The thermal structure of the troposphere and lower stratosphere during the movement eastward of several Pacific troughs is examined primarily from the standpoint of the distribution of baroclinity within a vertical plane extending across the northwestern and north central United States. Baroclinity is defined and then expressed in a form suitable to the potential-temperature cross-sections employed in this study. Dominating features of the thermal field are two types of baroclinic zones: (1) broad and essentially non-frontal zones which form the leading and trailing edges of deep, rapidly moving cold domes in the middle and upper troposphere; (2) narrow, frontal type zones comprising the leading or trailing edges of either slowly-moving, low-level cold domes or rapidly-moving, upper-level ones. There is evidence that the non-frontal baroclinic zones are equally as important, both dynamically and synoptically, as the frontal ones.