According to The World Meteorological Organization Commission for Instruments and Methods of Observation, “Wind is the greatest source of disturbance in precipitation measurement.” As early as 1769, William Heberden wrote about the variation of precipitation with height. He observed over the course of 1 year that a rain gauge on the ground recorded 22.61 inches, on top of a nearby house recorded 18.14 inches, and on top of Westminster Abbey recorded 12.10 inches. We now know that these differences in observations are directly related to the increased velocity of wind with height and gauge under-catch as a function of wind speed. Furthermore, we know that wind speed increases exponentially with height above ground with the exponent a function of ground surface roughness.
This post is an excerpt from the Belfort Instrument Engineering Guide to Siting Precipitation Gauges. Click the button below to download the free guide now (PDF).
The ideal site selection would have trees, shrubs or other objects that act as a wind break surrounding the gauge, but they would not be so tall as to block the normal flow of blowing precipitation. Furthermore, an ideal site would be on relatively flat land to reduce localized eddies caused by terrain. There should be no large obstacles adjacent to the gauge that would block or deflect precipitation around the gauge. Finally, the gauge should be sited in a way that minimizes collection of drifting snow, blowing debris, and splashing rain.
The ideal forest site would have trees cut down so that the nearest trees are at a distance greater than 4 times the height of the remaining trees (US NOAA/CRN criteria). It would have intervening shrubbery and bushes cut to a height not to exceed the height of the gauge inlet orifice. The U.S. Forest Service installation in Howland, Maine, as shown in the precipitation gauge figure, meets these criteria.
Open Flat Land Sites
The ideal open flat land site would be surrounded by cleared surfaces with a slope not to exceed 1/3 (an elevation change of 1 unit over a horizontal distance of 3 units) or 19 degrees (NOAA/CRN and WMO specification) as shown in the NOAA CRN sites.
No Large Obstacles
The ideal precipitation gauge site would not have any obstacles that subtend more than a 10 degree arc when looking from the base of the gauge as it will block precipitation or cause eddies that will impact collection efficiency.
Minimize Collection of Drifting Snow, Blowing Debris and Splashing Rain
All precipitation gauges will measure blowing snow as a precipitation event. Weighing type precipitation gauges will weigh leaves and blowing debris from a freshly harvested field as if it were accumulative precipitation. The small diameter inlet orifices used in manual and tipping bucket gauges will become easily fouled by blowing debris and under record precipitation. Gauges sited too close to the ground in hard surface areas can collect rain splashes and record them as precipitation.
- Minimizing the effect of blowing snow requires siting your gauge in a way that snow collected on adjacent trees or ridges does not blow into the collection orifice. In areas where there is significant accumulation of snow, the gauge inlet should be at least one meter above the highest amount of historic snow accumulation as shown in the WMO test site in the mountains of Switzerland.
- In the spring, blowing seeds and pollen from trees can be anticipated in areas of deciduous trees and some conifers. This debris can be recorded as precipitation in weighing type gauges and can also foul the inlets of tipping bucket and manual gauges. In the fall, blowing leaves can be a significant source of error for all types of precipitation gauges. Siting gauges away from or above blowing seeds, pollen or leaves will minimize the problem. One of the most common reference gauge site designs is called a “Pit Gauge Site.” It is designed to locate the inlet gauge orifice at the surface of the ground as shown and thereby eliminate wind-related catchment errors. However, it must be continuously monitored to remove blowing debris and carefully constructed to prevent splash-back from nearby surfaces.
- Siting gauges in areas of low grass will reduce the maximum drop splash to less than 1/3 of a meter. Rocks or other hard surfaces can cause rain drop splashes to reach a height of 1 meter and contribute to errors in precipitation readings. Gauges at the USCRN sites are sited on a 3 foot high pole with the inlet orifice at 1.66 meters (65.35”) above ground in areas where snow accumulation does not require additional height. At this height drop splashes from hard surfaces should not affect the accuracy of precipitation measurements.