Species’ thermal tolerances are used to estimate vulnerability to climate warming, but few studies consider the role of the hydric environment in shaping physiological responses to warming. As environments become hotter and drier, organisms respond by limiting water loss to lower the risk of desiccation; however, reducing water loss may produce trade-offs that lower thermal tolerances if respiration becomes inhibited. Here, we measured the sensitivity of evaporative water loss (EWL) and critical thermal maximum (CT_max) to humidity in response to short term changes in precipitation. We collected click beetles (Elateridae) near Oulu, Finland from two sites that were sampled immediately after rain and several days after rain. In the laboratory, we acclimated beetles to wet or dry humidity treatments (30%, 50% relative humidity), and then performed thermal tolerances experiments that measured EWL and CT_max at two different acute humidity conditions (wet and dry) in a fully balanced experimental design. Click beetles sampled immediately after rain events had a significantly higher CT_max and EWL than beetles measured several days after rain. Analysis of covariance indicated that EWL was negatively associated with CT_max, such that individuals with high EWL exhibited a lower CT_max. We found a significant interaction between the acclimation and acute humidity treatments, indicating a high degree of plasticity in CT_max over times scales ranging from minutes to days. The correlation between EWL and CT_max identifies the need to study thermal tolerances from an ‘whole-organism’ perspective that considers trade-offs between physiological traits. Moreover, our experiments revealed a high degree of plasticity that complicates CT_max as a straightforward proxy of climate vulnerability.