Noctilucent clouds are tenuous cloud-like phenomena in the upper atmosphere. They are made of ice crystals and are only visible in a deep twilight. Noctilucent roughly means night shining in Latin. They are most commonly observed in the summer months at latitudes between 50° and 70° north and south of the equator. These clouds can be observed only during local summer months and when the Sun is below the horizon for the observer, but while the clouds are still in sunlight. They are the highest clouds in Earth's atmosphere, located in the mesosphere at altitudes of around 76 to 85 kilometres (47 to 53 mi). Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon; there is no confirmed record of their observation before 1885, although they may have been observed a few decades earlier by Thomas Romney Robinson in Armagh. Noctilucent clouds can form only under very restricted conditions during local summer; The occurrence of noctilucent clouds appears to be increasing in frequency, brightness and extent.