All You Ever Wanted to Know About Choosing a Rain Gauge

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The Ultimate Guide to Rain Gauges

  1. Guide to rain gauges: types, sizes, benefits and drawbacks

  2. Practical guide to determining Rainfall Rate and Rain Intensity Error

  3. Rain Gauge Accuracy Tables

  4. Rain gauge accuracy and WMO/NWS standards

  5. Rain rate intensity classification

  6. Rain drop size and speed of a falling rain drop

  7. Raindrop Shape & Size Calculator (NEW)

  8. WMO CIMO/TECO Guide to Instruments and Methods of Observation, 2018 edition - Volume V, chapter 6 on the Measurement of precipitation.

  9. The correct and wrong way to mount your weather station

  10. Aerodynamics of rain gauges and error due to wind (coming soon)

Rain Gauge Types by Measuring Principle

Rain gauge measuring principle (rain gauge type) determines the general accuracy, reliability and maintenance requirements of the rain gauge. There are many different measuring principles. Bellow we list the most popular ones. If you would like others listed, please let us know in the comments bellow:

  • Self-emptying tipping bucket rain gauges, or TBR for short, are the most popular type of rain gauge despite having some drawbacks, many of which may be a thing of the past with the introduction of a new self-balancing tipping-bucket mechanism in the new MeteoRain line of rain gauges by BARANI DESIGN Technologies. Historically very popular designs include Pronamic, Davis Instruments and RainWise rain gauges among the many types.

  • Weighing rain gauges have the advantage of being able to offer very high rain resolution, good rain accumulation accuracy and short term precipitation intensity measurement at least in the laboratory. Practical experience has shown that maintaining them can be quite burdensome and wind induced pressure errors and vibration errors along with non-horizontal mounting errors and most significantly debris (leaves, insects, …) that falls into the weighing pan errors are a source of endless problems and frequent maintenance is a must. Nevertheless they remain highly popular with meteorological offices since their single biggest benefit is their consistent measurement of rain accumulation not affected by rain intensity, which has been the pitfall of tipping bucket rain gauges until the MeteoRain IoT wireless series from BARANI, which feature a built in intensity correction factor for each tip of the tipping bucket which is transparently reported to the user.

  • Siphoning rain gauges have the advantage precise total rain measurement for long lasting rain storms. For short rainstorms they introduce a large error due to starting error from water left over from the previous storm that is stored in the siphon and consequently also larger evaporation of stored water from the siphon and the tipping-bucket. Since siphons accumulate and then flush water into their tipping bucket counter under the siphon, the siphoning flow rate and not the true rain rate is measured, thus all information about rainfall intensity is lost. It is also important to keep the siphon clean with regular dissassembly and maintenance, making them less desirable for remote installations. Some examples include Texas Electronics rain gauges or TB4 from Kisters that combine a siphon with a tipping bucket mechanism.

  • Acoustic precipitation measuring principle like Vaisala WXT520, WXT530, WXT532 and WXT536 use is a rough estimate of rain fall rate based on the impact strength of raindrops hitting a metal surface. Needless to say, accumulated rain amounts are only very rough estimates based on manufacturer calibration equations. With this principle the stated 5% accuracy will be quite hard to reach outside of a laboratory and I would put it in the same category of “rain sensors” with the following RG-11. (Vaisala has quietly acknowledged this by removing the "5% “rainfall cumulative accumulation accuracy 5%*” as found on the WXT520 from the follow-on Weather Transmitter WXT530/WXT532 datasheets. Subsequently they reintroduced the following without any upper limit on the error: Field accuracy for daily accumulation: Better than 5%, weather dependent). In an independent evaluation “The Impact of Wind on Precipitation Measurements from a Compact Piezoelectric Sensor” by Enrico Chinchella, Arianna Cauteruccio, and Luca G. Lanza, published in the Journal of Hydrometeorology in February 2024 (Vol. 25, No. 2, pp. 339–352; DOI 10.1175/JHM-D-23-0180.1). It analyzed the Vaisala WXT520 with computational fluid dynamics (CFD) and reported “measured precipitation is overestimated by up to 400% under the influence of wind.“ Thus it is only a rain sensor. Its advantage is however low maintenance and simple operation.

  • Optical rain sensor like the RG-11 Hydreon Optical Rain Sensor is not a rain gauge and for this purpose it is the least reliable form of accumulated rain measurement with errors of up to 37% recorded by our testing. It is a rain sensor. Its advantage is however low maintenance and simple operation.

Rain gauge sizes

Rain gauges are for the most part divided into the following size classes based on rain catchment opening area. The most popular size for professional meteorological use is 200 cm² which has been informally accepted as it is the minimum acceptable size with a circular opening for reasonably accurate precipitation measurement. A circular catchment opening for a rain gauge is the standardized shape since it presents a consistent catchment area shape to windblown precipitation independent of rain and wind direction.

REFERENCES:

Rain gauge accuracy and WMO/NWS standards