Rain gauge according to Hellmann

Analog rain gauges are open-topped, cylindrical vessels equipped with a readable scale. To increase accuracy, the inlet opening is often funnel- shaped to capture a larger amount of precipitation or reduce the impact of evaporation .

Rain gauge according to Hellmann

In meteorology, the Hellmann rain gauge is the most commonly used type in German-speaking countries. It was developed by the Prussian meteorologist Gustav Hellmann in 1886. This rain gauge is made of stainless steel or zinc sheet and has a collection area of ​​200 cm² defined by a sharp-edged brass ring, in accordance with World Meteorological Organization standards. The rainwater flows through a funnel into a collecting can in the lower part of the rain gauge, thus protecting it from evaporation. For measurement, the collected rainwater is poured into a measuring cylinder, which allows measurements to be made to an accuracy of 0.05 mm (1/20 mm). If snowfall is expected, a so-called snow cross is placed in the upper part to protect the falling snow from subsequent turbulence caused by strong winds. For measurement, the entire rain gauge is replaced with a second identical one and placed in a cool room with the lid closed until the solid precipitation has changed into the liquid state and can be measured as usual. Alternatively, a previously measured amount of water can be added to the solid, collected precipitation, which must be subtracted again after the measurement is completed.

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

There are also recording versions of the Hellmann rain gauge, which record the temporal progression of the rainfall over a period of 24 hours to a month without auxiliary power (pluviographs). Here, the water also flows into a collecting can containing a float that transfers the water level to a recording arm and a recording sheet mounted on a recording drum. After a rainfall depth of 10 mm, the collecting can empties, allowing even larger amounts of rainfall to be recorded.

Digital rain gauges

Rocker in a rain gauge

Automatic rain gauges usually use a "tipping bucket" or a tipping bucket, also called a seesaw. In both systems, a bowl fills with rainwater. When it reaches a certain weight, it tips downwards and empties. The amount of precipitation can be calculated from the number of tipping movements. The tipping movements are often recorded electrically using a magnet on the tipping device and a reed switch fixed opposite. The fundamental difference between a tipping bucket and a tipping bucket lies in their design. The tipping bucket 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 tipping process, as can happen with a spoon.

A more recently developed measurement method is the weighing principle. Here, the precipitation falls into a collecting tray mounted on a sensitive scale. Since 1 ml of water weighs almost exactly 1 g, the amount of precipitation can be read directly without any calculations. The advantage of this system is that a heating device is unnecessary, since even solid precipitation can be measured immediately and does not have to be melted first. Another more recent measurement principle is the ultrasound measurement of the number and size of raindrops, hailstones, or snowflakes.