to monitor air traffic for any and all potential, dangerous activities and will continue to do so.

EPA is aware that some members of the public are concerned about unusual contrails they see in the sky or reports about geoengineering and dangerous chemicals being sprayed from jet aircraft at high altitudes. Some members of the public have been concerned when they see low flying planes spraying fields (known as crop dusting. See more info below). When witnessing these phenomena, it is reasonable to ask questions and inquire further to understand what you are seeing. More information on geoengineering. For more information on when chemicals are sprayed from airplanes for legitimate purposes, see below.

What is intentionally sprayed from airplanes?

Chemicals are sometimes intentionally sprayed from aircraft for non-nefarious purposes like firefighting or farming. But these are well documented occurrences and used for specific purposes. Releases of this type are from low-flying propeller aircraft, not high-altitude jets.

Here is some information on non-nefarious reasons chemicals may be released from low-flying propeller aircraft:

Agricultural spraying or crop dusting: Common in farming areas, low-flying propeller aircraft are used to spray pesticides, herbicides, or fertilizers. For more information on agricultural spraying, please visit: EPA Introduction to Pesticide Drift.

Firefighting: Planes sometimes drop fire retardants or water to suppress wildfires. The chemicals used are non-toxic but dyed red for visibility. More info can be found at Modular Airborne Fire Fighting Systems (MAFFS).

If you suspect illegal spraying, chemical exposure, or environmental damage, visit: Report Environmental Violations.

If an aircraft is flying unusually low or in a potentially unsafe way, report to the Federal Aviation Administration (FAA) at: FAA Hotline.

Are contrails related to geoengineering or weather modification?

The federal government is not aware of there ever being a contrail intentionally formed over the United States for the purpose of geoengineering or weather modification. Geoengineering encompasses a broad range of activities that intentionally attempt to cool the Earth or remove gases from the atmosphere. A subset of these activities is intended to cool the Earth by modifying the amount of sunlight reaching Earth’s surface. This is a subject currently going through intense domestic and international public debate, research, and scrutiny for good reason. More information about geoengineering and weather modification.

What is HAARP and is it related to contrails, geoengineering or weather modification?

The High-frequency Active Auroral Research Program (HAARP) is a high-power, high frequency transmitter used to study the ionosphere. The program was managed by the U.S. Air Force and U.S. Navy between 1990 and 2014, but responsibility for HAARP facilities and equipment was handed over to the University of Alaska-Fairbanks in 2015.

HAARP has been at the center of numerous conspiracies over the years. EPA is not aware of any scientific evidence that supports any claim that any nefarious activities are taking place. HAARP is not a classified program, and its Environmental Impact Study can be viewed by the public. The Federal Communication Commission (FCC), Federal Aviation Administration (FAA) and Alaska Department of Environmental Conservation (ADEC) are the three major permitting and licensing organizations with jurisdiction over the HAARP Research Site.

For more information on HAARP: FAQ | HAARP