At the same location the natural GDR is usually more or less constant over time. Figure 1 displays a typical time series of the GDR at the monitoring station 010590751 in the city of Schleswig for the time interval from 13. April 2011 to 15. April 2011. No rain-events, no snow-coverage, and no technical interferences are visible there.

Short period peaks of the GDR, up to a factor of 2, occur when radioactive progeny (daughter nuclides) of the radioactive noble gas radon (Rn-222) are deposited on the ground by rain. Such events are usually correlated with precipitation. Typically the increase is steep and the signal asymptotically returns to the normal level within a few hours. The typical half-life time of this decay is about 30 minutes.

In Figure 2, the effect of some rain-events in a time series taken at a monitoring station in the village of Todendorf is shown. Beside the GDR (black line), the time series of a corresponding weather radar signal (measured by the DWD, the German Weather Service) is shown (red line). The signal height of the weather radar is proportional to the density of clouds. A high density most likely indicates precipitation. Coinciding with the radar signal one can see the short-lived increase of the GDR.

Snowfall, like rain can also lead to an increase of the GDR, particularly if the ground previously was not covered by snow. In addition to short-term variations, the GDR often shows also long-term variations within the course of the year. A layer of snow or ice on the ground generally causes the GDR to decrease because the radiation originating from the ground is partly absorbed by the snow layer. With the onset of thawing, when the snow is melting, the GDR increases until it reaches its previous level again. Similarly, soil moisture, which in turn depends on the local geology, soil surface characteristics and on microclimate conditions, can lead to variations of the GDR of up to some 0.01 μSv/h (micro Sievert per hour).

Variations of the measured GDR

Radiological events or technical problems can cause variations of the GDR much larger than the natural effects described above. Technical problems (e.g. long periods of mains failure, uninterrupted power supply battery fault, degradation of a Geiger-Müller tube, etc.) as well as radiological events are recognised, when the GDR exceeds a given threshold-value. The data logger locally detects such variations and problems and sends an automatic warning. In the case of a threshold warning the operator on duty will be alarmed immediately if two such events occur within one hour and at less than 30km distance apart. During routine operation of the monitoring network, all measurement results exceeding the given thresholds (automatically labelled as: ''to be examined'') are checked by a human operator daily. Hence problems and radiological events will be noted immediately. Since the low-dose Geiger-Müller counters are very sensitive they are also very susceptible to many kinds of perturbations. Interferences in the measurement-chain (probe, data logger, voltage-supply) or a lightning event close to the measuring station can influence the GDR. In Figures 4 - 8, some interferences are presented that occur quite often.

A typical behaviour of a GDR time series from a broken low-dose Geiger-Müller-counter is shown in Figure 4. After normal behaviour, the values suddenly start jumping atypically and stepwise. In this particular case the malfunction is explained by a malfunction of the high dose tube arbitrarily disturbing the data. The data logger detects this failure as the breakdown of a counter tube and sends a corresponding spontaneous message to that monitoring network node of the BfS (Bundesamt für Strahlenschutz, Engl. Federal Office for Radiation Protection) that is responsible for this monitoring station.

If the data logger does not get signals from either the low-dose or the high-dose Geiger-Müller-counter, the calculation and subsequent transmission of GDR-values by the data logger is no longer possible. The most common cause for such a coincidental missing of signals from both Geiger-Müller-counters is a cut in the cable between probe and data logger. Another typical cause for such a failure is the breakdown of the probe's electronics.

At locations extraordinarily exposed to wind, it is possible that the probe oscillates on its pole. When the resonance frequency is met, the anode-wire of the low-dose Geiger-Müller-counter also oscillates and the distance between the anode-wire and the cathode becomes close enough for spontaneous gas discharges, i.e. sparks. These sparks mimic real signals induced by gamma rays and consequently are interpreted as such by the data logger. There is no regular pattern either in amplitudes or shape. The corresponding augmentations might stop after several hours and may start again after a while (see Figure 6). In order to fix this problem the probe is exchanged or a more massive standpipe is used. Eventually as a last resort, the probe is moved to a less exposed position.

Significant regular increase of the signal might be ''measured'' due to a so-called echo effect. The GDR, suddenly jumps to values roughly twice as high as before (see Figure 7). Typically, an hour later the GDR returns to the previous level. This phenomenon predominantly shows up at higher temperatures and with older Geiger-Müller-counters with old counting gas.

A regular gamma-ray induced pulse into the counter tube is then followed by one, sometimes even more echo pulses. This malfunction of the probe is counted by the electronics as a normal signal and thus, is giving an approximately doubled GDR-value. Another counter detects the occurrence of the echo due to the small time interval between both pulses and counts the number of echo signals separately. Thus the malfunction can be detected quickly and the probe can be replaced.

Sometimes single spikes up to several 0.1 μSv/h can be found in a GDR time series which do not follow any chronological regularity (see Figure 8). Most of such spikes come from electromagnetic perturbations captured by the cables between probe and data logger. With the newer, technically more advanced probes such spikes have become very rare.