Noctilucent CLouds
© FLPAMysterious 'night shining' clouds have a solar controller.
The comings and goings of noctilucent or "night shining" clouds in the extreme upper atmosphere may be linked to the sun's rotation.

Noctilucent clouds appear about 80 kilometres above the Earth in each hemisphere's summer. Their extent and brightness varies over days, weeks and years, but no one knows why.

Now Charles Robert of the University of Bremen, Germany, and colleagues think they have an answer. By measuring changes in the light reflected from the clouds, they found that the clouds appear to wax and wane in prevalence over a 27-day cycle. As the sun takes 27 days to rotate around its axis, the team suggest a link (Journal of Geophysical Research, DOI: 10.1029/2009jd012359).

The researchers suspect that the amount of ultraviolet light Earth receives from the sun explains the link. The sun's emissions of UV light are not uniform, so as it rotates UV-bright regions move in and out of view of Earth. Increased UV light may break down water molecules in the upper atmosphere, reducing cloud formation, says James Russell of Hampton University in Virginia, who was not involved in the study.

However, the sun's rotation does not explain all the variations seen, says Russell. An observed long-term increase in the number of these clouds may be due in part to the rise in atmospheric carbon dioxide levels, he says.