Dataset abstract Scattered sunlight measurements can be strongly affected by clouds, mainly due to multiple scattering effects that give rise to large uncertainties in the light path retrieval. In order to estimate cloud cover, we recorded images of the sky every 5 minutes to evaluate a cloud index, from 0 (clear sky) to 10 (completely overcast). This dataset presents the sky images in PNG (Portable Network Graphics) format recorded on board the R/V Akademik Tryoshnikov during the austral summer of 2016/17 as part of the circumnavigation expedition (ACE). Data coverage is from December 2016 until April 2017. These images form a supporting dataset to optical spectroscopy data (Benavent et al., 2020; DOI 10.5281/zenodo.3827443). The ship’s position can be matched with these images using the corrected cruise track (Thomas and Pina Estany, 2019; DOI: 10.5281/zenodo.3483166) or the GPS data provided with the spectroscopy data set (Benavent et al., 2020; DOI 10.5281/zenodo.3827443). Dataset contents YYYY-MM-DD_hh.mm.ss.png, data file, portable network graphics README.txt, metadata, text where YYYY-MM-DD_hh.mm.ss is the date and time at which the file was saved in UTC. Dataset license This dataset of raw optical sky images from ACE is made available under the Creative Commons Attribution 4.0 International License (CC BY 4.0) whose full text can be found at https://creativecommons.org/licenses/by/4.0/

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The Antarctic Circumnavigation Expedition (ACE) was made possible by funding from the Swiss Polar Institute and Ferring Pharmaceuticals. Support for the MAX-DOAS installation and data collection was provided by the SORPASSO project (also part of ACE) led by Prof. Rafel Simó.