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. 2019 Feb 4;9:1391. doi: 10.1038/s41598-018-37817-8

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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The two upper graphs (a) and (b) show the angular luminance distribution measured near Bonnievale, South Africa, for (a), the sky segment facing the southwest (red area in Fig. 3) and for (b) the sky segment facing the northeast (blue area in Fig. 3). Each graph shows clear night (blue diamonds) and overcast night (black circles) data. The variability slightly off zenith in the clear sky data originates from bright stars. Note the large difference in scale between the two upper graphs. The lower graph shows the cloud attenuation factor, for the two selected sky segments. Grey circles represent the north-eastern segment and black squares the sky segment facing the south-west.