## 4 Difference between Climate Normals and Climatologies
### Daytime and Nighttime Cloud Fraction  
Figures 4 and 5 show the similar results to Figure 1, but for the daytime and nighttime cloud fractions, respectively. Here, the differences between CN_WMO and the other variants of climatologies are clearly evident. The global mean daytime cloud fraction in CN_WMO is 0.62% and 0.79%, lower compared to CN_MB and Clim30, respectively (Figure 4). The differences are much stronger locally, and can even have different signs. Furthermore, CN_WMO could not be computed over the polar region in the northern hemisphere, extending as south as 45 degrees latitude. Figure 4: Same as in Figure 1, but for the daytime cloud fraction. In order to understand this data gap in CN_WMO and the differences between CN_MB and Clim30, one has to first note the definition of daytime cloud fraction in the CLARA-A3 climate data record. When the local solar zenith angles are less than 75 degrees, it is considered to be the daytime condition, and when they are larger than 95 degrees, it is considered to be the nighttime. Since the sun remains below the horizon in the Arctic in January, there is natural data gap resulting in the Arctic, as is visible in the Clim30 case in Figure 6 that shows the total number of years used to compute CN_WMO and Clim30, and the difference thereof. But, the data gap in CN_WMO extends even more southward to 45 degrees north. This is due to the fact that, in the latitude band between 45\({}^{\circ}\)N and 55\({}^{\circ}\)N, the solar zenith angle decreases every day throughout the January month, but remains below 75\({}^{\circ}\) in the first half of January. This means that the D5 and D11 conditions are not satisfied in this latitude band, as shown in Figure 7, resulting in the data gap. Figure 5: Same as in Figure 1, but for the nighttime cloud fraction. Figure 6: The number of years, out of total 30, used to compute CN_WMO and Clim30 in the case of daytime cloud fraction and the difference thereof. The computation of CN_WMO is further influenced by the orbital drift of NOAA satellites [52]. For example, the afternoon NOAA satellites have drifted significantly from their sun-synchronous orbits, causing increasingly delayed local observational times. The orbital drift can affect the partitioning among daytime, twilight, and nighttime conditions more strongly at the higher latitudes during polar winters, since the daily rate of change of solar zenith angle is higher there, and just a few hours delay in the local observational times can cross over the daytime threshold for solar zenith angles. This is also clearly evident in Figure 7 between the latitude bands 30\({}^{\circ}\)N to 45\({}^{\circ}\)N between 1991 and 2000, when data were available from fewer satellites and the NOAA-11 (1991-1995) and NOAA-14 (1995-2000) drifted considerably towards the end of their lifespans. This resulted in the gradual loss of data, spatially extending even more southwards towards the later years of their life. Figure 7 further shows that the data loss in the case of daytime cloud fraction due to orbital drift has not been uniform over the rest of the globe. This inadequate sampling, together with different diurnal cycles of various cloud regimes, lead to different signs of the difference between CN_WMO and Clim30, as seen in Figure 4.  
It is also to be noted that in the latter two decades (2001-2020), the data from multiple NOAA and MetOp satellites are available to compute daytime cloud fraction; thus, the impact of the orbital drift of an individual satellite is not clearly visible anymore during this period. Similar conclusions can also be applied to the climate normals of nighttime cloud fraction, as shown in Figure 5, except in this case, it is the southern polar regions that are affected by the problems associated with the orbital drift and solar zenith angles, as well as the solar contamination in the thermal channels. Furthermore, in the case of nighttime cloud fraction (Figure 5), the differences between climate normals and Clim30 are limited to only a certain latitude band.