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. Nocturnal low-level stratus clouds (LLCs) are frequently observed in the atmospheric boundary layer (ABL) oversouthern West Africa (SWA) during the summer monsoon season. Considering the effect these clouds have on the surface energy andradiation budgets as well as on the diurnal cycle of the ABL, they are undoubtedly important for the regional climate.However, an adequate representation of LLCs in the state-of-the-art weather and climate models is still a challenge,which is largely due to the lack of high-quality observations in this regionand gaps in understanding of underlying processes.In several recent studies, a unique and comprehensive data set collected in summer 2016 during the DACCIWA(Dynamics-aerosol-chemistry-cloud interactions in West Africa) ground-based field campaignwas used for the first observational analyses of the parameters and physical processes relevant for the LLC formationover SWA. However, occasionally stratus-free nights occur during the monsoon season as well.Using observations and ERA5 reanalysis, we investigate differences in the boundary-layer conditions during 6 stratus-free and20 stratus nights observed during the DACCIWA campaign.Our results suggest that the interplay between three major mechanisms is crucial for the formation of LLCsduring the monsoon season: (i) the onset time and strength of the nocturnal low-level jet(NLLJ), (ii) horizontal cold-air advection, and (iii) background moisture level. Namely, weaker orlater onset of NLLJ leads to a reduced contribution from horizontal cold-air advection. This in turn results in weakercooling, and thus saturation is not reached.Such deviation in the dynamics of the NLLJ is related to the arrival of a cold air mass propagating northwards from the coast,called Gulf of Guinea maritime inflow.Additionally, stratus-free nights occur when the intrusions of dry air masses, originating from, for example, central or south Africa,reduce the background moisture over large parts of SWA.Backward-trajectory analysis suggests that another possible reason for clear nights is descending air, which originatedfrom drier levels above the marine boundary layer.

What controls the formation of nocturnal low-level stratus clouds over southern West Africa during the monsoon season?