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<br /> <br />-22- <br /> <br />The instrument used to measure the direction of the wind is the <br />wind vane. Usually it has a relatively long tail and is mounted on a <br />vertical axis. The lateral forces of, the wind on the tail cause the vane <br />to align itself with the horizontal component of the flow. Small mass <br />relative to tail area and a bifurcat~d, spread or split shape are incor- <br />porated into vanes designed for sensitive response to small changes in <br />,J direction, especially with light winds. Omni-directional wind vanes-- <br />/ that is, those mounted in gimbals so that they may point in any direction-- <br />afford three-dimensional measurements of eddy patterns. By convention, <br />wind direction is defined as the direction from which the wind blows. <br /> <br />Most anemometers represent a compromise between sensitivity and the <br />weight and sturdiness 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 <br />anemometer. <br /> <br />Upper-air instruments <br /> <br />Speed and direction of winds aloft are measured by observing the <br />motion of some visible object being pushed along by the force of the <br />wind. B.Y far the most common object is a small lighter-than-air, balloon <br />that is released at regular observation times, and observed by single or <br />double theodolites or by radar. Other objects include a puff of smoke, <br />a visible meteor trail, chaff thrown from an airplane, or a cloud. <br /> <br />Location and installation of measuring equipment <br /> <br />As observed, most wind speeds and directions are not instantaneous <br />values but average values over a short interval of time. Wind speed <br />varies substantially with height above the land surface and this varia- <br />~ion is a complex and not very well understood function of the roughness <br />of the land surface and the "stability" of the air, as well as of the <br />speed itself. <br /> <br />Recently, the Weather Bureau has relocated the wind instruments of <br />all its airport stations at a uniform height of 20 feet above the land <br />surface. The relation of velocities measured at this standard height <br />to those at other heights is expressed approximately by the following <br />e~pirical formula. <br /> <br />V20 <br />\ <br /> <br />! 20, ')\~ <br />~ \-;;- <br /> <br />(1) <br /> <br />in which <br /> <br />V20 <br /> <br />Vh <br />l/n <br /> <br />= velocity at height 20 feet <br /> <br />= velocity at height h (between 20 and 1,000 feet) <br /> <br />= a power ranging from 1/1 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 />