Noctilucent Cloud

Noctilucent clouds are tenuous, cloud-like phenomena in the upper atmosphere of Earth, made of ice crystals up to 100 nanometres across, among the smallest ice particles found in the atmosphere. They form at roughly 76 to 85 kilometres altitude, higher than any other cloud type, mostly in polar regions between about 50 and 70 degrees latitude, and need water vapor, dust particles and temperatures below about minus 120 degrees Celsius to form; counterintuitively this coldest mesospheric air occurs in summer because of seasonal wind patterns. They stay invisible in daylight and are only seen during astronomical twilight near sunrise or sunset, when an observer on the ground is already in the shadow of Earth but the clouds themselves, much higher up, are still lit by the sun.