INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS JQS 



Isotherm. — A line on a chart or diagram drawn tlarongh places or points hav- 

 ing equal temperatures. 



Isotlierinal layer. — (See Stratosphere.) 



Land and sea hreezes. — The breezes that, on certain coasts and under cer- 

 tain conditions, blow from the land by night and from the water by day. 



Lapse rate. — The rate of decrease of temperature in the atmosphere with 

 height. 



Lenticular cloud. — A cloud having approximately the form of a double- 

 convex lens. Clouds of this sort may be formed at the crests of standing waves 

 in the atmosphere such as are often induced by mountain ranges ; usually 

 they represent a transitional stage in the development or disintegration of one 

 of the more well-known cloud types. 



Lightning. — A disruptive electrical discharge in the atmosphere, or, gen- 

 erally, the luminous phenomena attending such a discharge. 



Light pillar. — A form of halo, consisting of a column of light, vertical or 

 nearly so, extending from or through the sun or moon. Called a sun pillar, 

 or a moon pillar, as tlie case may be. 



Line squall. — A more or less continuous line of squalls and thunderstorms 

 marking the position of an advancing cojd front. 



Looming. — An apparent elevation of distant objects by mirage. 



Low. — An area of low barometric pressure, with its attendant system of 

 winds. Also called a barometric depression or cyclone. 



Mammatocumuliis. — A form of cloud showing pendulous sack-like protu- 

 berances. 



March. — The variation of any meteorological element in the course of a day, 

 year, or other interval of time ; e. g., the diurnal march of temperature ; the 

 annual march of barometric pressure. 



Marine climate. — A type of climate characteristic of the ocean and oceanic 

 islands. Its most prominent feature is equability of temperature. 



Maximum. — The highest value of any element occurring during a given period. 



Meniscus. — The curved upper surface of a liquid in a tube. 



Meteorograph. — A self-registering apparatus which records simultaneously 

 the values of two or more meteorological elements. Certain types of meteoi^o- 

 graph are connected, electrically or otherwise, with some of the standard in- 

 struments at meteorological stations. These record conditions at the earth's 

 surface only. Other types are carried aloft by airplanes and free balloons. 



Meteorology. — The science of the atmosphere. 



Microbarograph. — An instrument designed for recording small and rapid 

 variations of atmospheric pressure. 



Miimar.— iiiee Bar.) 



Minimum. — The lowest value of any element occurring during a given 

 period. 



Mirage. — An apparent displacement or distortion of observed objects by 

 abnormal atmospheric refraction. Sometimes the images of objects are in- 

 verted, magnified, multiplied, raised, or brought nearer to the eye than the 

 object. Refraction layers in the atmosphere often assume the appearance 

 of fog. (See Refraction Phenomena, pages SO to 82.) 



Mist. — A very thin fog, in which the horizontal visibility is greater than I 

 kilometer, or approximately 1,100 yards. (This is the definition laid dowis 

 by the International Meteorological Organization.) In North America the word 

 is often used synonymously with drizzle or fine rain. (See also part V.) 



Mock fog. — A simulation of true fog by atmospheric refraction. 



Mock sun. — (See Parhelion.) 



Monsoon. — A wdnd that reverses its direction with the season, blowing more 

 or less steadily from the interior of a continent toward the sea in winter, 

 and in the opposite direction in the summer. 



Nephoscope. — An instrument for measuring the movement of clouds. 



Neutral point. — The term applied in a special sense to any point at which 

 the axis of a wedge of high pressure intersects the axis of a trough of low 

 pressure. Also called "saddle point." 



Nimbostratus. — A form of cloud. (See part V.) 



Noct'ilucent clouds. — Luminous, cirrus-like clouds sometimes visible through- 

 out the short nights of summer ; supposed to be clouds of dust at great alti- 

 tudes shining wdth refiected sunlight. Such clouds were observed during 

 several summers after the eruption of Krakatoa (1883), and are still occa- 

 sionally reported. 



