The Vapor Pressure Deficit (VPD) is a key climatological variable that relates to atmospheric dryness, vegetation productivity and terrestrial water and carbon fluxes. It is often interpreted to be a direct reflection of the atmospheric moistening by land evaporation, or its absence, and hypothesized to play a central role in land-atmosphere interactions. Here, we show that a large part of VPD results from the diurnal heating of the lower atmosphere, so that it is tightly linked to the Diurnal Air Temperature Range (DTR). We use an analytical expression for DTR that mechanistically links it to changes in the heat content of the lower atmosphere, which we then use to estimate VPD using the slope of the saturation vapor pressure curve. When applied across continents, our approach reproduces observed spatial and temporal variability in VPD with R²of 0.9 and 0.8 respectively. It captures observed responses of VPD to solar radiation, clouds, and soil water stress across diverse climate and moisture regimes. It also explains the characteristic hump-shaped relationship between evaporation and VPD as an emergent consequence of changes in DTR during transition from energy- to water-limited conditions. Our findings suggest that much of the observed VPD variability and its responses can be explained by the thermodynamic controls on lower-atmospheric heating, rather than moistening. This explanation of VPD variations implies that VPD largely responds to land surface energy partitioning, with important implications for interpreting VPD-ecosystem relationships, land-atmosphere feedbacks and evaluating aridity trends under global warming.