Studies of the nucleation of new particles in the atmosphere are usually concentrated on the formation of sulfuric acid particles. However, there exist also other trace gas species with the nucleation capability in the atmosphere. In this paper the formation of water-ammonium chloride particles via homogenous heteromolecular nucleation in the atmospheric conditions is studied by using classical nucleation theory. This study is focused especially at the high relative humidities (90-99%), that can be found in the atmosphere during cloud or fog processes. The Gibbs free energy for the formation of a stable cluster is found to be insensitive to the variations in relative humidity. Instead, according to the results the nucleation rate is dependent on relative humidity, although the influence of relative humidity on the nucleation rate is relatively weak at the studied relative humidity range. The significant nucleation of stable water-ammonium chloride clusters via the studied path is concluded to be possible only in the polluted atmosphere during cloud processes. In the clean (for example remote sea) areas the significant nucleation of aqueous ammonium chloride particles is found to be highly impropable.