Noctulucent Clouds - Relatively unusual wavy, thin, bluish-white clouds that form at altitudes of about 80 to 90 km.
[Air: the nature of atmosphere and the climate; Micheal Allaby; p73; 1992; Curtis Garrat Limited; New York.]
[Cloud investigation by satellite; Richard Scorer; p621; 1986; Ellis Horwood Limited; England.] 
Atmospheric Chemistry Glossary