Friday, 10 June 2011
Noctilucent Cloud Season has started!
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, Skåne, Southern Sweden.
The formation of NLC is related to that of PMSE, which we discussed last week.
Photo: Thomas Ulich.