The adhesion forces between a smooth spherical particle and flat surfaces of alumina, silver, and titanium-coated Si wafers were measured with an atomic force microscope (AFM) under various humidity conditions. The results showed that there is a discrepancy between the experimentally observed and the theoretically predicted values of capillary adhesion forces. The reason for the discrepancy is explained considering the relative humidity of the surrounding atmosphere and the surface roughness profiles of the contacting surfaces. Two geometrical configurations to define the contact region profile are suggested. The equations to calculate the capillary adhesion forces were modified using the new geometries. The discrepancy was largely eliminated using the modified equations.