When upper atmospheric temperatures cool, our department occasionally receives calls regarding the lingering contrails formed from aircraft exhaust. Below is some information about contrails courtesy of National Aeronautics and Space Administration, National Science Foundation, National Weather Service, and National Oceanic and Atmospheric Administration. For more information, see the links below or other scientifically-based information available on the internet.
What Are Contrails?
A contrail is the condensation trail that is left behind by a passing jet plane. Contrails are clouds, usually cirrus, formed when water vapor condenses and freezes around small particles (aerosols) that exist in aircraft exhaust. Some of that water vapor comes from the air around the plane; and, some is added by the exhaust of the aircraft. The exhaust of an aircraft contains both gas (vapor) and solid particles. Both of these are important in the formation of contrails. Some elements of the exhaust gasses are not involved in contrail formation but do constitute air pollution. Emissions include carbon dioxide, water vapor, nitrogen oxides, carbon monoxide, hydrocarbons such as methane, sulfates, and soot and metal particles.
What Conditions Must Be Present for Contrails to Be Formed?
This trail of clouds normally forms at very high altitudes (usually above 26,000 ft) where the air is extremely cold (less than -40ºC). The temperature needed to create contrails depends on the ambient air pressure, temperature and humidity outside of the aircraft, and on the ratio of water vapor and heat released into the atmosphere by the aircraft exhaust. That is why sometimes you can see them and sometimes you can’t. The most common times to see contrails are in the spring and fall. The contrail cloud formation is similar to the cloud you see on cold days when you exhale and "see your breath."
In order for a contrail to form, there must be enough moisture in the high levels of the atmosphere for the ice crystals to form around the airplane exhaust. If the upper atmosphere is very dry, contrails will not easily form, or will be of the short-lived type. Persistent contrails can last for hours to days, and spread over thousands of square kilometers, becoming indistinguishable from naturally occurring cirrus clouds.
Why Is There Concern About Contrails?
Contrails, especially persistent contrails, represent a human-caused increase in high thin clouds in the Earth's atmosphere, and are likely to be affecting climate and ultimately our natural resources. Scientists are concerned about contrails because predicted increases in air-traffic could result in a continued increase in cloud cover.
Where Can I Get More Information?
The National Oceanic and Atmospheric Administration (NOAA) website contains additional information on contrails at http://www.wrh.noaa.gov/fgz/science/contrail.php?wfo=fgz .
Eric Betterton, University Distinguished Professor, Hydrology and Atmospheric Sciences, University of Arizona provided additional information on contrails.