Here's an interesting question I received today from Magy in Ashland. She asks: "What exactly causes contrails and can they change the climate?"
To answer this question, lets first identify what a contrail is. A contrail is the condensation trail that is left behind by a passing jet plane. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. Vapor pressure is just a fancy term for the amount of pressure that is exerted by water vapor itself (as opposed to atmospheric, or barometric, pressure which is due to the weight of the entire atmosphere above you). The mixing occurs directly behind the plane due to the turbulence generated by the engine. If condensation (conversion from a gas to a liquid) occurs, then a contrail becomes visible. Since air temperatures at these high atmospheric levels are very cold (generally colder than -40 F), only a small amount of liquid is necessary for condensation to occur. Water is a normal byproduct of combustion in engines.
This cloud formation is very similar to the process that occurs when you breath on a cold winter day and you can see your own breath in the form of a "cloud". You may have noticed that on some days this "cloud" you produce lasts longer than on other days where it quickly disappears. The length of time that a contrail lasts is directly proportional to the amount of humidity that is already in the atmosphere. A drier atmosphere leads to a more short-lived contrail, while an atmosphere that has more humidity will lead to longer-lived contrails. However, if the atmosphere is too dry, no contrails will form. Occasionally a jet plane, especially if ascending or descending, will pass through a much drier or more moist layer of atmosphere which may result in a broken pattern to the contrail, with it appearing in segments rather than in one continuous plume.
Contrails can be found over most of the planet. Now that jet plane traffic, both civilian and military, can be at anyplace over the globe at anytime, contrails are becoming more and more common. This picture was taken by the NOAA-12 satellite as it passed over portions of Europe in 1995. It is very obvious from this color enhanced satellite image that the atmosphere was very conducive to the development of contrails on this date (5 April 1995) and that these contrails were long-lived enough to accumulate with many criss-cross patterns over the same heavily traveled portion of air space.
Contrails have been recorded throughout the history of jet plane travel. Many reports exist from World War II of situations where the accumulations of contrails was so extensive that pilots were unable to keep visual contact with neighbor or enemy planes during combat. Contrails have been recorded from the Sahara Desert to the South Pole indicating that contrails are not constrained to only populated regions of the Earth.
Contrails were difficult to study before September 11, 2001, because they normally cross each other by the hundreds. But with planes grounded for three days following the terrorist attacks.
During that same time, David Travis, a University of Wisconsin atmospheric scientist, studied daily temperature ranges across the country and found that the average range increased, with the largest changes in the Northeast, the Midwest and the Pacific Northwest, where air traffic is usually heaviest. Although this complicates global warming arguments, it offers further evidence that contrails do affect climate in some way.
Contrails typically cover up to five percent of the sky over Europe and the U.S. But as air travel grows in popularity, Travis warns they could increase by a factor of six by 2050. More clouds would mean cooler temperatures. We'll have to wait and see. What do you think? Post your comments and keep those great questions coming!