Report and Recommendations of IAU
 (International Astronomical Union) Commission 50 (Identification and
 Protection
   of Existing and Potential Observing Sites), prepared by F.G. Smith,
   UNESCO Subvention -1977
     - DG/2.1/414/44:
     <dd><dd>-Interference from Aircrafts (report excerpt): Condensation
     trails from aircrafts and the cloud
     build-up which often results from them, have very serious effects
     on atmospheric transparency.
     Aircraft lights may interfere directly with astronomical
     observations. It is essential that
     regular air routes, and where possible individual aircraft, should
     avoid the sky visible from
     observatory sites, since exhaust products and heat may affect
     conditions for astronomical
     observations.
     <dd><dd>-Recommendation: <em> Aircrafts should not be allowed in
     the sky more than 10 degrees
     above the horizon, as viewed from the observatory site, nor within
     5 km horizontal distance
     from the site </em> .