METHODS OF MEASURING PRECIPITATION FOR USE WITH THE AUTOMATIC WEATHER STATION
ILLINOIS STATE WATER SURVEY URBANA
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Results of conferences with authorities in the field of percipitation are reviewed. It was concluded that accurrate measurement of smow is difficult. Factors that lead toward inaccuracy in collector-type gages and which need further investigation are 1 aerodynamic neutrality provided by something over a wide range of wind speeds 2 rim effect 3 internal circulation within a gage 4 snow capping of gage 5 rim formation and 6 climate peculiarities such as drifting snow and blowing snow. An instrument of precipitation detection does not present as many difficulties. A review of 34 investigations of wind effect indicated that experiments with shields in low-speed wind tunnels have been shown that conditions for aerodynamics neutrality in the vicinity of the gage orifice are approximated when shields are used. Field results for snow measurement showed that shielded gages catch between 80 and 90 under average conditions of wind speed. The usefulness of a shield decreases as the wind velocity increases above 20 mph. A review evaluation was made of 13 percipitation detectors ombroscopes. Only 3 designs, the Barnothy and Bell heated coil, the Adlam water collector, and the Sarcohterm snow collector, were suitable for both rain and snow detection in their present form. The Barnothy-Bell heated coil method appeared most desireable.