Rain gauge accuracy and WMO/NWS standards

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Confusion reigns when comparing different rain gauges. First one must consider rain gauge opening size and then rain gauge resolution and also rain gauge accuracy. For a detailed description and help in choosing a rain gauge see the following guide: Guide to Rain Gauges: Types, Sizes, Benefits and Drawbacks

  • Rain gauge size determines the repeatability and also the accuracy of a rain gauge. The larger the opening, the lower the statistical error in measurements in real-world applications. Always choose the largest size that your budget allows.

  • Rain gauge resolution determines the smallest amount of precipitation that one is able to measure, the overall accuracy of accumulated rain measurement and the accuracy of rain intensity (precipitation rate) measurements.

  • 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 like leaves and insects that fall into the weighing pan 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.

Rain gauge resolutions or minimum recordable amount of rain

Automatic rain gauges are not only differentiated by the aforementioned rain opening sizes but mainly by the minimum amount of rain they are able to measure. This is their “Resolution” and is the minimum rain amount to cause and increment in the rain reading.

Mechanical tipping bucket and siphon type rain gauges are usually found in the following rain resolutions:

  • 0.01” inch resolution is equivalent to 0.25 mm and is a standard minimum rain gauge resolution set by the NWS and NOAA.

  • 0.2 mm resolution is a WMO standard and is now more frequently being replaced by 0.1 mm resolution in professional meteorological applications for synoptic observations.

  • 0.1 mm resolution is also a WMO standard and is being more and more frequently requested by meteorological departments.

  • Other non-standard resolutions range from 0.25 mm to 1 mm. Coarser (> 1 mm) resolution is typically not used since it is not able to produce a reading in many short rain events.

High-resolution rain gauges (benefits & drawbacks)
0.1 mm (0.004”) and finer resolution

  • Higher/Finer resolution rain gauges have lower limits on maximum rain rates they can measure accurately. This applies to tipping bucket, siphoning, and even weighing rain gauges.

  • High-resolution rain gauges have higher errors in the accumulated rain measurement (despite datasheet claims) since each raindrop represents a significant percentage of its measurement increment (resolving amount which dictates its resolution).

  • High-resolution rain gauges require more regular maintenance and are more susceptible to wind-induced vibrations along with dirt and debris accumulation in their measuring mechanism, siphon, or weighing pan. With higher resolutions, the weight and volume of each dirt particle represent a larger percentage of their measurement increment.

  • Despite the above limitations, high-resolution rain gauges have significant benefits for meteorology for local precision measurements as well as for synoptic observations since they require lower rain amounts to produce an acceptable level of accuracy due to their higher resolution. In these applications, regular maintenance is usually not an issue. Calculating error from rain gauge resolution follows in the next section and rain gauge accuracy in terms of drops of rain is found here.

  • For higher resolution rain gauges, it is highly recommend that the rain catchment are be at a minimum of 8 inches (324.7 cm²) or larger to minimize measurement mechanism or measurement principal errors due to individual drops of rain. See the rain gauge accuracy tables below for an idea of what 1% accuracy looks like in terms of the number of rain drops.

Lower-resolution rain gauges (benefits & drawbacks)
0.2 mm (0.01”) and coarser resolution

  • Lower resolution rain gauges can more precisely measure larger accumulated rain amounts since each raindrop represents a small percentage of its measurement increment (resolving amount which dictates its resolution).

  • For lower accumulated rain amounts, they have higher uncertainty (and error) since the mechanism may be partially filled when the rain begins or stops without registering this excess rain amount. That is why their benefits are realized in measurement of 10 mm of rain or higher. Calculating error from rain gauge resolution follows in the next section.

  • Lower resolution rain gauges require less regular maintenance and are usually more robust and reliable since dirt and debris accumulation in their measuring mechanism, siphon, or weighing pan does has a smaller proportion to the resolving rain amount. With higher resolutions, the weight and volume of each dirt particle represent a larger percentage of their measurement increment.

  • For agricultural use, lower resolutions are perfect since farmers are interested in the water that seeps bellow the ground surface where plants can access it. Trace amounts of rain, which lower resolution rain gauges ignore, are of no benefit in agriculture since they usually evaporate from the ground surface anyway. Lower maintenance, better dirt resistance and higher reliability of lower-resolution rain gauges are also of benefit since regular maintenance is usually ignored in farming applications.

  • Very low resolution rain gauges (greater than 0.2 mm or 0.01”) only give very rough estimates of rain and are mainly for hobby use.

Calculating basic rain gauge uncertainty (error)

The basic formula to determine the minimal amount of rain for a given accuracy follows:

  • Divide the rain gauge resolution by the % accuracy desired: 0.2 mm / 2% = 10 mm or more of accumulated rain is required to reduce the rain gauge resolution error to less than 2%.

  • Adding in a datasheet accuracy of 1% results in a ± 3% minimum uncertainty of accumulated rain amount.

  • Since there are other uncertainties which we did not consider yet, like wind blown rain uncertainty, the total uncertainty and measurement error will be larger.

  • For more details on the complex and complete calculation of rain gauge measurement uncertainty and error, please see the WMO CIMO/TECO Guide to Instruments and Methods of Observation, 2018 edition - Volume V, chapter 6 on the Measurement of precipitation.

Determining rainfall rate and practical rain intensity error

For practical advice on determining accurate precipitation intensity (rate) and how to choose a rain gauge for accurately determining rain rates, please see the following guide: Practical Guide to Determining Rainfall Rate and Rain Intensity Error

Rain gauge error in terms of missed drops of rain

A standardized drop size as used for laboratory and medical purposes is defined as 1/20th of a milliliter (also cubic centimeter = 1 ml). One drop is exactly 0.05 ml. Rain gauge accuracy tables show how accuracy in terms of missed drops of rain.

  • 200cm² rain gauge

    • 0.1 mm resolution equals 2 lm (40 standard drops) of water

    • 0.2 mm resolution equals 4 lm (80 standard drops) of water

  • 8” rain gauge

    • 0.004” inch resolution equals 3.25 lm (65 standard drops) of water

    • 0.01” inch resolution equals 8.25 lm (165 standard drops) of water

  • 400cm² rain gauge

    • 0.1 mm resolution equals 4 lm (80 standard drops) of water

    • 0.2 mm resolution equals 8 lm (160 standard drops) of water

  • 500 cm² rain gauge

    • 0.1 mm resolution equals 5 lm (100 standard drops) of water

    • 0.2 mm resolution equals 10 lm (200 standard drops) of water

Rain Gauge Accuracy Tables

Rain gauge accuracy tables show how accuracy, in percent (%), is related to measurable amounts of rain so that one can quickly judge whether a manufacturer is providing correct or incorrect values on their datasheets. It seems to be the case that many meteorological instruments from even the “most reputable” manufacturers exaggerate their rain gauge accuracy with respect to real world accuracy in outdoor conditions.

As raindrop size and speed increases, its shape pancakes and deforms due to aerodynamic forces until it breaks appart into 2 or more smaller drops. Image courtesy of: Pbroks13 [CC BY (https://creativecommons.org/licenses/by/3.0)]

As raindrop size and speed increases, its shape pancakes and deforms due to aerodynamic forces until it breaks appart into 2 or more smaller drops. Image courtesy of: Pbroks13 [CC BY (https://creativecommons.org/licenses/by/3.0)]

Rain drop size and rainfall intensity

Rain drop size and shape varies based on rain intensity and rainfall speed. As a raindrop grows in size within a cloud, its terminal velocity increases until it breaks apart when aerodynamic forces overcome the surface tension that holds it together as shown in the accompanying illustration. Drop breakup causes it to immediately slow down.

Typical rainfall drop sizes will range from 0.001 ml (Ø 0.6 mm) to 0.3 ml (Ø 4 mm) with higher rain intensities producing larger rain drop sizes and larger rain drop size variability.

Rainfall intensity is classified according to the rate of precipitation which is given as amount of rainfall per unit of time. According to MANOBS (Manual of Surface Weather Observations) the following general categories are used to classify rainfall intensity:

  • Light rain - less than 2.5 mm/h (<0.1”/hr) or 0.04 mm/min rain rate, which for a 0.2 mm resolution rain gauge is 12 full tipping buckets per hour (12 pulses/hr).

  • Moderate rain - rain rate of fall is 2.6 to 7.5 mm/h (0.1 to 0.3”/hr) or (0.04 to 0.125 mm/min), which for a 0.2 mm resolution rain gauge equals 13 to 37 full tipping buckets per hour (13 to 37 pulses/hr).

  • Heavy rain - rain rate is greater than 7.6 mm/h (>0.3”/hr) or (>0.125 mm/min) which for a 0.2 mm resolution rain gauge equals 38 or more full tipping buckets per hour (38+ pulses/hr).

  • Violent rain: Can sometimes be used to characterize precipitation rates greater than >50 mm/hr (>1.97 in/hr) (>0.83 mm/min)which for a 0.2 mm resolution rain gauge equals 250 or more full tipping buckets per hour (250+ pulses/hr, 4.2+ pulses/minute).

References

  1. Microphysics of Clouds and Precipitation by Pruppacher and Klett, (1978, Reidel, Boston), pp. 316-19

  2. Beard, K.V. Terminal Velocity and Shape of Cloud and Precipitation Drops [pdf]. Journal of the Atmospheric Sciences (May 1976): 851-864.

  3. Holladay, April. Falling raindrops hit 5 to 20 mph speeds. Wonderquest. Albuquerque: 19 Dec 2001.

  4. MANOBS (Manual of Surface Weather Observations)

  5. Accuracy of Siphoning Rain Gauges Gert Luyckx, Jean Berlamont Hydraulics Laboratory, University of Leuven, Kasteelpark Arenberg 40, B-3001 Heverlee; Phone: +32 16 321655; Gert.Luyckx@bwk.kuleuven.ac.be

 
BARANI DESIGN TECHNOLOGIES IS A MANUFACTURER OF PROFESSIONAL RAIN GAUGES, WEATHER STATIONS AND METEOHELIX PERSONAL WEATHER STATIONS

BARANI DESIGN TECHNOLOGIES IS A MANUFACTURER OF PROFESSIONAL RAIN GAUGES, WEATHER STATIONS AND METEOHELIX PERSONAL WEATHER STATIONS