Sky images recorded during the austral summer of 2016/17 as part of the Antarctic Circumnavigation Expedition (ACE). ***** 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. ***** Original data collection ***** A Logitech HD web-camera (model C270) was installed on the starboard side of the foredeck of the R/V Akademik Tryoshnikov throughout the expedition. An image of the sky was taken every five minutes and saved onto the local computer. The camera was purposefully positioned at an angle to maximise the coverage of sky in the image. 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). ***** Data processing ***** These images have not undergone any processing. Image files were copied to the on-board storage during the expedition, then to network attached storage and finally object storage post-cruise. 138 images (0.65% of the total recorded) were removed from the dataset before publication because they contained images of people. ***** Quality checking ***** These images have not undergone any quality checking. ***** Standards ***** These images do not conform to any standards. ***** 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 contact ***** Alfonso Saiz-Lopez, Department of Atmospheric Chemistry and Climate, Spanish National Research Council (CSIC). ORCID: 0000-0002-0060-1581. Email: a.saiz@csic.es ***** 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/ ***** Dataset citation ***** Please cite this dataset as: Benavent, N., Garcia-Nieto, D., Cuevas, C.A. and Saiz-Lopez, A. (2020). Sky images recorded during the austral summer of 2016/17 as part of the Antarctic Circumnavigation Expedition (ACE). (Version 1.0) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.3827445