A rain gauge measures liquid precipitation over a set period. The accuracy of the data collected is highly dependent on where the gauge is positioned, as nearby objects and environmental factors can skew the readings. Finding the best place for a rain gauge involves a careful balance of avoiding obstructions and mitigating environmental interference to ensure the collected rainfall truly represents the area’s precipitation. The goal is to install the instrument in a location that provides a clear, open path for rain to fall directly into the collector.
Optimal Clearance from Obstructions
The placement of a rain gauge must prioritize an open, unobstructed view of the sky to prevent a phenomenon known as the “rain shadow” effect. Tall objects, such as buildings, fences, trees, and large shrubs, disrupt the natural path of falling rain and create air turbulence. This turbulence can deflect raindrops away from the gauge, leading to an underestimation of the actual rainfall.
To minimize this interference, a standard rule involving the height of the obstruction is applied. The horizontal distance from the gauge to any permanent, vertical structure should be at least two to four times the height of that object. This guideline is sometimes referred to as the 45-degree rule, ensuring the gauge is outside the cone of influence created by the obstruction. For example, if a nearby tree is 20 feet tall, the gauge should be placed a minimum of 40 feet away, and ideally up to 80 feet away. Positioning the gauge too close to a building can lead to both under-reading due to wind deflection or over-reading if the structure causes rainwater to splash or channel toward the instrument.
Height and Level Requirements
The vertical positioning of the rain gauge is equally important for obtaining accurate measurements. The instrument’s opening should be mounted at a specific height above the ground to avoid splash-back while remaining low enough to minimize wind-related errors. A common recommendation for the height of the gauge’s opening is between 2 and 5 feet above the ground surface.
Mounting the gauge at this height prevents water droplets from hard surfaces, like bare soil or concrete, from splashing into the collector and artificially inflating the reading. Professional meteorological standards often suggest a height of approximately 4 feet (1.2 meters) to strike a balance between avoiding ground splash and reducing wind turbulence. The gauge must be secured to a sturdy post or dedicated mount that is perfectly stable and resistant to movement from wind or accidental contact.
The precision of the measurement depends on the gauge being perfectly horizontal, or plumb, to ensure that precipitation is collected evenly across the entire opening. A slight tilt can cause a significant error in the volume measured, especially in gauges that use a tipping mechanism. Using a bubble level during installation and periodically checking the mount’s stability are necessary steps to maintain the instrument’s required level orientation.
Mitigating Environmental Interference
Beyond fixed obstructions, environmental conditions can introduce measurement errors that must be addressed through careful placement. Wind is a major factor, as strong winds can create localized eddies and turbulence that physically blow rain away from the gauge’s opening, leading to an undercatch. While a completely wind-protected location is difficult to find, the gauge should be situated away from areas where wind naturally channels, such as narrow passageways between structures.
Another consideration is the surface beneath the gauge, which should not radiate heat or cause excessive splash. Surfaces like asphalt, concrete, or rooftops absorb solar energy, and the resulting heat can cause collected rainwater to evaporate quickly, reducing the recorded total. The ideal surface is a natural one, such as short grass or gravel, which helps absorb heat and minimizes the potential for splash-up into the collector.
Finally, the gauge must be kept clear of artificial water sources that could contaminate the reading. This includes ensuring that the instrument is not placed near sprinkler systems, downspouts, or any location where water runoff from a roof or deck might splash into the collector.