Photos

Noctilucent CLouds [NLC] - Hanstholm [Denmark]
Noctilucent Clouds are extremely high altitude seasonal clouds over the polar regions of Earth. They are very thin, like Cirrus Clouds, but are so faint that they are never visible in the day. They are called "Noctilucent" because they shine a bluish-white while it is dark with stars visible above. They are the highest clouds in the Earth's atmosphere, located in the mesosphere at altitudes of around 85 km, and are visible only when illuminated by sunlight from below the horizon while the ground and lower layers of the atmosphere are in the Earth's shadow. Noctilucent clouds are not fully understood meteorological concepts. Clouds generally are not able to reach such high altitudes, especially under such thin air pressures.