The equilibrium constants for water oligomers ranging from dimers to cyclic hexamers are determined using Wertheim's theory of associating systems. In the present model for water, the pair potential has a spherical hard core, and tetrahedral hydrogen bonds which are represented by an energy parameter and an interaction volume. On the basis of the present theory, one predicts that in earth's troposphere, water dimers and perhaps trimers may contribute to the absorption of solar radiation, but concentrations of higher oligomers are too low to influence the optical properties of the earth's atmosphere.