_Mountain and Valley Breezes._--The breezes that, in mountainous regions, normally blow up the slopes by day (_valley breeze_) and down the slopes by night (_mountain breeze_). _Nephoscope._--An instrument for measuring the movements of clouds. _Nieve Penitente._--Fields of pinnacled snow found on certain high mountains in tropical or subtropical regions. _Nimbus._--The rain cloud. (See Chapter VI.) _Noctilucent Clouds._--Luminous, cirruslike clouds sometimes visible throughout the short nights of summer; supposed to be clouds of dust at great altitudes shining with reflected sunlight. Such clouds were observed during several summers after the eruption of Krakatoa (1883) and are still occasionally reported. _Normal._--The average value which, in the course of years, any meteorological element is found to have on a specified date or during a specified month or other portion of the year, or during the year as a whole. Also used as an adjective in such expressions as “normal temperature,” etc. Thus, for any station at which records have been maintained for many years, we may compute the normal temperature of January 1, the normal pressure of February, the normal rainfall of the year, etc. The normal serves as a standard with which values occurring in a particular year may be compared in order to determine the _departure from the normal_. _Nucleus._--A particle upon which condensation of water vapor occurs in the free atmosphere in the form of a water drop or an ice crystal. _Oblique Arcs of the Anthelion._--A rare form of halo, consisting of intersecting arcs, usually white, passing through the anthelion or the place where the anthelion would occur if visible. _Ozone._--An allotropic form of oxygen, which occurs transiently in small quantities in the lower atmosphere, and is supposed to be permanently present and relatively abundant at high atmospheric levels. _Painter._--A dirty fog frequently experienced on the coast of Peru. The brownish deposit from it is sometimes called “Peruvian paint.” _Paragrêle._--A hail rod. _Paranthelion._--A halo phenomenon similar to a parhelion, but occurring at a distance of 90 degrees or more in azimuth from the sun. The solar distance of the ordinary paranthelia is 120 degrees. (Analogous phenomena produced by the moon as source of light are called _parantiselenæ_.) _Paraselene._--(Plu. _paraselenæ_.) (See _parhelion_.) _Paraselenic Circle._--(See _parhelic circle_.) _Parasitic Clouds._--The name formerly given to clouds capping the summits of mountains. _Parhelic Circle._--A halo consisting of a white circle passing through the sun and parallel to the horizon. A similar phenomenon in connection with the moon is called a _paraselenic circle_.