Contrails (condensation trails behind jet aircraft) are becoming a menace to observational astronomy. They drift through the telescope's field-of-view, and cause unexpected dips in the brightness of the star (or galaxy, or ..). This can lead to gross errors in the interpretation, unless the true cause of the dip is discovered.

With this concern in mind, I gave a talk the symposium "Preserving the Astronomical Sky", in Vienna, July 12-16, 1999 (meeting co-sponsored between IAU and the United Nations). This was well received, and the written version will appear in the conference proceedings.

Some of the sources, from which I composed my talk can be seen here .

Upon second thought, it appears that contrails are a problem, not only for professional astronomy, but also for the amateur astronomer, and for the general public. Contrails present an eye-sore, but worse, perhaps, is the fact that they often stay visible for many hours, during which they degrade into large cloud systems, which are almost impossible to tell from natural clouds. Contrails, hence, may influence the general cloudiness of the Earth, thereby its energy balance. Night-time clouds are most important: they act to retain the outgoing thermal radiation, whereby they heat the Earth, -first locally, then globally. This is the vicious circle called the 'greenhouse effect'. The time scale for this may be shorter than comfortable, perhaps a few decades.

Holger Pedersen

Guest researcher, Copenhagen University Observatory

Last revision: March 3, 2000