INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS IQS 



is an ordinary mercurial thermometer. The latter has its bulb covered with 

 muslin or other fabric, which is either permanently wet or is wetted before use. 

 In some psychrometers there is only one thermometer, readings being taken both 

 before and after moistening the bulb. In the aspiration psychrometer the air 

 is drawn past the bulb by a revolving fan. 



Pumping. — Unsteadiness of the mercury in the barometer caused by fluctua- 

 tions of the air pressure produced by a gusty wind, or due to the oscillation 

 of a ship. 



Purple light. — The purple or rosy glow observed over a large area of the 

 western sky after sunset and the eastern sky before sunrise; it lies above the 

 bright segment that borders the horizon. 



Pyrheliometer. — An instrument that measures solar radiation by its heating 

 effects. 



Radiation fog. — Fog characteristically resulting from the radiational cooling 

 of air near the surface of the ground on calm clear nights. 



Rain. — Drops of water falling from the sky. (See also part V.) 



Rainboio. — A luminous arc formed by the refraction and reflection of light 

 in drops of water. (See part VI.) 



Rainfall. — A term sometimes synonymous with rain, but most frequently used 

 in reference to amounts of precipitation (including snow, hail, etc.) 



Rain, gage. — An instrument for measuring rainfall. 



Reduction. — As applied to meteorological observations, generally means the 

 substitution for the values directly observed of others which are computed 

 therefrom, and which place the results upon a comparable basis. 



Refraction. — Astronomical refraction, change in the apparent position of a 

 heavenly body due to atmospheric refraction ; terrestrial refraction, change in 

 the apparent position of distant terrestrial objects due to the same cause. 



Relative humidity. — (See Humidity.) 



Representative observations. — Those which give the true or typical meteoro- 

 logical conditions prevailing in an air mass ; hence they must be relatively 

 uninfluenced by local conditions. 



Rcscau. — A collection of meteorological stations operating under a common 

 direction or in the same territory. An international reseau is a group of sta- 

 tions in different countries cooperating for any purpose. The reseau mondial is 

 a world-wide system of selected stations, the observations of which may be 

 utilized in studies of the metorology of the globe. 



Ridge. — A relatively narrow extension of an anticyclone or high-pressure area 

 as shown on a weather chart. 



Rime. — 1. Hoarfrost. 2. A rough or feathery coating of ice deposited on 

 terrestrial objects by fog. (The second meaning is the one now used in 

 technical literature). 



Saddle.—- (8ee Col.) 



St. Elmo's fire. — A luminous brush discharge of electricity from elevated 

 objects, such as the masts and yardaniis of ships, lightning rods, steeples, etc., 

 occurring in stormy weather. Also called corposant. 



Saturated adiahatic lapse rate. — A rate of decrease of temperature with 

 height equal to the rate at which an ascending body of saturated air will cool 

 during adiabatic expansion. The value of the latter, unlike the case for un- 

 saturated air, is not the same under all conditions. However, under no 

 circumstances is it greater than the dry adiabatic rate. It varies inversely 

 with the temperature. 



Saturation. — The condition that exists in the atmosphere when the partial 

 pressure exerted by the water vapor present is equal to the maximum vapor 

 pressure possible at the prevailing temperature. 



Scarf cloud. — A thin cirruslike cloud which often drapes the summits of tal" 

 ctimulonimbus clouds. 



*SV'« breezes. — (See Land and sea breezes.) 



Secondary. — A small area of low pressure on the border of a large or "pri 

 mary" one. The secondary may develop into a vigorous cyclone while the 

 primary center disappears. 



Semicircle. — The "dangerous semicircle" of a cyclonic storm at sea is the half 

 of the storm area in which rotary and progressive motions of the storm rein- 

 force each other, and the winds are also directed in such a way as to drive 

 a vessel running before the wind across the storm track ahead of the advancing 

 center. The other half is called the "navigable" semicircle. 



