Rain Gauge Differences
Date: Summer 2014
We have two rain gauges, side-by-side in an open area with no real differences in how they are mounted and with no obstructions. One is a "professional rain gauge" that has a funnel which sits in the 1.5 inch diameter cylinder inside of a four inch diameter cylinder designed to catch rain over one inch. The other is an old fashioned glass tube rain gauge which sits in little mounting bracket and is about 3/4 inch diameter. The smaller tube consistently collect more rain. Example: on Saturday the little tube registered 1.5 inches and the big professional tube registered 1.25 inches. When I asked the company they said the professional gauge was more accurate, but they couldn't tell me why. If the smaller gauge showed less rain, I'd understand it better but it always shows more.
Can you explain?
Not seeing the setup I might suggest two possibilities. First there is some way the mounting of the simple glass tube is channeling water into the tube, but the difference in measurements seems to argue against that. Secondly, the manufacturer may be correct and the less sophisticated gauge is off. If there is a proportion between the two, say one is always 20% larger than the other you might begin to consider that.
Or you could put out a sprinkler, the old fashioned back and forth type, and compare the two gauges in a controlled manner. Your problem reminds me of the old saying, "A man with one watch knows the time. A man with two watches is never quite sure".
Take care and I hope this helps.
At least hypothetically, the larger gauge should be more accurate just because it is collecting over a larger area that would better represent an average rainfall. If rain were a uniform mist it should make no difference, but because it come in droplets (sometimes quite large) randomly distributed, the smaller the area of the collector, the higher the odds that the small area will sporadically see more or less hits than the "average" for an area. Carry it to an extreme for example, if your small collector was a tube 1/8" in diameter, there would be occasions when that would collect 0 raindrops in a short shower, other times when a direct hit from a single drop would register as large amount of rainfall.
If the smaller gauge is _always_ registering more, then there is likely something wrong or not being accounted for in one design or the other. The first issue would be whether the gauges are accurately marked vertically. Another issue would be if the gauge were slightly conical, e.g., one wider at the top than at the bottom would read high, and one wider at the bottom would read low. A way to verify the accuracy of the gauges is to measure the diameters of the collection cylinders multiple times in various directions and calculate the area from the average diameter. Calculate a volume or mass of water that would be equivalent to a set amount of rainfall, say 1" rain per square inch of collection area, measure that out with a balance or graduated cylinder, and directly pour into the final collection vessel.
Other possibilities would be water getting stuck on the walls of the cylinders, allowing for evaporation before it gets measured (e.g., right after a rain, are more water droplets visibly stuck on the wall of one collector or the other?), or evaporation from the bottom of the collection cylinder, although you can minimize the latter by measuring as soon after the rain stops as is possible.
It is hard to imagine any one of these things accounting for more than a few percent difference, although if they all bias the same way maybe you can get a 20% (0.25" out of 1.25") total impact.
One last thing that I thought of that would especially impact the smaller cylinder, is if the glass used is a bit thick, then the actual collection area may be larger than the inner diameter of the cylinder; if it is a rounded lip on the top edge, everything inside of the highest point of the edge would fall inwards. In that case if you make an ink print of that edge and measure that diameter, you will get a somewhat more accurate measurement of the collection area than if you just measure the cylinder inner diameter.
If all these things check out and the gauges both look like they should be accurate, then just track the measurements for both over time, In the long term, they should average out to the same values. The small collector should occasionally read high as well as low, so if you look at dozens to hundreds of events, there should be a distribution of times when it reads lower than the large gauge as well as higher (and/or about the same).
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Update: November 2011