The structure and evolution of the atmospheric boundary layer (ABL); under clear-sky fair weather conditions over mountainous terrain is; dominated by the diurnal cycle of the surface energy balance and thus; strongly depends on surface snow cover. We use data from three passive; ground-based infrared spectrometers deployed in the East River Valley in; Colorado’s Rocky Mountains to investigate the response of the thermal; ABL structure to changes in surface energy balance during the seasonal; transition from snow-free to snow-covered ground. Temperature profiles; were retrieved from the infrared radiances using the optimal estimation; physical retrieval TROPoe. A nocturnal surface inversion formed in the; valley during clear-sky days, which was subsequently mixed out during; daytime with the development of a convective boundary layer during; snow-free periods. When the ground was snow covered, a very shallow; convective boundary layer formed, above which the inversion persisted; through the daytime hours. We compare these observations to NOAA’s; operational High-Resolution-Rapid-Refresh (HRRR) model and find large; warm biases on clear-sky days resulting from the model’s inability to; form strong nocturnal inversions and to maintain the stable; stratification in the valley during daytime when there was snow on the; ground. A possible explanation for these model shortcomings is the; influence of the model’s relatively coarse horizontal grid spacing (3; km) and its impact on the model’s ability to represent well-developed; thermally driven flows, specifically nighttime drainage flows.