A World Meteorological Organization (WMO) publication, (Sevruk et al 1989) indicates that there are over 50 different types of national standard precipitation gauges used by 136 countries throughout the world. “Precipitation is the only meteorological variable that does not have a standard instrument approved.” (Groisman,P.Ya 2012). There is so much variability in the gauge geometry, measurement technique and siting requirements that it is almost impossible to correlate precipitation data from one country to the next. Measurement inaccuracies in total annual precipitation are estimated to be as high as 70% (Groisman,P.Ya 2012). If this well respected group of scientists has been unable to agree on what type of precipitation gauge is most effective for meteorological applications how are you to make a selection decision. This summary is intended facilitate your decision process by giving you factual information about 3 types of precipitation gauges commonly used by government agencies responsible for collecting meteorological and hydrological information.
There are three types of precipitation gauges commonly used as measurement standards throughout the world:
- Manual Precipitation Gauges
- Automatic Precipitation Gauges
- Automatic Tipping Bucket Precipitation Gauges
In a 2010 WMO survey by Nitu and Wong of 54 WMO members, 18% of respondents used automatic gauges and 82% used manual gauges. Of the automatic gauge users in this study, 82.9% were tipping bucket gauges and 16.2% were weighing gauges.
Each if properly maintained and sited may provide accurate indications of precipitation levels at that location.
Historically these gauges have a variety of geometries and ranged in size from 4 inch (102mm) inlet diameter by 6 inches (153mm) high to 20 inch (508mm) inlet diameter by 4 foot (1219mm) high. Today the WMO standard inlet diameter is 6.28” (15.95 cm) or an orifice area of 200 cm2. In the United States the historic National Weather Service (NWS) standard inlet diameter is 8” or 12” (324 cm2 or 729 cm2 inlet area). They are made of several materials designed to withstand environmental wear over extended periods of time including, brass, stainless steel, and certain ultra violet protected plastics. Most are designed to mount on a free standing pole with the inlet orifice 1 to 2 meters above ground although several are designed for surface mounts. For over 135 years Belfort Instrument Company has manufactured precipitation gauges that meet the highest standards in the industry and currently manufactures each of the three types of precipitation gauges to meet the requirements of the NWS and WMO. We will use the three types of Belfort Gauges as examples in this analysis as it is not intended to be a comparison of gauge manufacturers.
What type of precipitation gauge meets your requirements?
After reviewing the above cited advantages and disadvantages for each type of gauge; an orderly selection process will usually be dictated by resource constraints and measurement conditions.
In the case of precipitation gauge selection the primary resource constraints are:
- Availability of trained maintenance personnel
- Procurement and Installation Cost
- Available electrical power at the gauge site
The site measurement conditions that impact the gauge type decision are:
- Maximum Annual Precipitation
- Maximum Rain Rate
- Maximum Snow Rate
Once a gauge type is selected based on resource constraints and measurement conditions selection of a gauge design that meets programmatic accuracy and reliability requirements will require a more detailed comparison of available manufactures offerings and siting criteria, subjects of another Belfort application note.
- B. Sevruk and S. Klemm, WMO Instruments and Observing Methods Report #39, 1989
- P.Ya. Groisman, Precipitation, Hydrological Cycle , Vol . II, Encyclopedia of Life Support Systems , 2002-2012.
- NOAA, “The Tipping Bucket Rain Gauge”, 1995 www.nws.noaa.gov/asos/tipbuck.htm,
- WMO, Guide to Meteorological Instruments and Methods of Observation, WMO No. 8, 2008
- Nitu, R. and Wong, K , WMO Report 102, CiMO Survey on National Summaries of Methods and Instruments for Solid Precipitation measurement at Automatic Weather Stations, 2010