| This would permit the study of so-called noctilucent clouds, the highest in the atmosphere, which are thought to have an important role in climate change. |
| Evidence of these changes comes from sightings of noctilucent clouds, very unusual clouds seen only in polar regions and known to be in the mesosphere. |
| White Knight Two and SpaceShipTwo, says Dr Stern, could fly more experiments to noctilucent clouds in six months than NASA has managed in 40 years. |
| Based on historical records of significant noctilucent cloud development in the skies over Europe following the event, some scientists contend that a comet caused the explosion. |
| At these times the clouds are visible and are therefore referred to as noctilucent clouds. |
| May's black ash from the Eyjafjallajökull eruption in Iceland might just have a silver lining... Look north at twilight this month, and you may find yourself seeing one of the sky's most ghostly phenomena: noctilucent clouds. |
