Publisher Summary This chapter reviews basic thermodynamic relations and presents typical temperature and humidity distributions in the planetary boundary layer (PBL) or atmospheric boundary layer (ABL). Various factors and processes influence the vertical distribution of air temperature in the ABL, including the type of air mass and its temperature just above the PBL, which depend on the synoptic situation and the large-scale circulation pattern; thermal characteristics of the surface and submedium, which influence the diurnal range of surface temperatures; and net radiation at the surface and its variation with height, which determine the radiative warming or cooling of the surface and the PBL. The factors influencing the specific humidity or mixing ratio of water vapor in the PBL include specific humidity of air mass just above the PBL, type of surface, its temperature, availability of moisture for evaporation and transpiration, the rate of evapotranspiration or condensation at the surface, and the variation of water vapor flux with height in the PBL. One of fundamental relationship between the commonly used thermodynamic variables is the equation of state or the ideal gas law. Among the dozen variables used by meteorologists that directly or indirectly express the moisture content of the air, the most often used is the specific humidity (Q), defined as the ratio of the mass of water vapor to the mass of moist air containing the water vapor.