The physical origin of the water-related atmospheric continuum absorption is examined. Existence of double water molecules (dimers) in equilibrium water vapor at room temperature is proved by direct spectroscopic experiment supported by ab initio calculations. It is demonstrated that diluting water vapor by air does not essentially reduce the dimer abundance. Numerous earlier studies have predicted the presence of water dimers in the atmosphere and their influence on chemical reactions, homogeneous condensation and the Earth's radiation balance. Our results provide experimental proof of the presence of dimers in the atmosphere, thus enabling a detailed study of their role in natural processes. Prospects for future research are discussed.