The effects of variations in diffusion coefficient, surface properties of the composite (α, ε, h), panel tilt, ground reflection, and geographical location on the moisture concentration profiles and average moisture content of composite laminates have been studied analytically. A heat balance which included heat input due to direct and sky-diffuse solar radiation, ground reflection, and heat loss due to reradiation and convection was used to determine the temperature of composites during atmospheric exposure. The equilibrium moisture content was assumed proportional to the relative humidity of the air in the boundary layer at the surface of the composite. Condensation on the surface was neglected. Histograms of composite temperatures were determined and compared with those for the ambient environment. These temperature changes and the resulting changes in the relative humidity at the panel surface caused the moisture content in the surface plies to cycle on a daily basis to a depth of up to about 430 µm. The average moisture contents calculated by assuming equal diffusion coefficients, D, for absorption and desorption are in good agreement with those calculated accounting for typical changes in D due to cyclic wetting and drying.