Abstract The plume buoyancy framework used in previous work is applied to understand the processes that lead to moist tropical heat waves (HW). Lag composite analysis applied to land and ocean regions that often experience moist heat waves reveals that buoyancy is typically positive prior to the HW, becoming negative during HW onset due to an increase in saturation deficit, in agreement with previous work. The saturation deficit anomaly is mostly a result of warming in the lower free troposphere and drying at midlevels. It exhibits a nearly upright structure that extends from the surface to the upper troposphere. This structure motivates the use of a column saturation deficit budget to understand HW onset processes. It is found that subsidence driven by deviations from weak temperature gradient balance drives the increase in column saturation deficit, with horizontal MSE advection playing a smaller but non‐negligible role.