Using High-Speed Photography to Study Undercatch in Tipping-Bucket Rain Gauges

Abstract

o better understand the undercatch process associated with tipping-bucket rain gauges, a high-speed camera normally used in determining the structure of lightning was employed. The photo rate was set at 500 frames per second to observe the tipping of the bucket in a commonly used tipping-bucket rain gauge. The photos showed detail never seen before as the bucket tipped from one side to the other. Two fixed rain rates of 19.9 mm h?1 (0.78 in. h?1) and 175.2 mm h?1 (6.90 in. h?1), the minimum and maximum available, respectively, were used.The data from four tips at each rain rate were examined. The results show that the time from the beginning of a tip to the time the bucket assembly is horizontal?defined as the period during which undercatch occurs?was an average of 0.450 s for the eight cases. The average time for a complete tip was 0.524 s; thus, the vast majority of the time of a tip, 86%, is spent in undercatch mode. Because there was no apparent dependence of these times on rain rate, it should be possible to apply an accurate linear correction for undercatch as a function of rain rate given the time that undercatch occurs during a tip. Over all eight tips, the undercatch was found to be 0.98% for the 19.9 mm h?1 rate and 8.78% for the 175.2 mm h?1 rate. The procedure used to estimate the undercatch is described. Slow motion videos of the tipping of a bucket are available online.