Tuesday, September 10, 2019

The Rain Gauge - How Can Something So Simple Be So Complex?

The rain gauge. At its most basic it is just a straight-sided cylinder, with a bottom, of course. Like the mousetrap, someone is always trying to build a better one. A simple web search of "rain gauge" will display a gallery of images of different types of rain gauges.


While the basic concept for a rain gauge is simple, the complications start to come in with calibration, measurement, and siting/exposure. Add to that the increasing complexity when mechanical and electronic components become part of the measurement process and the potential for measurement errors greatly increases. There are weighing bucket rain gauges, which measure the amount of precipitation by weighing the water and recording it on a revolving chart (and now digital storage). There are also weighing rain gauges that utilize more complex technology and software to measure precipitation.

An older weighing bucket rain gauge. The recording drum is located behind the door in the bottom.

Tipping bucket rain gauges utilize a small "bucket" that tips and triggers a signal every time one one hundredth of an inch of rain is collected. Optical rain gauges utilize photo diodes, laser, or infrared to detect drops and measure rainfall rate and intensity. Acoustic rain gauges have sensors that detect the acoustic signature for each drop size on the sensor surface. From this data the drop size distribution can be determined, and from the the rainfall rate and accumulation.


Tipping bucket rain gauge

As we all know, weather also has an effect on precipitation measurement. Strong winds affect collection efficiency, typically resulting in an under-catch of rain, and even more so for snow. Speaking of winter, rain gauges that rely on collecting liquid water must be heated if mechanical or electronic, or the frozen precipitation must be melted before it can be measured, for example, as with the CoCoRaHS gauge and the NWS 8-inch standard rain gauge. Siting and exposure all can affect the measurement with any of the rain gauges mentioned, not just how close the rain gauge is to nearby objects that might affect the rain gauge catch, such as trees and buildings, but also the height of the opening above the ground.  A rain gauge installed closer to the ground is slightly less susceptible to affects from strong winds compared to one at a greater height above ground.

So, what should be a simple, straightforward measurement (how much water is in the straight-sided cylinder) is affected by many factors. Even manual measurement is subject to observer error. The more complicated rain gauges, largely developed to make measurements where or when manual measurements are not practical, introduce the potential for a variety of other errors even though minimizing the human factor.

2 comments:

  1. I was surprised in my analysis of the Productive Alternatives gauge vs the new Outback Blue gauge that it was important to keep track of water temperature to compute its density accurately.

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  2. The effect of the water temperature may be important to the density measurement to three or four decimal places, but for measurement of precipitation to the nearest one one hundredth of an inch the change in the density of water over the range of temperatures we measure at is negligible.

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