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Aspects of the local wind system in a mountain valley were investigated for fine weather situations. In addition to customary wind observations and analyses, the quasi-conservative nature of potential temperature was exploited. It provides an indication of the transport of air masses, and its analysis supplies information on the local wind in the mountainous terrain. Changes of air masses due to mountain and valley winds can be identified in the time series of potential temperature. Criteria were developed for the determination of the onset of the mountain and valley winds based upon the time series of potential temperature. In this context, the potential temperature serves as an objective indicator of the air mass change independent of wind data and the associated uncertainties. The interpretation of the "front-lines" of the air masses yields insight to the dynamics of the valley wind, and the interaction of transport and pollution processes.