Noctilucent clouds occur at great heights of between 75 and 90 kilometers. They can be seen at twilight during the summer from between 50 or 65 degrees north or south. The clouds are very tenuous, have a bluish or silverish colour and consist of water ice of meteoric origin. In the northern hemisphere, the appearance of noctilucent clouds reaches its maximum in late June and early July. An increase of noctilucent clouds was observed the 1-st July 1908 and the days following the Tunguska event (see e.g. the paper, 18.895 Mb).