Noctilucent clouds, or night shining clouds, are tenuous cloud-like phenomena in the upper atmosphere of Earth. They consist of ice crystals and from the ground are only visible during astronomical twilight. They are most often observed during the summer months from latitudes between ±50° and ±70°.
Too faint to be seen in daylight, they are visible only when the observer and the lower layers of the atmosphere are in Earth’s shadow, but while these very high clouds are still in sunlight. Recent studies suggest that increased atmospheric methane emissions produce additional water vapor once the methane molecules reach the mesosphere – creating, or reinforcing existing noctilucent clouds.
They are the highest clouds in Earth’s atmosphere, located in the mesosphere at altitudes of around 76 to 85 km (249,000 to 279,000 ft).