Experiments on the effects of humidity in adhesion between rough surfaces have shown that the adhesion energy remains constant below a critical relative humidity (RHcr) and then abruptly jumps to a higher value at RHcr before approaching its upper limit at 100% relative humidity. A model based on a hierarchical rough surface topography is proposed, which quantitatively explains the experimental observations and predicts two threshold RH values, RHcr and RHdry, which define three adhesion regimes: (1) RH < RHdry, no stable water bridges form between the contacting Surfaces; (2) RHdry < RH < RHcr, water bridges form but are confined to the initial solid-solid contact regions; and (3) RH > RHcr, water menisci freely form and spread along the interface between the rough surfaces.