Rain gauge

Precipitation measuring device based on the weighing principle with draft shield

A precipitation gauge or rain gauge is an instrument for measuring the precipitate that has fallen in a given time interval. The snowfall is usually also recorded , as far as it melts to its water equivalent .

Other names are ombrometer ( Greek  ὄμβρος ombros 'rain' and -meter ), hyetometer (Greek ὑετός hyetos 'rain') or pluviometer ( Latin pluvia 'rain').

The precipitation gauge is part of every weather station . In meteorology it serves as an additional aid for weather forecasting and in climate science for the purpose of long-term statistics of precipitation.

history

The first western rain gauges were designed by Benedetto Castelli in 1639 and by Christopher Wren in 1661 . In fact, the development of the first pluviometer goes back to 1442 in what is now Korea under the then King Sejong .

Device types

A distinction can be made between analog and digital rain gauges.

In both cases, there are also variants with built-in heating, so that solid precipitation such as snow , hail or sleet can also be registered. The collecting funnel, the measuring device and the rain runoff can be kept frost-free with electrical heating foils and a temperature control.

Simple analog rain gauge

Rain gauge mark

Marking position x (mm) as a function of the amount of rain h (mm) for a conical rain gauge with a height of H = 320 mm

Simple analog rain gauges for home use are cylindrical vessels that are open at the top and are equipped with a readable scale. To increase the accuracy, the inlet opening is often in the shape of a funnel in order to catch a larger amount of precipitation or to reduce the influence of evaporation .

Conical rain gauges are mostly used on farms . They have the advantage that small amounts of rain can be read more precisely than large amounts of rain. The markings for small amounts of rain are further apart than the markings for larger amounts of rain. Such rain gauges typically have a height of z. B. . The marker position for the amount of rain is: ${\displaystyle H=320mm}$ ${\displaystyle x}$ ${\displaystyle h(mm)}$

${\displaystyle x={\sqrt[{3}]{3hH^{2}}}}$ 

With a conical rain gauge of height , a maximum of ${\displaystyle H}$

${\displaystyle h_{max}=H/3}$

be measured. With a rain gauge with a height of 320 mm, a maximum amount of rain of a little over 100 mm can be recorded.

Precipitation meter according to Hellmann

Precipitation meter according to Hellmann

In professional meteorology in German-speaking countries, the Hellmann precipitation meter is most often used, which was developed by the Prussian meteorologist Gustav Hellmann around 1886 . This rain gauge is made of stainless steel or zinc sheet and has a collecting area of ​​200 cm², limited by a sharp-edged brass ring, according to the standard of the World Meteorological Organization . The rainwater passes through a funnel into a collecting can in the lower part of the rain gauge so that it is largely protected from evaporation. For the measurement, the collected rainwater is filled into a measuring cylinder, which allows a determination to be made with an accuracy of 0.05 mm (1/20 mm). When snowfalls are expected, a so-called snow cross is placed in the upper part, which protects the falling snow from subsequent turbulence caused by strong winds. To measure, the entire precipitation gauge is exchanged for a second identical one and placed in a cool room with the lid closed until the solid precipitate has changed to a liquid state and can be measured as usual. Alternatively, a previously measured amount of water can be added to the solid, collected precipitate, which must be subtracted again after the measurement has been completed.

There is also a smaller version with a collecting area of ​​100 cm ² (without collecting can); In the mountains, rain gauges with a collecting area of ​​500 cm² are also used.

There are also registered versions of the Hellmann precipitation meter, which record the temporal course of the amount of precipitation without auxiliary energy over 24 hours to a month (so-called pluviographs). Here the water also reaches a collecting jug, in which there is a float, which transfers the water level to a writing arm and a writing sheet that is placed on a recording drum. After a precipitation height of 10 mm, the collecting can is emptied so that larger amounts of precipitation can also be recorded.

Digital rain gauge

Seesaw in a rain gauge

Automatic rain gauges usually use a "tilting spoon" or a tipping scale, also called a seesaw. In both systems, a bowl is filled with rainwater. At a certain weight, it tips down and empties. The amount of precipitation can be calculated from the number of tilting movements. The tilting is often detected electrically with a magnet on the tilting device and a reed contact fixed opposite . The fundamental difference between the tipping bucket and the tipping bucket lies in the design. The tilting spoon is shaped like a spoon and can only hold water on one side. The tipping bucket is more sensitive and has a two-part measuring chamber. This means that no raindrops are lost during the actual overturning process, as can be the case with a spoon.

A measurement method that has recently been implemented is the weighing principle. Here the precipitation arrives in a collecting tray, which is located on a sensitive scale. Since 1 ml of water weighs almost exactly 1 g, the amount of precipitation can be read off directly without a calculation. The advantage of the system is that there is no need for a heating device, since solid precipitates can also be measured immediately and do not have to be melted first. Another more recent measuring principle is the number and size of raindrops, hailstones or snowflakes measured by ultrasound .

Lineup

According to the guidelines of the German Weather Service, the height of the collecting area of ​​the precipitation measuring device is in the standard case 1 m above the ground. Due to the expected snow depth, the height at a station height over 500 m above sea level. NN at 1.5 m and at a station height of over 800 m above sea level. NN fixed at 2 m above ground.

For climatologically meaningful measurements, the precipitation gauge must be set up in a suitable location that is representative of the local climate . On the one hand, this must be sufficiently open so that even inclined precipitation can be reliably caught, on the other hand, a certain shielding from strong winds is necessary in order to keep the wind error caused by overflowing the collecting vessel as low as possible. Precipitation measurement, which is simple in principle, is subject to comparatively large errors in practice. In general, measurement uncertainties of 10 to 20% can hardly be avoided. Depending on the weather, the measurement results must be interpreted accordingly.

Differentiation from other devices

Devices for measuring the size distribution of raindrops and the time course are called disdrometers .

In contrast to rain gauges , rain sensors only determine whether and how much it is raining. The amount of precipitation is not measured.

literature

• Joachim Blüthgen: Allgemeine Klimageographie , Walter de Gruyter, 1980, ISBN 3-11-006561-4 , pp. 262–266 ( online book preview at GoogleBooks)
• Hannes Römer: Precipitation determination from remote sensing data , Section 1.3 (Conventional methods of precipitation measurement), GRIN Verlag, 2007 ( online book preview on GoogleBooks)

Web links

Commons : Rain gauge  - collection of images, videos and audio files

Wiktionary: Precipitation meter  - explanations of meanings, word origins, synonyms, translations

• Development and testing of a rain gauge with double tipping bucket
• World Meteorological Organization (WMO) website (in English)

Individual evidence

1. ↑ Renate Wahrig-Burfeind (Ed.): True. Illustrated dictionary of the German language . ADAC-Verlag, Munich 2004, ISBN 3-577-10051-6 , pp. 623 .
2. ^ John G. Robertson (Ed.): Robertson's Words for a Modern Age. A Cross Reference of Latin and Greek Combining Elements . Senior Scribe Publications, 1991, ISBN 0-9630919-1-3 , pp. 105 ( limited preview in Google Book search).
3. ↑ Duden online: Ombrometer
4. ↑ Duden online: Hyetometer
5. ↑ Duden online: Pluviometer
6. ↑ Deutscher Wetterdienst: Guideline for automatic part-time weather stations in the DWD - external edition (PDF; 192 kB), section 2.4.5, June 2017, accessed on April 30, 2019