Noctilucent clouds are the highest clouds in Earth's atmosphere, forming at the very edge of space.

Noctilucent clouds (NLCs) reside in the mesosphere, the coldest layer of Earth's atmosphere, where temperatures can plummet to -150 degrees Celsius (-238 degrees Fahrenheit). These clouds were first recorded in 1885 following the massive eruption of Mount Krakatoa, which released immense amounts of water vapor into the upper atmosphere. Unlike common clouds that form from water vapor clinging to terrestrial dust, NLCs form when water vapor freezes onto 'meteor smoke'—tiny particles left behind by disintegrating space debris.NASA's Aeronomy of Ice in the Mesosphere (AIM) satellite mission, launched in 2007, has provided extensive data on these phenomena. The AIM mission confirmed that NLCs are extremely sensitive to changes in atmospheric temperature and water vapor content, making them a 'canary in the coal mine' for climate change. Research indicates that as methane levels increase in the lower atmosphere, more water vapor is transported to the mesosphere, causing these clouds to appear more frequently and at lower latitudes than ever before.The distinctive electric-blue color of these clouds is due to the way the small ice crystals scatter sunlight, a process known as Rayleigh scattering. Because they are located so high up, they remain illuminated by the sun's rays even when the ground below is in total darkness. This geometric alignment occurs when the sun is between 6 and 16 degrees below the horizon, creating a spectacular glowing effect against the night sky.