Why do Vaisala RAINCAP® measurements differ from tipping bucket measurements?

The Vaisala RAINCAP^® is fundamentally different from tipping buckets and other rain collection devices. Each type of measurement device is subject to measurement error from various sources. Vaisala RAINCAP^® measurement errors are primarily due to random variations in the shape and velocity of raindrops caused by air movements. Tipping buckets, of which there are a wide variety of designs, are subject to errors such as evaporation, splashing of water, and wetting on the interior walls of the collector and container. Many meteorological agencies use correction factors to compensate for these sources of error. However, most tipping buckets are calibrated at only one level of rain intensity. When the measurement results are not compensated, substantial measurement errors may be present, particularly during heavy rainfall.

Please reference “WMO laboratory intercomparison of rainfall intensity gauges, Sept 2004…Sept2005 – Final Report” for a more complete treatment of this subject. See the Vaisala RAINCAP^® sensor brochure for more information on the theory of operation of the Vaisala RAINCAP^® sensor, including comparison measurements.