attention from Abbe and others, but as yet we do not know enough about them to give them a name; sometimes they are called nacreous, from their gentle, pearly luster, at other times noctilucent, because they shine at night; they have as yet only been observed in northern latitudes during the month of June and above the northern horizon. The most reasonable assumption is that they are so high up as to receive a little light from the twilight then prevailing over the Arctic regions. Otherwise they may be self-luminous or phosphorescent.
A few measurements have given these clouds altitudes of between 20 and 40 miles. But we believe that a more systematic search for such clouds and a more careful determination of their altitudes must be made before we can attempt to discuss the questions that will naturally arise in regard to them, for of course at present no one can understand how aqueous vapor can be carried up to those altitudes or exist there long enough to form a visible cloud. The eruption of Krakatoa did throw vapor particles up to middle altitudes, and these formed interesting optical phenomena, but not visible clouds. However, if such clouds do exist, and their motions can be determined, we shall hardly dare to assume that the motions represent any movement of the atmosphere as such, for the air is so rarefied that the cloud particles may be moving without reference to the wind.
CLAYDEN ARTHUR W., "Cloud Studies," London, 1905.
CLAYTON H. HELM, "Observations Made at the Blue Hill Meteorological Observatory. Discussion of the Cloud Observations," Cambridge, Mass., 1896. ( Annals of the Astronomical Observatory of Harvard College, Vol. xxx, Pt. IV.) Includes an extensive history of cloud nomenclature.
INTERNATIONAL METEOROLOGICAL COMMITTEE, "International Cloud Atlas," Paris, 1896. In French, English, and German. Colored plates. This is the official international classification of clouds, adopted by all countries; but several more detailed classifications, based upon this, have been proposed by cloud specialists.
VINCENT J., "Atlas des nuages," Bruxelles, 1907. ( Annales de l'Observatoire royal de Belgique, Nouvelle série, Annales météorologiques.)
Plates illustrating the International Cloud Classification are published by the U. S. Weather Bureau.