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<br />001481 <br /> <br />Thus l' asS1lllling an anemometer relocated from a fomer height of <br />100 feet to the standard height of 20 feet, and asSUming alsu a power <br />of 1/7 in the preceding equation, the standard 20-foot~high velocities <br />would be 0.8tillles the former 100~foot-high velocities. <br /> <br />. <br /> <br />Since the measured wind speeds strongly reflect the effects of site <br />and of the height of the instruDlent, proper exposure for the purposes in <br />mind is very important. For determining general movement of the air, the <br />exposure s;hould be as free as possible from obstructions to the flow. <br />Measuremei\ts in connection with evaporation studies are made near the <br />surface, sometimes at several heights. For special studies the inst~- <br />ments may :be mounted at various heights or at intervals on towers. <br /> <br />Consistency and reliability of records <br /> <br />Relatively few records of wind movement have been obtained over a <br />long term ,of years devoid of changes in location of the station, in the <br />surrounding environment, and in height of instruments above the land <br />surface. !.rhus, most records are likely to embody inconsistences, which <br />are diffie,tlt or impossible to identify ,and evaluate. COIIIlIIon sources of <br />inconsiste)'lcy include the following: <br /> <br />Although efforts, have been made to standardize height of <br />instr,uments, wide variations occur. The effect of variable <br />heigh~ of installation has been summarized. <br /> <br />;B.nd movement close to the land surface may be affected <br />greatly by growth of grass, crops, or b~sh. These small-scale <br />changes in environment should be considered in regard to records <br />of wind movement at an evaporation station. <br /> <br />;Larger-scale ohanges in environment must be considered in <br />regllI'j:l to all other stations. Many stations have been lo:cated, <br />in ci;ties, where continual const~ction and dest~otion of <br />buildings probably has affected the exposure of most anemometers <br />and wind vanes. The recent trend toward placing stations at <br />airports has not eased this situationj const~ction to accOIlIJllodate <br />the phenomenally rapid increase in air traffic has forced all-too- <br />frequent relocation of wind instruments. At another extreme, <br />logging or fire in a forested area can cause an ab~pt increase <br />in wind-speed measurements; then, all normal growth is reestablishi;3d, <br />the measurements revert gradually toward their former value. <br /> <br />Instr,umental errors in measurement of wind velocity and direction <br />usually ~ relatively SlIIall, excepting tho,se due to mechani,cal failure <br />or improp~ maintenance. Most types of anemometers lag due to inertia, <br />and to an;y' play in gear trains or dials--they do not respond instantly <br />to change a in wind velocity. Thus, rapid perturbations are smoothed and, <br />peaks of ahort duration may under-register substantially. The most common <br />anemometer, the cup type, registers too I1I\lch total movement during a <br />variable ~d becaUlle the cup wheel accelerates more readily than it <br />decelerates. <br /> <br /> <br />VI.3 <br />