Where to Put the Rain Gauge: A Siting Rule from Wind Undercatch, Height, and Obstacle Distance

A rain gauge almost always reads low in wind, and where it stands decides by how much. This paper computes that shortfall, called undercatch, for an unshielded cylindrical gauge from stated assumptions: wind increasing with height as a power law with exponent 0.14, an aerodynamic deficit of 1.2 percentage points for each metre per second of wind at the gauge rim, and an obstacle turbulence allowance that falls exponentially with distance measured in obstacle heights. For a moderate site with a wind speed of 5 m/s at 10 m, the rule that keeps total undercatch at or below 5 percent is simple: set the rim at 0.3 m with at least 4 obstacle heights of clearance (4.72 percent), or at 0.5 m with the same clearance (4.99 percent); a gauge at the common height of 1 m meets the bound only with 10 obstacle heights of clearance (4.40 percent) or with no obstacle at all (4.35 percent). For a windy site with 8 m/s at 10 m, no siting in the grid meets the bound: even the lowest gauge, at 0.3 m with no obstacle, already shows 5.88 percent, and with 10 obstacle heights of clearance it shows 5.93 percent. Common practice performs poorly by comparison: a roof mounting at 7 m shows 13.71 percent and a fence-post mounting at 1.5 m shows 9.27 percent, against 4.72 percent for the rule mounting, a difference of 71.9 mm and 36.5 mm respectively in a year of 800 mm of rain. All results are simulated, not measured.