108 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



Ulloa's ring. — 1. A glory. 2. A halo (also called Bouguer's halo), surround- 

 iug a point in the sky diametrically opposite the sun ; sometimes described as 

 a "white rainbow." 



y/-sliaped depression. — A trough of low barometric pressure bounded, on the 

 weather map, by V-shaped isobars. 



Vane. — A device that shows which way tlie wind blows ; also called weather 

 A^ane or wind vane. 



Vapor pressure. — The pressure exerted by a vapor when it is in a con- 

 fined space. In meteorology vapor pressure refers exclusively to the pressure of 

 water vapor. When several gases or vapors are mixed together in the same 

 space each one exerts the same pressure as it would if the others were not 

 present ; the vapor pressure is that part of the total atmospheric pressure 

 which is due to water vapor. 



Variability. — Interdiurnal variability is the mean difference between succes- 

 sive daily means of a meteorological element. 



Veer. — Of the wind, to shift in a clockwise direction ; opposite of "back." In 

 scientific practice this definition now applies to both hemispheres. 



Vernier. — An auxiliary scale for estimating fractions of a scale division wlien 

 the reading to the nearest whole division on the main scale is not suflBciently 

 accurate. 



Visibility. — The transparency and illumination of the atmosphere as affecting 

 the distance at which objects can be seen. It is usually expressed on a numeri- 

 cal scale. 



Warm air mass. — Broadly spealving, an air mass that is warm relative to 

 neighboring air masses. The term implies that the air mass originated in 

 latitudes lower than those in whicli it now finds itself and that it is, therefore, 

 warmer than the surface over which it is moving. 



Wa7-m front. — The discontinuity at the forward edge of an advancing cur- 

 rent of relatively warm air which is displacing a retreating colder air mass. 



Warm sector. — The area bounded by the cold and warm fronts of a cyclone. 



Waterspout. — A tornadolike vortex and cloud occurring over a body of water. 



Wave (listiirbancc. — A localized deformation of a front, which travels along 

 the front as a wave-shaped formation, and which generally develops into a 

 well-marked cyclone. 



Wcdf/c. — 1. A wedge-shaped area of higli barometric pressure as shown on a 

 weather chart. S.vnonymous with "ridge". 2. Applied to an air mass whose 

 advancing forward portion, from a three-dimensional standpoint, is shaped 

 lilve a wedge. 



Wet bulb. — (See Psychrometer.) 



Williiratv. — A sudden blast of wind descending from a mountainous coast 

 to the sea. (Especially applied to such blasts in the Straits of Magellan.) 



Wind. — Moving air, especially a mass of air having a common direction of 

 motion. The term is generally limited to air moving horizontally, or nearly 

 so ; vertical streams of air are usually called "currents". 



Wind rose. — 1. A diagram showing the relative frequency and sometimes also 

 the average strength of the winds blowing from different directions in a speci- 

 fied region. 2. A diagram showing the average relation between winds from 

 different directions and the occurrence of other meterological phenomena. 



Zodiacal lif/ht. — A cone of faint light in the sky which is seen stretching 

 along the zodiac from the western horizon after the twilight of sunset has 

 faded and from the eastern horizon before the twilight of sunrise has begun. 



B. COLLOQUIAL TERMS 



"Backstays of the sun.'' — A sailor's name for crepuscular rays extending 

 downward from the sun. 



Bagnio. — The name current in the Philippines for a tropical cyclone. 



Bora. — A cold wind of the northern Adriatic, blowing down from the high 

 plateaus to the northward. Also, a similar wind on the northeastern coast 

 of the Black Sea. 



Brave west tvinds. — The boisterous westerly winds blowing over the ocean 

 between latitudes 40° and 50° S. This region is known as the "Roaring 

 Forties." 



Bull's-eye. — 1. A patch of clear sky at the center of a cyclonic storm ; the 

 "eye of the storm". 2. A small isolated cloud seen at the beginning of a bull's- 

 eye squall, marking the top of the otherwise invisible vortex of the storm. 



