Clouds are among the most important and least understood components of the Earth System. Their ubiquitous, highly variable and inhomogeneous presence contributes to increasing the difficulties in assessing their impacts on the radiative energy budget, especially at high latitudes. Measurements of cloud characteristics such as optical thickness and droplet effective radius can be obtained with in situ and ground-based instruments, but such measurements are very sparse in polar regions. This work presents the application of different methods for cloud optical thickness estimation by means of a ground-based UV-VIS-NIR spectrometer recently installed at the Thule High Arctic Atmospheric Observatory, Greenland (THAAO, www.thuleatmos-it.it, 76.5°N, 68.8°W). The retrieval of droplet effective radius is left for a second part of this work, which is underway and exploits spectral measurements at longer wavelengths (1000-2200 nm). © 2024 Author(s).