Contrail Science

Forrest M. Mims III

As I have pointed out from time to time in The Citizen Scientist, contrails provide an important topic for various kinds of amateur scientific studies.

During World War Two, US Army Air Forces bombers flying over Europe were sometimes revealed by the condensation trails (contrails) their engines produced. These straight-as-an-arrow, artificial clouds pointed directly to their sources high overhead. They allowed enemy fighter pilots to quickly find their targets.

Contrails caused other kinds of mischief, as when some of the pilots in a formation of aircraft had their view blocked by contrails from the aircraft ahead.

For these reasons, military meteorologists studied contrails to learn how they formed so they could be predicted in advance.

The meteorologists learned that contrails form when water vapor condenses on tiny particles in hot engine exhaust when the sky is both moist and well below freezing. The end result is a cloud composed of tiny ice crystals.

Figure 1. Spreading contrails over Lausanne, Switzerland, in November 2006 caused by passenger jets crisscrossing Europe. Photograph by Forrest M. Mims III.

Atmospheric scientists still study contrails today but for a very different reason. Their concern is that persistent contrails can spread across the sky like cirrus clouds. They then block sunlight during the day and the escape of heat during the night.

Dr. David Travis of the University of Wisconsin studied an episode of spreading contrails over the Midwest. He discovered that the contrails caused a daytime cooling of from 3.6 to 7.2 degrees.

I often experienced this phenomenon during winter when we lived in Albuquerque, New Mexico. My science and writing “office” was a tiny metal storage building that stayed cold during mornings when contrails blocked the sun.

Switzerland is in the center of Europe and is cross-crossed by airplanes flying between many cities on that continent. Often there are so many contrails in the sky that they merge together and block considerable sunlight.

Some years ago while I was teaching a course on atmospheric science in Lausanne, I measured the reduction in sunlight caused by contrails and found it closely matched the reduction caused by cirrus clouds. I linked up with Dr. Travis, and we published a short scientific paper about this finding.

After the terrorist attacks of 2001 grounded commercial aircraft for several days, Travis noticed the absence of contrails in the sky. He found that the daily temperature range across the US was about two degrees wider than during the days before and after the attack. This occurred because the contrail-free skies allowed the temperature to become warmer during the day and cooler at night.

While some scientists disagree, these and other studies indicate that contrails may cause a net warming in areas under heavy jet traffic. Fortunately the effect is not global, but it can affect large areas of Europe, the Atlantic Ocean, the US and elsewhere during days when contrails spread across the sky.

Where I live in South Texas, contrails spread across the sky only occasionally and mainly during winter. An interesting exception occurred recently when Hurricane Dolly passed nearby along with thick tropical clouds and contrails.

Comments about editorials and any other content in The Citizen Scientist are welcome and should be sent to "Backscatter." Please include your name and the title and date of the article or column about which you are writing. Comments may be lightly edited for style and grammar. Please indicate if you do not want your comment published. Editor.