When the sky becomes colorful on warm summer evenings, it’s mostly due to noctilucent clouds. These are clouds that are illuminated by the sun, which is located below the horizon at the end of twilight. They consist of accumulations of ice crystals from the mesosphere and are located at a height of 81 to 85 km (for comparison: "normal" clouds reach a maximum height of 13 km). Only here earth's atmosphere reaches the absolute temperature minimum.
When and where can noctilucent clouds be seen?
In Central Europe, the noctilucent clouds can be observed from the beginning of June to the end of July. The summer solstice causes the sunlight to reflect on the underside of the clouds and "color" the clouds. In the sky the clouds are mostly seen in a northerly direction.
How do night clouds form?
When the sun is between 6° and 16° below the horizon, you can see the colored clouds. Therefore, the clouds present themselves brightly shining for us as observers on the earth, although the sun has actually already set. Depending on the position of the sun, the color facets can range from yellow to silvery.
How the ice crystals are formed has not yet been finally clarified. In 1885, the noctilucent clouds were first sighted, but interpreted as the result of a volcanic eruption. Studies still exist today that show that the natural event is associated with global warming. The phenomenon of noctilucent clouds has not yet been sufficiently investigated to know which of the theoretical approaches is correct.