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-21- 001252 WIND Measurements and types of equipment Surface instruments The speed, direction and gustiness of the wind near the land surface are measured by anemometers and wind vanes installed on poles or towers at or near the site of the instruments used to measure other meteorological elements such as precipitation, temperature, and humidity. The height of the installation is determined by the purpose of the measurement. For example, winds near the surface are required in connection with evapo- ration studies, but even for aviation or climatological purposes the height of installation is usually no more than a few tens of feet. The instruments commonly used for measuring wind speed fall into three types, as follows: Rotational Vertical-axis rotating cups, horizontal-axis windmill or propeller. Pressure Pitot-static, Venturi-static, pressure plate, or bridled cups. Heat exchange Hot wire or thermistor, kata- thermometer. The majority of installations are of the rotating-cup type. The pro- peller type usually is for portable or hand-held use. Pitot and Venturi tubes measure pressure gradient of the air flow. The ordinary pressure- plate type hangs on a horizontal axis perpendicular to the wind and is swung from the vertical by the wind's force. The normal-plate type remains perpendicular to the air flow and its deflection measures the pressure of the wind against it. The bridled-cup type is non-rotating and measures the torque resulting from the wind force. Hot-wire types measure either the variations in current required to maintain the wire temperature or the variations in resistance of the wire as its temperature is cooled qy the air flow. Kata-thermometer anemometers measure the cooling power of the air flow ventilating the bulb of a thermometer; wind speed is determined from the time required to depress the tempera- ture from 400 c to either 380 C or 350 c. The gustiness, or short-period fluctuations of wind speed may be measured by any of the anemometer types. Some are more suitable than others depending on the scale of the variations it is desired to measure.