Mechanisms of the small time scale spectral variability of OB-supergiants are investigated. The main factor of appearance of variable details in the absorption parts of the profiles of resonance lines in the spectra of these stars was supposed to be a motion of dense clumps in the atmosphere. The inhomogeneities were considered as the parts of dense clumps, forming the discrete absorption components (DACs) in the line profiles. In the framework of the given model the profiles of UV resonance lines C IV and Si IV in the spectra of OB-giants and supergiants are calculated in the SEI-approximation. Parameters of the atmospheres and clumps have been estimated. The possibility to explain the appearance of DACs in the profiles of resonance UV lines C IV and Si IV and their temporal evolution are demonstrated.