The Ultimate Guide to Rain Gauges

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

   • Rain gauge sizes and diameters

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 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.

• Siphoning rain gauges tend to have a rising error with the increase in rain intensity, much like most tipping bucket rain gauges. Since I do not have experience with siphoning rain gauges, I can only theorize that the fact that they are significantly less popular than tipping bucket rain gauges may suggest certain disadvantages in maintenance, use or cost. Some examples include Texas Electronics rain gauges that combine a siphon with a tipping bucket mechanism.

• Acoustic precipitation measuring principle like Vaisala WXT520, WXT530 and WXT532 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.)

• 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.

• 200 cm² catchment area rain gauges seem to be most popular worldwide among meteorological departments and have a diameter of 15.96 cm (Ø 6.28 in). Examples include BARANI DESIGN MeteoRain™ 200, Lambrecht, Pronamic Professional, Stevens and others.

• 8 inch diameter rain gauge (Ø 20.32 cm) is standard in the USA and recommend by the NWS. It has a 50.33 in² (324.7 cm²) opening catchment area (funnel orifice). Exmaples include Texas Electronics, RainWise, Hyquest SRG and others.

• 400 cm² catchment area rain gauges are required by many meteorological departments and have a diameter of 22.57 cm (Ø 8.89 in). Examples include BARANI DESIGN MeteoRain™ 400, Delta-OHM HD2013, NESA PL400 and others.

• 500 cm² catchment area rain gauges are less popular due to their size and have a diameter of 25.23 cm (Ø 9.93 in). Examples include EML SBS500, MPS System TRwS514 and others.

• 1000 cm² catchment area rain gauges are the largest size and have a diameter of 35.7 cm (Ø 14.06 in). Examples include EML SBS1000 and others.

• Other non-standardized sizes and shapes exist for agricultural and amateur meteorological use such as the EML Aerodynamic 7.87”, Davis Aerocone 214 cm², Pronamic Small Rain Sensor, Acurite rain gauge collector, and Netatmo rain gauge.

REFERENCES:

Rain gauge accuracy and WMO/NWS standards