Detailed analyses are presented of the temperature and pressure fields of the planetary boundary layer (PBL) and their seasonal variability over the western United States and over the Plateau of Tibet (Qinghai-Xizang Plateau). Over the United States these analyses rely on 850 mb data, augmented by surface data. Over Tibet the 600 mb surface adequately describes the PBL. The effects of a “plateau monsoon” appear prominently during winter and summer over both plateaus. Together with continental monsoon effects they help to shape prominent circulation features, such as the low-level jet stream (LLJ) over Texas and Oklahoma. The complex, seasonal characteristics of precipitation regimes over the North American continent can be explained, to a large extent, by considering these monsoonal changes in the PBL, especially over the mountains. Abstract Detailed analyses are presented of the temperature and pressure fields of the planetary boundary layer (PBL) and their seasonal variability over the western United States and over the Plateau of Tibet (Qinghai-Xizang Plateau). Over the United States these analyses rely on 850 mb data, augmented by surface data. Over Tibet the 600 mb surface adequately describes the PBL. The effects of a “plateau monsoon” appear prominently during winter and summer over both plateaus. Together with continental monsoon effects they help to shape prominent circulation features, such as the low-level jet stream (LLJ) over Texas and Oklahoma. The complex, seasonal characteristics of precipitation regimes over the North American continent can be explained, to a large extent, by considering these monsoonal changes in the PBL, especially over the mountains.