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Although efforts h~ve been made instruments, wide vari~tions occur. of inst~llation has been summ~rized. to standardize height of The effect of variable height 001253 -23- Thus, ~ssuming an anemometer relocated from ~ former height of 100 feet to the standard height of 20 feet, ~nd assuming also ~ power of 1/7 in the preceding equ~tion, the standard 20-foot-high velocities would be 0.8 times the former 100-fbot-high velocities. Since me~sured wind speed is affected strongly by features of the site and by height of the instrument, proper exposure for the purposes 'in mind is very important, For determining gener~l movement of the air, the exposure should be ~s free ~s possible from obstructions to the flow. Measurements in connection with ev~poration studies ~re m~de ne~r the l~nd surf~ce, sometimes ~t several heights. For special studies the instruments m~y be mounted ~t various heights or ~t intervals on towers. Consistency ~nd reliability of records Relatively few records of wind movement have been obt~ined over a long term of ye~rs devoid of ch~nges in location of the station, in the surrounding environment, ~nd in height of instruments above the land surface. Thus, most records are likely to embody inconsistencies, which are difficult or impossible to identify and ev~luate. Common sources of inconsistency include the following: Wind movement close to the land surface m~y be affected greatly by growth of grass, crops, or brush. These small-scale changes in environment should be considered in regard to records of wind movement at an evaporation station. Larger-scale changes in environment must be considered in regard to all other stations. Many stations have been located in cities, where continual construction and destruction of buildings probably has affected the exposure of most anemometers and wind vanes. The recent trend toward placing stations at airports has not e~sed this situation; construction to ~ccommodate the phenome- nally rapid increase in air traffic has forced all-too-frequent relocation of wind instruments. At another extreme, after a forested area is logged or burned, wind-speed measurements can increase abruptly; then, as norm~l growth is re_established, the measurements revert gradually toward their former value. Instrumental errors in measurement of wind velocity and direction usually are relatively small, excepting those due to mechanical failure or improper maintenance. Most types of anemometers lag due to inertia, and to any play in gear trains or dials--they do not respond instantly to changes in wind velocity. Thus, rapid perturbations are smoothed and peaks of short duration may under-register substantially. The most common anemometer, the cup type, registers too much total movement during a variable wind because the cup wheel accelerates more readily than it decelerates.