Noctilucent cloud

Noctilucent clouds ( NLC abbreviation of Engl. Noctilucent clouds ) are collections of ice crystals above the mesosphere in the mesopause. There, the absolute minimum temperature of Earth's atmosphere is achieved. They appear in a level 81-85 mi - in contrast to the clouds forms the troposphere, maximum reach a height of 13 km. Most sightings in Central Europe there from early June to late July (ie during the months around the summer solstice) in the twilight towards the north as shining fibrous clouds and are not to be confused with the polar stratospheric clouds.

On 26 April 2007, the Earth observation satellite Aeronomy of Ice in the Mesosphere (AIM ) was started to investigate the phenomenon of luminous clouds.

Morphology

Due to their great height they can be illuminated after sunset from the slanting sun below the horizon, and thus brought to the " lights ". They become visible when the sun is between 6 ° and 16 ° is below the horizon. The sun's light is then reflected from the noctilucent clouds, while the sky is otherwise already almost dark. Your coloring depends on the position of the sun and can range from yellow to silvery nacreous.

Formation

The origin of the nuclei is not yet finally resolved. The noctilucent clouds were first described in 1885, two years after the volcanic eruption of Krakatoa and interpreted as a consequence of the eruption. To the surprise of the scientists, they were observed in the following years and decades. Even if aerosols have at times contributed from the Krakatau eruption leading to the formation of noctilucent clouds, there must be another and consequently are no permanent source of nuclei. Today the view that this is material that is released when burning out of meteors predominates. In fact, shooting stars light up typically in the exact altitude on where to stay, the noctilucent clouds. The well encountered there metal atom layers are the product verglühter meteoroids.

Due to the low density occur in pressure and temperature differences in the mesosphere at times to strong atmospheric turbulence and currents. They cause a rapid mixing of the gases that rise from the lower atmosphere and water vapor transport from the stratosphere above the mesosphere to very cold mesopause where they condense on the existing crystal nuclei (eg, dust particles).

This rapid air mass transport by convection in the presence of water typically causes a nearly adiabatic expansion of the gas mixture, thereby locally much lower temperatures (down to -140 ° C) may occur in relation to the normally prevailing in the mesopause temperature of -85 ° C. This spatially limited temperature anomalies are associated with the presence of nuclei obviously an important prerequisite for the formation of noctilucent clouds.