WMO QUESTIONNAIRE ON RECORDING PRECIPITATION GAUGES
B. Sevruk and J. Michaeli*
Atmosphere and Climate ETH, Winterthurerstr. 190, CH- 8057 Zurich, Switzerland, 0041 1 6355235:
Sevruk@geo.umnw.ethz.ch: * Meteorological Service P.O. Box 25, Bet Dagan 50250, Israel: jacobm@ims.gov.
The Commission of Instruments and Methods of Observation, CIMO, of the World Meteorological
Organization, WMO aims the global standardisation of meteorological observations and improving
the long-term quality of measurements. With this in view CIMO prepared in collaboration with the
first author a questionnaire on the common (national-standard), manual precipitation gauges in
1987-1988. 136 (out of 160) countries responded. Sevruk and Klemm (1989) published the results.
They showed that there were more than 50 types of manual, national standard precipitation gauges
used at that time all around the world. They differ considerably in design, shape, size and material.
The orifice area varies from 7 to 1000 cm2, most gauges having an area of 100-200 cm2. Materials
used for gauges were primarily galvanised iron sheets and copper but plastic was popular as well.
The installation height varies between countries from 0.2 to 2.0 m. Up to 90 countries use
installation heights of 1 m and less. 2.0 m was used only in the countries of the former Soviet Union
and 0.3 m in the former British Empire. Seven types of gauges are permanently fitted with a
windshield of different design. The most widely used gauges appear to be the German Hellmann
gauge, followed by the Chinese type and the English (Snowdon) Mk2 gauge. These types account
for one-half of all precipitation gauges but they are concentrated in an area of only 31x106 km2. The
replies showed that in 50 countries changes in national standards such as the replacement of the
long-used type, lowering or increasing the installation height took place in the last 50 years. This
can affect the quality of measurements and cause inhomogeneities in the precipitation time series.
The most decisive change was related to the automatisation of meteorological observations and the
replacement of a certain number of manual, standard precipitation gauges with the recording
precipitation gauges, RPG in some developed countries. Yet it is more difficult to measure and
process precipitation intensity in the national networks than daily totals of precipitation. The
demand for such data has resulted in various instrument design and operational data processing and
archiving procedures but not much has been done to analyse long-time series of data and to
investigate the RPG reliability and accuracy of the measurements. To get an insight also into this
matter CIMO prepared in collaboration with the author and experts from the WMO Commissions of
Hydrology and for Agricultural Meteorology, the second CIMO questionnaire, this time on RPG. It
was sent to all Members of the WMO (some 180 countries plus five territories). 118 responses from
109 countries were received. In some countries (Austria, Costa Rica, Italy, Jamaica, Libya,
Seychelles, Turkey and Venezuela) there are separate hydrological, meteorological and military
RPG networks and each of them have send the completed Questionnaire. Consequently, there are
more responses than countries. All in all, there are nine groups of questions including the type of
RPG, manufacturer, measuring system applied, total number of RPG of a given type used in the
country, methods of recording and data transmission, further orifice area size and installation
heights, heating and windshield application etc. One of the crucial questions was if there is need to
organise intercomparisons of RPG measurements. Such an international intercomparison provides
important material on the accuracy, reliability and corrections of the measurements of a given type
of RPG as can be seen from a paper presented in this volume (Chvila et al., 2002). Up to now, the
CIMO/WMO organised two intercomparisons of precipitation measurements and participated in one
(Sevruk and Hamon, 1984; Sevruk, 1993; Goodison et al., 1998).
The interest is quite a strong one in 50 countries which agreed not only to participate but
even to organise such an international intercomparison on RPG measurements. Almost all countries
(105 out of 109) find the intercomparisons useful and only 15 (out of 109) decided not to
participate. The interest from developing countries might reflect both the importance and problems
of precipitation intensity measurements for the agriculture and the forecasts of floods in the arid,
sub-tropic and tropic climate regions. Other questions focused on more technical matters as shown
in Table 1.
Table 1. System of measuring, number of gauges and countries with most recording gauges.
System of measuring
Number
Number of Countries with most recording gauges
of gauges manufacturers
Float system
15.044
20
5000 China, 3050 Mexico, 820 Venezuela, 750
Russian Federation, 660 Colombia, 580 India,
Tipping-bucket system
13.481
38
5000 China, 1316 Japan, 1100 Australia, 1000
France, 960 UK, 460 Rep. Korea, 358 Canada
Weighing system
748
7
172 Canada, 125 Turkey, 122 Brazil, Drop
counter system
176
2
89 Germany, 45 Spain, 26 R. Korea, 15 Turkey
Optical gauges
165
2
104 Sweden and 61 Saudi Arabia
In addition, there is a hybrid gauge of drop counter and tipping bucket as used in Hong Kong in China.
All in all, there are about 30.000 RPG as used in 109 countries. Considering the 70 countries which
did not responded the Questionnaire (e. g. the USA with at least 3500 RPG, s. Sevruk and Klemm,
1989), the global number can amount to more than 40.000. This is roughly one-fifth of all standard
gauges of 200.000 as assessed by Sevruk and Klemm (1989). This number will increase
considerably in the next future. In the above-mentioned numbers the RPGs as used by private and
communal organisations are not included. The total numbers of the float RPG and tipping-bucket
RPG are more or less the same but the number of the modern weighing RPG is very modest, indeed.
Yet in the last two years the number increased to some thousands! Only such gauges are capable to
report five minutes intensities as recommended by the WMO. Most RPGs are to be found in
developing countries as shown in Table 1.
The number of manufacturers (local and international) is unexpectedly high (Table 1). All in
all it amounts to 72 (38 for tipping bucket, 20 for floater, 7 for weights etc.). The forerunners are
German companies (Lambrecht supplied 33 countries; Thiess, 16, Fuess, 11 and Ott, 4 countries),
followed by the Casella (UK), 43 countries, Precis Mechanique (France) 11 and Siap (Italy) 9
countries. In many countries different types of RPG from different manufacturers are used side by
side. Most RPGs originate from the Chinese Companies, the Shanghai Meteorological Instrument
Factory and the Tianjin Meteorological Marine Instrument Factory (5000 gauges each).
The oldest method of precipitation recording, the pen on time chart is still used in most
countries (94), followed by the modern data logging (58) and combined with other methods.
Punched in paper tape is used only in five countries (Armenia, Canada, Indonesia, Mongolia,
Philippines). Still, the most used method of data transmission is the post (77 countries), followed by
telephone (57), radio (35) and satellite (16). But data loggers, telex, e-mail, telegraph, cell phones
and PC networks are also used. Usually different methods are combined in the same country.
Table 2 shows a great variety of installation conditions. Considering installation heights
differences exist not only among the countries but also in the same country. In many cases
installation height, IH, depends on the gauge type used. For instance, in 47 countries different types
of RPG and IH are applied. The IH ranges from 30 to 600 cm. Three different IH are used in
Mexico, (60, 124 and 600 cm), Australia (30, 100 and 300 cm), Germany (100, 150, 200 cm),
Greece (120, 140, 180 cm), Sweden (150, 175, 200), Venezuela (120, 150, 180 cm) etc. In 15
countries, IH of 30 cm and in 19 countries, around 200 cm is in use. Probably, the most frequent IH
is that of 100 cm (26 countries), followed by 200 cm (19 countries) and 150, 180 and 200 cm (44
countries). In 19 countries IH of less than 60 cm is applied.
Table 2. Number of sizes of orifice areas [cm2] and installation heights [m] of precipitation gauges
Orifice area 100 200-250 314-340
400-470
500-550
650-980
1000-2000
Number (171) 26
51
26
20
27
12
9
Installation height
0.3-0.5
0.5-1.0
1.0-1.5
1.5-2.0
2.0-6.0
Number (166)
26
52
58
26
4
Number in brackets refers to the total number of sizes of orifice areas and installation heights, respectively.
There is a tendency to use a bigger size of the gauge orifice area than in the case of standard gauges
(Sevruk and Klemm, 1989). An orifice area of 2000 cm2 is used in 4 countries (Madagascar,
Morocco, Rwanda and Venezuela); 1000 cm2 in 8 countries, 650-750 (11 countries); 500 (27); 400
(14); 320 (23); 200 (37) and around 125 cm2 in 17 countries. In some countries (Cyprus, Czech
Republic, Hong-Kong) RPG with different orifice areas, up to four sizes are used. Almost 101
countries do not use any type of windshield. It is deployed only in Belgium, Canada, Switzerland
and Sweden (modified Nipher type), Denmark, Republic of Korea (Alter type) and in Kazakhstan
and Estonia (Tretyakov type). In Canada and Sweden two types, the Nipher and Alter windshields
are used. All in all, the heating is applied for snow measurement, at least partly, in 29 countries.
The results of the Questionnaire show that there is a large variety of construction parameters
of RPGs including measuring system, orifice area size, installation height etc. as used by national
meteorological services not only world wide but even in the same country. This indicates that there
are good reasons for international RPG intercomparison measurements as shown also by Chvila et
al. (2002) and in the same time it confirms the need for such intercomparisons as has explicitly been
expressed by replies of almost all countries. The reasons are shortly discussed below:
(i) Precipitation measurements using elevated can-type gauges are subject to systematic
errors mostly due to wind field deformation above the precipitation gauge orifice, wetting and
evaporation losses, splash-out and splash-in, blowing snow etc. An elevated precipitation gauge
systematically distorts the wind field and forces the wind speed to increase over the gauge orifice
(blocking effect). Due to this adverse wind action, some of the lighter precipitation particles are
borne away before reaching the gauge and are lost for the measurement. The wind-induced loss
depends on wind speed, the weight of precipitation particles, i. e. the intensity of precipitation and
the gauge construction parameters. An example for RPG is show by Chvila et al. (2002). The use of
windshield and small installation height of the gauge can help to reduce the wind speed in the level
of the gauge orifice. For shielded gauges the wind-induced loss for snow can be reduced to one-half
of its value for unshielded gauges and to 70 % for mixed precipitation (Goodison et al., 1998).
Summing up, the wind-induced error is small for large intensities, small installation heights, gauges
with windshield or as placed at protected gauge sites. In contrast it is large for small intensities,
large installation heights, unshielded gauges and gauges placed at exposed sites. The wind-induced
loss amounts on average to 2 - 10 % of measured values of rain and up to 60 % of snow for
unshielded gauges and wind speeds greater than 4 ms-1 (Goodison et al., 1998). In addition to
weather conditions, the wetting loss depends on age, material of the inner walls and the depth of the
gauge collector relative to its diameter. The shape of the gauge orifice rim can have a certain effect
too. The evaporation loss depends also on the gauge construction parameters as shown by Sevruk
and Klemm (1989).
(ii) Due to the systematic measurement error, precipitation gauges of different construction
parameters including the size of orifice area, the shape of orifice rim, the use of windshield etc.
show different precipitation figures (Sevruk, 1997). This might also be the case even if the gauge is
of the same construction but from different manufacturers or as installed at different height at the
same site near to each other. Because there are different types of RPG as used in the same or
different countries, the global and local precipitation intensity data sets are hardly compatible
(Sevruk, 1994). Precipitation figures and intensities among the countries as well as in the same
country show systematic differences, according to the type of RPG, the installation height used and
the degree of gauge site exposure. To eliminate these spatial inhomogeneities of precipitation time
series, the performance of RPG has to be checked and the precipitation measurements corrected.
Similarly, an exchange of different types of RPG at the same gauge site or moving the gauge to a
new gauge site with different exposure can cause also inhomogeneities. For this reason, the WMO
recommends to carry out intercomparison measurements of a given gauge type with the WMO
reference standard.
(iii) Correction procedures are based on field intercomparison measurements using the
WMO reference standards (Sevruk and Hamon, 1984; Goodison et al., 1998, Chvila et al., 2002).
Recently, the numerical simulation as described by Nespor and Sevruk (1999) is used to derive
correction procedures as well. The results of both methods agree well Sevruk and Nespor (1998).
(iv) No WMO reference standards exists for the RPG.
(v) Because of non-consistent data sets only very few attempts have been made up to now to
compile global or large-scale climatologies of precipitation intensities.
(vi) The national meteorological services of some of developed countries attempt in the next
years to reorganise the precipitation measurement networks, which consist of conventional (manual)
gauges and RPG and to increase the number of RPG. Specific attention should be given to
climatological applications. For this aim they need a new better type of RPG as has been in service
in the last 30 years. Mostly it was the tipping-bucket gauge. This type of gauge has a number of
shortcomings. Usually, the reporting interval exceeds 5 minutes. Moreover the snow measurements
are recorded with delay needed to melt the snow by heating. In addition, the loss due to evaporation
is considerable. It is not calibration-free, has frequent failures and generally shows less precipitation
than the common types of gauges (Sevruk, 1996). The operational costs are high. The results of a
WMO intercomparison of RPG measurements would give a good guidance for the selection of the
most reliable and modern type of RPG to be used in national meteorological networks.
The Questionnaires provides a global overview of very useful metadata, which are otherwise
difficult to become. It shows that there is a great variety in instruments and methods of observations
of precipitation intensity used not only worldwide but also in the same country. This variety exceeds
by far the variety in manual standard precipitation gauges. In addition, the recording precipitation
gauges have generally larger orifice areas and installation heights than the standard gauges. This
indicates that the systematic error of precipitation measurement using recording precipitation gauges
is larger than that of standard gauges. This has serious consequences for the measurement accuracy
and consistency of local and global time series. Quite recently a WMO Expert Meeting on
Precipitation Intensity Measurement and Processing was organised with the aim to discuss the
following subjects (WMO, 2001):
(i) Field intercomparisons of recording precipitation gauge measurements; (ii) The WMO
reference standard of recording precipitation gauges measurements; (iii) Correction procedures of
the systematic error of precipitation measurements; (iv) Procedures for manually extracting digital
data from charts; (v) Quality control of data; (vi) Data representation formats; (vii) New types of
self-calibrating recording precipitation gauges based on electronics balance. The recommendations
will be used to organise WMO Precipitation Intensity Intercomparison Measurements. An example
of such an intercomparison is described in this Volume by Chvila et al. (2002).
Chvila, B,, Sevruk, B. and Ondras, M. (2002). The wind-induced loss of precipitation measurement of small time
intervals. (See this Volume).
Goodison, B. E., Louie, P.Y.T. and Yang, D. (1998). WMO solid precipitation measurement intercomparison. World
Meteorol. Org., Geneva, Switzerland, WMO/TD-No. 872, 212 pp.
Nespor, V. and Sevruk, B. (1999). Estimation of wind-induced error of rainfall gauge measurements using a numerical
simulation. J. Atmos. Ocean. Tech, 16(4), 450-464.
Sevruk, B. (1993). WMO precipitation measurement intercomparisons. In: B. Sevruk and M. Lapin (co-editors):
Precipitation Measurement and Quality Control. Proc. Symposium on Precipitation and Evaporation, Vol. 1,
207 p., Slovak Hydrometeorological Institute, Bratislava and Swiss Federal Institute of Technology,
Department of Geography, Zurich.
Sevruk, B. (1994). Spatial and temporal inhomogeneity of global precipitation data. In: M. Desbois and F. Desalmand
(Eds.). Global Precipitation and Climate Change. NATO ASI Series, Vol. I 26, 219-230, Springer Verlag.
Sevruk, B. (1996). Adjustment of tipping-bucket precipitation gauge measurements. Atmos. Research, 42, 237-246.
Sevruk, B. and Hamon, W. R. (1984). International comparison of national precipitation gauges with a reference pit
gauge. WMO/CIMO Instruments and Observing Methods Rep., No. 17, 135 pp.
Sevruk, B. and Klemm, S. (1989). Catalogue of national standard precipitation gauges. World Meteorological
Organization, Instruments and Observing Methods Rep., WMO/TD-No. 313, 50 pp., 1989.
Sevruk, B. and Nespor, V. (1998). Empirical and theoretical assessment of the wind-induced error of rain measurement.
Wat. Sci.Tech., 37(11), 171-178.
WMO (2001). Expert Meeting on rainfall intensity measurements. Bratislava, Slovakia 23 to 25 April, 2001.
WMO/CIMO, Geneva.