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<br />- 38 - <br /> <br />1.5 x 1~-3 inches of wate~. Howeve~, the sensitivity of the tipping bucket <br />-4 <br />located at G~eenwate~ was inc~eased to 5.7 x 10 inches or water. <br />At higher elevations, where the precipitation was generally in the form <br />or onuw, optical. snow-rate sensors (manufactured by F.. Bollay Assoc., Model <br />No. 2-004) were operated alongside the wei~hing buckets. These instruments <br />detect individual airborne ice particles as they pass through a li~ht beam <br />1 meter in length which is focused on a photo-resistor. The signal thus <br />prod?ced is conditioned electronically and recorded on a ~trip chart as an <br />instantaneous value of the water equivalent ,of the snowfall rate. The <br />inst'rument can, resolve about 0.025 inches of water per hoUr with a time <br />resolution of 2 minutes. Some saturation may occur with precipitation rates <br />in excess of q.2 inches per hour due to more than one ice particle being in <br />the. light beam at the same time. The instrument was modified by adding a <br />black plate 23 em in diameter beh~nd the light source. This terminates the <br />field of view of the photo-resistor and eliminates erroneous readings due to <br />reflections from objects beyond the designated path (e.g. reflections from <br />snowbanks, buildings, trees). <br />The original calibration of the optical'snow-rate sensor was,ca~ied out <br />in the Colorado Rockies where the following empirical relationship was derived <br />(E. Bollay Assoc., Inc., 1968): <br /> <br />I" :: 0.3 d <br /> <br />(2.1) <br /> <br />where, r is the precipitation rate in inchhr-l and d is the deflection of <br /> <br /> <br />the optical snow-rate sensor expressed as a fraction of full-scale. From <br /> <br /> <br />simultaneous measurements of r (from the wei~hing buckets) and d we have <br /> <br />I <br />I <br /> <br />I <br />I <br />I <br />1- <br /> <br />1 <br />I- <br />I- <br />I,. <br />1- <br /> <br />I- <br />I- <br /> <br />I- <br />I- <br />1- <br /> <br />I- <br />I- <br /> <br />1- <br />