Sightings of these clouds are confined to the summer months and may be anticipated perhaps 3 or 4 times in an average year in Shetland. They are normally a northern phenomenon, generally being confined to latitudes north of about 50 degrees - though they are increasingly being seen further south. Sightings as brilliant as these are rare and they were a truly beautiful sight for anyone fortunate to see them. "Noctilucent" means "luminous at night" and they are only visible when lit from below after sunset, with the ground and lower atmosphere in darkness.