_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. _Qobar._--A dry fog or heat haze of the upper Nile region. (Also spelled _kobar_ and _gobar_ and occasionally applied to a hazy condition of the atmosphere in other parts of the world.) _Rain Balls._--Mammato-cumulus. _Rainbow._--A luminous arc formed by the refraction and reflection of light in drops of water. (See Chapter X.) _Rainfall._--A term sometimes synonymous with _rain_, but most frequently used in reference to amounts of precipitation (including snow, hail, etc.). _Rain Gauge._--An instrument for measuring rainfall. _Rain Tree._--A mythical tree which is alleged to exude copious showers of water even in the driest weather. (“Rain tree” is also the common name of an ornamental tree variously known to botanists as _Albizzia saman_, _Pithecolobium saman_, _Enterolobium saman_, and _Samanea saman_.) _Recoloration._--(See _afterglow_.) _Recurrence._--The alleged tendency of any particular type of weather to occur at about the same period every year, independently of and generally in contrast to the regular march of the seasons. _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 Wind._--In aeronautics, the motion of the air with reference to an aeroplane or airship moving through it. _Réseau._--A collection of meteorological stations operating under a common direction, or in the same territory. An _international réseau_ is a group of stations in different countries cooperating for any purpose. The _Réseau mondial_ is a world-wide system of selected stations, the observations of which may be utilized in studies of the meteorology of the globe. _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.) _Roaring Forties._--(See _brave west winds_.) _Saddle._--(See _col_.) _Saint Elmo’s Fire._--A luminous brush discharge of electricity from elevated objects, such as the masts and yardarms of ships, lightning rods, steeples, etc., occurring in stormy weather. Also called corposant. _St. Martin’s Summer._--One of several names given in Europe to a mild period in autumn, corresponding approximately to the Indian summer of North America. _Scarf Cloud._--A thin cirruslike cloud which often drapes the summits of tall cumulo-nimbus clouds.