_Visibility._--The transparency and illumination of the atmosphere as affecting the distance at which objects can be seen. It is often expressed on a numerical scale. (This term was formerly applied by British meteorologists to a state of unusual clearness of the atmosphere, regarded as a weather prognostic.) _V-shaped Depression._--A trough of low barometric pressure bounded on the weather map by V-shaped isobars. _Waterspout._--A tornadolike vortex and cloud occurring over a body of water. _Weather Well._--(See _blowing well_.) _Wedge._--A wedge-shaped area of high barometric pressure. _Wet-bulb thermometer._--(See _psychrometer_.) _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 Aloft._--The wind blowing at high levels as distinguished from that near the earth’s surface. _Wind Rose._--1. A diagram showing the relative frequency and sometimes also the average strength of the winds blowing from different directions in a specified region. 2. A diagram showing the average relation between winds from different directions and the occurrence of other meteorological phenomena. _Winter Day._--A day on which the temperature does not at any time rise above the freezing point. _Wool-pack Cloud._--Cumulus. Transcribers’ Notes Punctuation, hyphenation, and spelling were made consistent when a predominant preference was found in this book; otherwise they were not changed. Simple typographical errors were corrected; occasional unbalanced quotation marks retained. Ambiguous hyphens at the ends of lines were retained. In versions of this book that do not support accented letters, “Canon” (with a tilde over the middle “n”) is spelled “Canyon”. Page 103: For consistency with the preceding enumerated definitions, the Transcriber added extra spacing after the last of them, just before “We have given”. Page 348: In some versions of this eBook, the chemical symbols for methane and phosphine are represented as CH4 and PH3. Page 367: “The scale runs from 0 = calm to = hurricane” is missing the value for hurricane, which is “12”. End of the Project Gutenberg EBook of Meteorology, by Charles Fitzhugh Talman