Abstract The trade wind subcloud layer is an important structural component of the atmosphere. Its thermodynamic variability has long been characterized using simple frameworks, of which mixed-layer theory is the simplest kind. Past studies qualitatively support such a description, yet the adequacy of mixed-layer theory as a quantitative description has not been tested. Here we use observations collected during the Elucidating the Role of Clouds–Circulation Coupling in Climate (EUREC 4 A) field campaign to test this framework and evaluate our understanding of the trade wind subcloud layer. We find evidence for a transition layer separating the mixed-layer and subcloud-layer tops. The presence of such a finitely thick transition layer with vertical gradients complicates the application of mixed-layer theory, which assumes an abrupt gradient, or “jump” at the subcloud-layer top. This ambiguity introduces effective parameters and motivates their estimation through a Bayesian methodology. Results from this Bayesian inversion further reflect a finite-depth entrainment zone. We find that subcloud-layer moisture and heat budgets close for synoptic variability and a monthly campaign mean, yielding a campaign-mean residual of 3.6 W m −2 for moisture and 2.9 W m −2 for heat. Surface wind speed variability influences the subcloud-layer depth and fluxes, yet thermodynamic variability above the subcloud-layer top emerges as the primary control on subcloud-layer moisture and heat variability. Given that this simple theoretical framework can explain observed variability, it offers an appealing framework for evaluating larger-scale models that must parameterize the processes regulating this fundamental part of the atmosphere.