AIRLINER CONTRAILS, the thin line-shaped ice clouds formed from water vapor in exhaust gases, account for about 0.1% of the worldwide cloud cover, and as much as 0.5-2% over parts of Europe and the Eastern Northern Atlantic. A group of atmospheric scientists (Patrick Minnis, NASA Langley Research Center, p.minnis@larc.nasa.gov) have made of a study of these clouds in order to forecast their possible future radiative forcing effect, that is, the amount by which contrails would enhance (through greenhouse action) the solar and earth-emitted infrared radiation retained on Earth. The conclusion: between 1992 and 2050 contrail cloud cover will increase by a factor of 6. During this period the contrail fraction of anthropogenic radiative forcing may increase from about 1% (1992) to 2 or 3%. These are global estimates; regional averages (such as for the northern temperate zone) will be greater still. ( Minnis et al., Geophysical Research Letters, 1 July 1999.)