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<br /> <br />naB 79 <br /> <br />VI. WIND <br /> <br />Measurements and types of equipment <br /> <br />Surface insttuments <br /> <br />The speed, direction and gustiness of the wind near the land surface <br />are measured by anemometers and wind vanes installed on poles or towers at <br />or near the site of the insttuments used to measure other meteorological <br />elements s\ICh as precipitation, temperature, ahd humidity. The height of <br />the installation is determined by the purpose of the measurement. For <br />example, winds near the surface are required in connection with evaporation <br />studies but for aviation or climatological purposes the height of installa- <br />tion is usually a few tens of feet. <br /> <br />The instruments cOllUllonly used for measuring wind speed fall into three <br />types, as follows: <br /> <br />Rotational~- <br /> <br />vertical-axis rotating cups, <br />horizontal-axis windmill <br />or propeller. <br /> <br />Pressure-- <br /> <br />pitot-static, Venturi-static, <br />pressure plate, or bridled cups. <br /> <br />Heat exchange-- <br /> <br />hot wire or thermistor, kata- <br />thermometer. <br /> <br />The majority of installations are of the rotating-cup type. The <br />propeller type usually is for portable or hand-held use. Pitot and Venturi <br />tubes measure pressure gradient of the air flow. The ordinary pressure- <br />plate type. hangs on a horizontal axis perpendicular to the wind and is swung <br />from the vertical by the wind's force. The normal-plate type remains pe:('pen- <br />dicular to the air flow and its deflection measures the pressure of the wind <br />against it, The bridled-cup type is non-rotating and measures the torque <br />resulting from the wind force. Hot-wire types measure either the variations <br />in current required to maintain the wire temperature or the variations in <br />resistance of the wire as its temperature is cooled by" the air nOl{. <br />Kata-thermometer anemometers measure the COOling power of the air flow <br />ventilating the bulb of a thermometer; wind speed is determined from the <br />time required to depress the temperature from 400 c to either 380 C or <br />350 c. <br /> <br />The gustiness, or short-period fluctuations of wind speed may be <br />measured bY any of the anemometer types. Some are more suitable than <br />others depending on the scale of the variations it is desired to measure. <br /> <br />VI.I <br />