Noctilucent clouds

Noctilucent clouds (NLC) are thin clouds of a silvery white which in some summer nights can be seen in northerly directions near the horizon. Contrary to other types of clouds which maximally reach altitudes of about 13 km, noctilucent clouds appear at an altitude of about 83 km They are only visible when the sun is between 6� and 16� below the horizon Then the noctilucent clouds are still lit by the sun while the sky is already dark. Noctilucent clouds only form when the temperature of the mesopause is very low. These low temperatures normally occur between mid May and mid August. Especially in June and July noctilucent clouds can be observed in some nights. In our region, they normally reach an elevation of about 20� above the northwesterly to the northeasterly horizon. In special cases they can be observed near the zenith even in our regions.

Obviously noctilucent clouds consist of frozen water. The formation of ice from the very low concentration of vapour present at an altitude of 83 km requires very low temperatures of less than 140K. Furthermore dust particles are required as condensation nuclei or so-called water cluster ions form because of the water molecules being dipoles.

From measurements taken we know that the temperatures necessary for the formation of ice are reached only between June and August due to the inter-hemispheric circulation In summer there are also stronger winds transporting the ice particles over longer distances The ice particles are supposed to exist for some hours until they sublimate again due to sinking down to warmer regions of air or being transferred to more southerly regions. This is also a hint to possible rapid changes inside the NLC.

Long-term influences are much more difficult to be followed after. A connection to solar activity is obvious because changes in ultraviolet radiation have an influence on chemical reactions and also the temperature is subject 1o systematic changes. But there is no clear evidence for noctilucent clouds to be really more frequent during the time of solar maximum.

An increase in the concentrations of methane or carbon dioxide might also be responsible for a higher NLC activity as temperatures in the mesopause could more often drop sufficiently for the formation of noctilucent clouds.

Another connection is presumed between NLC and Polar Mesospheric Clouds (PMC). which are lying over the poles during the whole summer. If we presume that PMC move to the south. The NLC might be frayed ends of the PMC cover. Long-term trends, however, can only be derived from observations made over a period of several years or even decades. For this reason we call observers to participate in this programme.

An observation report should contain the following facts

Shapes distinguishing four basic types with subtypes and categories for more complex structures:

Complex structures

Additionally you should take notice of the conditions during your observation, especially "normal" clouds and/or mist. Sure reports on negative observations are also important!

Photographs are of special importance. Any miniature camera with a ~ will do very well. A film of medium speed (ISO 100 or 200) and a good-quality lens (about f/2) require exposures between 2 and 10 seconds according to the brightness of the NLC and of dusk or dawn. When the NLC are very bright, even the exposure meter will react. The best way is making a short series with different exposures (e.g. 15s, 8s, 4s, 2s, is) When there are complex types of NLC, it is worth while to make a series every 5 minutes; otherwise a series every 15 minutes will do. Detailed description recording of all exposures is very important in any case to be able to determine direction and speed of the movement of the NLC later.