Friday, 10 June 2011

Noctilucent Cloud Season has started!

It's Friday, and time for an interesting picture. This time the photo is of Noctilucent Clouds, which appear in the mesopause region. The polar summer mesopause is the coldest part of the Earth's atmosphere with temperatures around 130 K at about 88 km altitude. Here, ice particles are formed, which then sediment downwards and grow in the process. By the time they reach about 85 km altitude, they have grown to sizes of around 20 nm and clouds of such particles can be seen by naked eye in suitable conditions.

While the noctilucent clouds, or NLC as they are commonly known, appear only in polar regions, they can be observed there only in the very beginning and end of the polar day. However, south of the Arctic Circle, the clouds are visible at night, when they are illuminated from below by the setting sun, and this sunlight is reflected back to the observer.

These clouds are very thin, and almost always they exhibit a lot of structure, looking almost like the surface of the sea with waves running across. Indeed, these structures are caused by so-called gravity waves, which are created in the lower atmosphere and which then travel upwards into the upper atmosphere. Gravity waves are actively studied, because they are a key player of the energy transfer across atmospheric layers.

The photo above was taken on 27th June 2008 at N 56°21.7', E 13°28.5', i.e. Fagerhult, Sk√•ne, Southern Sweden.

The formation of NLC is related to that of PMSE, which we discussed last week.

Photo: Thomas Ulich.

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