How do you determine the amount of precipitation that has fallen and what problems can arise when measuring?
Precipitation is water that falls from the atmosphere and reaches the ground, in liquid or solid form – such as rain, snow, sleet or hail. Knowing the amount of precipitation occurring from place to place over a certain period of time is of great importance, for example for agriculture, forestry or water management. How much water benefits the soil and is therefore available to the plants or how large is the runoff in the watercourses? For this reason, and above all because the amount of precipitation often varies greatly across the area, there are a relatively large number of measuring stations. Government agencies alone, including the German Weather Service and some state authorities, record precipitation at around 4,000 locations in the republic.
The classic measuring device is the Hellmann rain gauge. It consists of a tubular metal housing into whose upper opening, which is located at a height of 1 meter, the precipitation falls and enters a collecting vessel via a funnel. The area of the opening is uniformly 200 square centimeters. In this classic design, the weather observer has to empty the vessel at regular intervals into a measuring glass that has a scale that can be used to directly read the total amount of precipitation (in millimeters or liters per square meter) since the last observation. If precipitation occurs in the form of snow, sleet or hail, this may need to be melted first.
In addition to the total amount of precipitation during an observation period, the temporal course of individual precipitation events is also of interest. Registering devices that can be operated automatically are used for this purpose. For example, a tipping scale can be attached between the funnel and the collecting vessel, which in turn consists of two small collecting trays (see our illustration). The water first drips from the funnel into one of the two bowls. When it is full, the scales tip, the bowl empties and the precipitate begins to fill the other bowl, and so on. For most devices, a quantity of 0.1 mm of water is required for a tipping process. With the help of an electrical circuit, the tipping processes are counted and in this way a value for the amount of precipitation per unit of time is calculated. Since the tipping scale only works with liquid water, the measuring device is heated in order to immediately melt any solid precipitation that may be present.
Depending on the weather situation, more or less large errors occur when measuring precipitation. Large deviations from the actual values can occur, especially in strong winds and therefore unfavorable flow conditions in the measuring device’s collection area. In addition, particularly at high temperatures and strong sunlight, some of the collected precipitation can be lost through evaporation. The design of the devices and the choice of installation location attempt to keep errors as small as possible. With a brightly painted and almost closed housing, the wind is to be kept out and excessive heating is to be prevented on the other hand. The attached funnel reduces the only opening to the environment to a minimum. To ensure that precipitation (particularly hail) does not fall into the collecting area, its edge is beveled inwards. In order to reduce snow blowing out, you can insert a so-called snow cross into the collecting opening in winter. The choice of location is also important for measurements that can be compared with one another. In the best case scenario, the rain gauge should be twice as far away from obstacles as they are high so that the precipitation reaches it as unhindered as possible.
February 7, 2024 (release date)