Rain gauges are the standard instruments for measuring precipitation, but their accuracy is not absolute. Collecting rainfall is subject to factors that can skew the final measurement. Accuracy depends on the instrument’s design, the immediate environment, and the care taken in its placement and upkeep. Understanding these variables allows for a more realistic interpretation of the data provided.
Different Ways to Measure Precipitation
The accuracy of precipitation measurement begins with the instrument’s design. The standard manual rain gauge, often an eight-inch diameter collector, is a reliable reference for basic accumulated rainfall. It involves a direct, static measurement of the water volume collected, relying on a funnel directing water into a calibrated inner cylinder, which is manually read by an observer.
The automated tipping bucket rain gauge is a common alternative, designed for continuous, remote data logging. This device uses a small, seesaw-like mechanism that tips after collecting a fixed volume of water, typically equivalent to 0.2 mm or 0.01 inches of rainfall. Each tip generates an electronic pulse, providing near real-time data on precipitation rate and accumulation.
A limitation of the tipping bucket design is its tendency to undercount during intense rainfall events. When rain falls rapidly, water continues to flow into the funnel while the bucket is tipping, causing unmeasured rain and resulting in underestimation. During very light rain, the gauge may fail to record precipitation until enough water accumulates to trigger a tip. Calibration drift in mechanical parts can also introduce errors, sometimes reaching five to ten percent of the reading if not regularly checked.
Environmental Factors That Distort Readings
Even a perfectly calibrated gauge faces environmental challenges that can distort its readings. Wind is the most significant factor causing measurement error, as it creates turbulence around the gauge’s collecting orifice. This turbulence results in “undercatch,” where the wind blows raindrops away from the opening, causing the gauge to record less precipitation than actually fell.
For liquid precipitation, wind bias causes an undercatch error ranging from two to ten percent, increasing with wind speed. This error is severe for solid precipitation, like snow, where undercatch can be as high as ten to fifty percent. Evaporation is another source of error, where collected water is lost from the gauge, especially in hot, dry climates or when the period between readings is long.
Splash-in and splash-out errors also affect accuracy, particularly if the gauge is placed too close to the ground or on an ungridded surface. Raindrops striking the ground or nearby objects can splash into the collection funnel, artificially inflating the reading. Conversely, water can splash out of the funnel’s opening, leading to an underestimation.
Optimizing Gauge Placement and Maintenance
The largest improvements in a rain gauge’s accuracy come from careful placement and routine maintenance. Proper siting is necessary to minimize wind effects and obstruction interference. The gauge should be installed in an open area, far from obstructions like buildings, trees, or large shrubs that could block or redirect rainfall.
A rule for placement is to position the gauge away from the nearest obstruction by a distance at least twice the obstruction’s height. For example, a ten-foot tall tree should be at least twenty feet away. The collecting orifice should be mounted at a standard height, typically one meter (about three feet) above the ground, to avoid ground splash while minimizing wind effects.
The gauge must be perfectly level to ensure the collected precipitation is measured uniformly across the collection surface. An unleveled gauge causes an inconsistent collection area and skewed results. Routine maintenance is necessary to sustain accuracy, including regularly removing debris like leaves, insects, or dirt that could clog the funnel or interfere with the tipping mechanism. Keeping the gauge clean and ensuring electronic components are functioning helps guarantee reliable measurement.