Guide to rain gauges: types, sizes, benefits and drawbacks
/Size, shape and rain resolution
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.
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 and the accuracy of short-term rain intensity measurements.
A finer resolution rain gauge will have a lower limit on the maximum rain rate it is able to measure accurately, which holds true for tipping bucket, siphoning and even weighing rain gauges.
For rain gauges of the same size and type, a finer resolution rain gauge will in general also have a higher error in the accumulated amount of rain measurement since each raindrop will represent a larger percentage of the rain gauges resolving amount (resolution).
Rain gauge accuracy
If a rain gauge has 1 % accuracy listed on its datasheet, it should mean that for all rain rates within its measuring range, light to heavy rains, the maximum difference between the real amount of rain that flows through its funnel and what is recorded by it will be within 1 % (1 % less or 1 % more). In other words, it is the difference between the amount of rain collected by a graduated cylinder with the same opening size and the amount recorded by the rain gauge over the same time interval.
To learn why so many rain gauges overstate their accuracy in their datasheets, please see “Rain Gauge Accuracy and WMO/NWS standards.” It explains how few drops it takes to cause a 1 % or 2 % rain measurement error. Also see a Practical guide to determining Rainfall Rate and Rain Intensity Error for more info on rain gauge error due to rain gauge resolution.
Types of rain gauges
Rain gauge measuring principle (rain gauge type) determines the general accuracy, reliability and maintenance requirements of the rain gauge.
Graduated cylinder rain gauge (also called a standard rain gauge or a manual rain gauge) is highly reliable but one must get wet in the rain to read off rain information in real-time. If one can wait until it stops raining and has the patience, time and persistence to manually read off rain accumulations, this type of rain gauge can be the most accurate and reliable. Error from evaporation loss in warm climates can be an issue if rain accumulations are recorded only once per day.
Self-emptying tipping-bucket rain gauges are the most popular type of rain gauge. New designs include the BARANI DESIGN MeteoRain and historically very popular designs include Pronamic, Davis Instruments and RainWise rain gauges among the many types.
Just about all tipping bucket rain gauges use a teeter-totter (seesaw) dual tipping bucket mechanism with a magnet to activate a reed switch when the mechanism flips from one side to the other due to accumulated rain in its buckets/spoons. Without external influences, each bucket flip will happen when a precise amount of rain water accumulates and shifts the mechanism’s center of gravity to the opposite side of its pivot point. For a 200 cm² rain gauge with a 0.2 mm resolution, each bucket-full equals precisely 4 ml or 80 drops of water. Due to the mechanism’s reliance on small shifts in the center of gravity, the tipping bucket mechanism is sensitive to residual water drops remaining in each bucket, dirt accumulation in the buckets, vibrations and mounting angle when not mounted completely horizontal. All these effects introduce errors of measurement which can be sizable, depending on the mechanism design.
A new type of self-balancing tipping bucket mechanism has been designed by BARANI DESIGN Technologies for their MeteoRain line of rain gauges which alleviates the tipping bucket mechanism’s sensitivity to residual water drop errors, high rain rate errors, vibration induced errors and errors due to gravity when the rain gauge is mounted not completely horizontal.
Pronamic, to the best of our knowledge, is the only manufacturer to successfully design a single spoon tipping bucket which offers the benefit of improved measurement consistency and accuracy compared to the see-saw dual bucket mechanism with respect to errors from residual drops of water remaining on the spoon/bucket surface.
In most cases the magnetic reed switch is electrically a “normally open” (NO) type which means that an electrical pulse is created only when the mechanism flips from one side to the other, thus ensuring very low current consumption and almost infinite electrical life time (MTBF). Delta-OHM, to the best of our knowledge, is one of the few manufacturers who have chosen to use a “normally closed” (NC) reed switch for their rain gauge electrical connection.
Weighing rain gauges are a rising trend of the past 20 years, first developed in 1992 by MPS System of Slovakia. They 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 wind induced pressure errors and vibration errors along with non-horizontal mounting errors and most significantly debris (leaves, insects, …) that fall into the weighing pan are a source of endless problems and frequent maintenance is a must.
Most weighing rain gauges have to be emptied manually by staff and in winter climates are susceptible to the weighing pan freezing. In hot climates, they are susceptible to large evaporation errors, thus oil is often added to create an thin oil layer on the accumulated water’s surface to limit evaporation. Extensive post processing of data from weighing rain gauges is required to achieve usable rain data and causes high power consumption and is a source of potential errors.
Just like acoustic and optical rain sensors, weighing rain gauges are active sensors and require a continuous supply of electrical energy unlike tipping bucket and siphoning rain gauges which require no power. Thus autonomous operation is highly reliant on a reliable source of energy and thus they are not particularly suited for IoT or agricultural applications.
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. Whats important to note is that this 5% error is only achievable in laboratory controlled conditions since it “does not include possible wind induced error.” (Vaisala has quietly acknowledged this by removing the 5 % “rainfall cumulative accumulation accuracy” from the follow-on to the WXT520 Weather Transmitter, the 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.
BARANI DESIGN TECHNOLOGIES IS A MANUFACTURER OF PROFESSIONAL RAIN GAUGES, WEATHER STATIONS AND METEOHELIX PERSONAL WEATHER STATIONS