Noctilucent Clouds (IMAGE) Optica Caption These are tenuous noctilucent clouds, which form high above the Earth and only under very specific atmospheric conditions, as seen at twilight from Salakas, Lithuania. Accompanying the issue is an online photo gallery. Credit Image courtesy of Audrius Dubietis/Applied Optics Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.