MANUAL PRECIPITATION GAUGES:
Manual precipitation gauges are the oldest form of precipitation measurement device and the least
expensive. It is still the most commonly used precipitation measurement gauge even though it must be
maintained daily or more frequently to obtain accurate and reliable data.
How a manual precipitation gauge works: In its simplest form a manual precipitation gauge is a right
circular cylindrical container of a precise diameter than can be mounted in a way that collects
precipitation. It is then removed and weighed to enable calculation of the liquid equivalent
precipitation. A more advanced manual precipitation gauge is the Belfort Model 5- 400 precipitation
gauge. It was designed in the early 20th century to meet the requirements of the U.S. National Weather
Service specification No. 450.2301. It has four component parts. An 8 inch diameter funnel that funnels
water into a 2 inch diameter collector, that are held in place by an 8 inch diameter outer overflow tube
that is supported by a metal frame. It has a capacity of 2 inches (50 mm) in the collector and 20 inches
or 500 mm in the overflow container. The unit measures with a resolution of 0.01 inches (0.25 mm)
using the calibrated measuring stick. The inlet orifice is 32 inches or 0.81 meters above ground with the
mounting legs provided with the unit. Thousands of these are currently in use throughout the world
where the cost of daily maintenance and manual recordation of precipitation events is not prohibitive.
2 “D. Tube Funnel Outer Overflow Tube
Calibrated Measuring Stick
Assembled Gauge on Stand
By funneling the precipitation into the smaller diameter tube the amount of evaporation error is
minimized and the measurement is amplified by the difference in area between the inner collector and
the outside diameter of the funnel collector in this case a factor 0f 16. (An increment of 0.01 inches (0.254 mm) of
precipitation would increment 0.16 inches (4.06mm) on the calibrated measuring stick).
Advantages:
1. Low acquisition cost and no electrical power requirement unless heated.
2. Low maintenance cost, no replacement part requirement, right circular cylinder geometry
minimizes wind induced catchment error
3. Low evaporative loss and due to small inlet restriction of funnel and frequent maintenance.
4. It does not require oil/antifreeze
5. Long operating life-many units are in field after 50 years of operation
6. Minimal contamination errors as unit must be cleaned with each measurement
7. High resolution, if well maintained, 0.01 inch (.25 mm) possible
Disadvantages:
1. High operating cost requires manual data recording and emptying of catch cylinders, often twice
a day or more during heavy precipitation periods.
2. Poor solid precipitation measurement as snow or ice will block inlet at restriction, unless heated
or attended and the snow or ice melted and measured.
3. Some loss of measurement data possible during data collection period.
4. Will not record short term rate of precipitation
5. Wetting losses as much as 0.2 mm per observation may materially impact accumulative
measurement. (WMO Guide, 2008)
6. Data must be observed by trained personnel and accurately recorded