Noctilucent clouds are bright cloudlike atmospheric phenomena visible in a deep twilight. The name means roughly night shining in Latin. They are known as polar mesospheric clouds when observed from a satellite in space. They are most commonly observed in the summer months at latitudes between 50° and 70° north and south of the equator.

They are the highest clouds in the Earth's atmosphere, located in the mesosphere at altitudes of around 75 to 85 kilometers (47 to 53 mi). They are normally too faint to be seen, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in the Earth's shadow. Noctilucent clouds are not fully understood as meteorological phenomenon. Clouds generally are not able to reach such high altitudes with such thin air pressures.

-Wikipedia