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<br />001480 <br /> <br />The instrument used to measure the direction of the wind is the wind <br />vane. Usually it has a relatively long tail and is mounted on Il. verticlll. <br />axis. The lateral forces of the wind on the tail cause the vane to align <br />itself with the horizontal component of the flow. Small mass relative to <br />tail area and a bifurcated, spread or split shape are incorporated into <br />vanes designed for sensitive response to small changes in direction, <br />especially with light winds. Omni"'directional wind vanes--that is, those <br />mounted in gimbals so that they may point in any direction--afford three- <br />dimensional measurements of eddy patterns. By convention, wind direction <br />is defined as the direction ~ which the wind blows. <br /> <br />Most anemometers represent a compromise between sensitivity and the <br />weight and. B~diness suitable for all ranges of wind speed from that of <br />a mere puff to that of a hurricane. No single type is ideally suitable <br />for the entire range, yet few stations have more than one type of anemometer. <br /> <br />Upper-air instruments <br /> <br />The wind speed and direction aloft are measured by observing the <br />motion of some visible object being pushed along by the force of the <br />wind. By far the most common object is a small lighter-than-air blll.loon <br />that is re~eased at regular observation times, and observed by single Or <br />double thepdolites or by" radar. Other objects inClude a puff of SJl(oke, a <br />visible meteor trail, chaff thrown from an airplane, or a cloud. <br /> <br />. Location and installation of measuring equipment <br /> <br />As ob$erved, most wind speeds and directions are not instantaneous <br />values but average values over a short interval of time. Wind speed varies <br />substantially with height above the land surface and this variation is a <br />complex and not very well understood function of the roughness of the surface <br />and the stability structure of the air, as well as of the speed i tsel:f. <br /> <br />The Weather Bureau has started to relocate the wind instruments of all <br />its airport stations at a uniform height of 20 feet above the land surfaee. <br />The relation of velocities measured at this standard height to those at <br />other heights is expressed approximately by the following empirical fOI'l\lUla. <br /> <br />. <br /> <br />\{20 =0 2~ \1 <br />)h <br />Yh i <br />./ <br />velocity at height 20 feet <br /> <br />in which: \/20 = <br /> <br />\Jh = velocity at height h <br /> <br />l/n = a power ranging from 1/7 for a strong wind under <br />a dry-adiabatic condition, through 1/3 for an <br />average wind and intermediate stability, to <br />1/2 for a light wind and a stable condition. <br /> <br />,j.' <br /> <br />