A ten-year comparison of daily precipitation from the 4" diameter clear plastic rain gauge versus the 8" diameter metal standard rain gauge

For ten years side-by-side measurements using 5 different unshielded rain gauges have been taken at the Fort Collins Weather Station on the campus of Colorado State University. This poster shows the results of comparing precipitation from the commonly used, low-cost and well-liked 4"- diameter high-capacity plastic rain gauge compared to the National Weather Service official Standard Rain Gauge. Monthly and annual totals are summed and compared for each year. The 4" gauge consistently accumulates more precipitation, with the 10-year average totaling 3% more than the Standard Rain Gauge. Differences are similar in all months of the year and for both snow and rain. The differences for individual daily totals are most often small -- either 0.00 inches or 0.01". The differences appear not to be a calibration error or bias but simply the result of the metal funnel on the standard rain gauge requiring nearly 0.01" of precipitation just to wet the funnel before any drops reach the measurement cylinder. Larger differences occur infrequently but are typically associated with snow and with heavier rain events. The 4" gauge tends to report less precipitation for storms that include hail, suggesting that hail may bounce out of the smaller gauge. Overall, this study supports the continued use of the 4"-diameter gauge as a lower cost alternative to the Standard Rain Gauge.