The effects of increasing relative humidity (RH) on fluidization/defluidization are investigated experimentally and understood via particle-level predictions for the resulting capillary force. Experimentally, defluidization is found to be more sensitive to small changes in RH than fluidization. This sensitivity is captured by a new defluidization velocity U-df, which characterizes the curvature of the defluidization plot (pressure drop vs. velocity) observed between the fully-fluidized (constant pressure drop) and packed-bed (linear pressure drop dependence on velocity) states; this curvature is indicative of a partially-fluidized state arising from humidity induced cohesion. Plots of U-df vs. RH reveal two key behaviors, namely U-df gradually increases with a relatively constant slope, followed by an abrupt increase at RH similar to 55%. Furthermore, the bed transitions from Group A to Group C behavior between RH of approximately 60-65%. From a physical standpoint, these macro-scale trends are explained via a theory for capillary forces that, for the first time, incorporates measured values of particle surface roughness. Specifically, a model for the cohesive energy of rough surfaces in humid environments shows the same qualitative behavior as U-df vs. RH for RH <55%, unlike predictions of the cohesive force. Furthermore, the abrupt transition at RH similar to 60-65% is explained via the previously observed onset of liquid-like water adsorption, rather than crystal/ice-like adsorption, onto glass surfaces. (C) 2016 American Institute of Chemical Engineers AIChE J, 62: 3585-3597, 2016