An analysis of the annual pattern of daily sunshine duration measured in Krakow using the Campbell-Stokes sunshine recorder and the CSD3 sunshine duration sensor revealed that the overall sunshine duration outlooks had similar patterns, but the automatic sensor typically showed higher values, especially on those days with the maximum solar radiation (spring and summer). The latter is explained by the high sensitivity of the automatic sensor, which is capable of recording direct radiation even when clouds briefly receded exposing the Sun’s disc, which the traditional sunshine recorder cannot do. A similar effect is noted when the Sun is low over the horizon. Indeed, when the Sun is less than 3–5o over the horizon, i.e. a short time after sunrise or before sunset, it delivers less than the CS threshold radiation intensity (279.2 W m−2), and the heliograph records no sunshine. In this same situation, the electronic sensor already starts to react at a radiation intensity of 120 W m−2. For this reason, on cloudless days in the warm half of the year, the CSD3 tends to record nearly 2 h more sunshine than does the Campbell-Stokes sunshine recorder. In winter, the Campbell-Stokes sunshine recorder’s sphere may be frozen over and record no sunshine on a sunny, but cold day. In autumn and winter, when the Sun is low and predominantly covered by Stratus-type clouds, the CS often fails to register any sunshine while CSD3 does, thus producing sunshine values higher than the traditional instrument.
However, there are also days when the CSD3 records up to 1 h less sunshine. This is possible when a series of quickly moving Cumulus-type clouds cover and uncover the face of the Sun in rapid succession. A Campbell-Stokes sunshine recorder will show a continuous burned track with a local narrowing (on average 0.9–1.0 h), while a CSD3 records less sunshine (0.6–0.8 h). An even greater difference occurs with more vertically developed clouds, such as Cumulus congestus. In addition, with thick, layered clouds of higher tiers (Altostratus and Cirrostratus), which cause scattered radiation, the Campbell-Stokes sunshine recorder records sunshine while the CSD3 does not. This is due to the low level of direct radiation involved. The latter instrument’s design principle means that higher levels of scattered radiation result in lower indicators of sunshine. The effect of Altocumulus translucidus results in sunshine being recorded by the CSD3 and in a thin track on the Campbell-Stokes sunshine recorder’s recording strip, but the values indicated by the two instruments are often different.
The greatest differences (both positive and negative) between sunshine duration values measured with the CS and CSD3 occur in the presence of clouds of different tiers (Cirrus, Cirrocumulus, Cirrostratus, Altocumulus, Stratocumulus and Cumulus), especially in spring and in September.
Comparison of the hourly intervals of sunshine duration makes it apparent that the two devices show the highest agreement in summer at midday when the sky is cloudless or only covered by Cirrus clouds. Both instruments also show agreement in recording no sunshine on fully cloudy days with stratus clouds (Altostratus, Stratocumulus or Stratus opacus).
The results obtained in Krakow confirm the finding of Kerr and Tabony (2004) that the greatest differences between the measurements taken by these two instruments occur in the presence of scattered clouds and depend on the elevation of the Sun over the horizon. No differences were found with the Sun high over the horizon during either cloudless or fully cloudy weather. The results are also compatible with patterns described in The Weather Observer’s Handbook by Burt (2012), who wrote that when compared to CS, the CSD3 sensor tended to record more sunshine in winter, but less in summer. Pokorný and Vaníček (2007) compared sunshine duration values recorded at Czech weather stations using CS sensors and automatic sensors. Although the automatic instruments used by the Czech weather service (DSU12 SD4 and SD5) were different to those used in Krakow, the authors clearly confirmed that the introduction of automatic sensors broke the homogeneity of the sunshine duration record. Similarly, as shown in the study by Kejna and Uscka-Kowalkowska (2006), monthly and annual sunshine duration values in Krakow are greater from AUTO than from CS sensors. This may also be caused by the specific local conditions in Poland, including a high proportion of stratus clouds and would warrant a further study involving different locations and different weather types. Indeed, all of the studies cited above noted the need for this due to the difficulty of the topic and the need to calculate credible conversion ratios. Obtaining reliable ratios would be very difficult due to the fact that the differences in the sunshine duration values from the two instruments are far from constant and depend on a range of factors, such as the type of clouds, elevation of the Sun and local conditions.