Noctilucent clouds are very high altitude clouds that occur 82km up in the atmosphere in summer 1-2 hours after sunset or before sunrise. 'Noctilucent' means 'night shining'. They are becoming more common, brighter and visible from increasingly low latitudes. It is likely that they are linked to climate change. Increased carbon dioxide levels actually leads to cooling at this altitude. Meteoritic dust might serve as seed particles on which ice forms, leading to the appearance of the noctilucent clouds. They are more common at times of minimum solar activity when heating from flares is reduced.