Clumping in stellar winds of hot stars is a possible consequence of radiative-acoustic instability appearing in solutions of radiative-hydrodynamical equations. However, clumping is usually included to stellar atmosphere modeling and radiative transfer calculations in a highly approximate way via a global free parameter called the clumping factor. Using different values of clumping factors many researchers succeeded to fit the observed spectra better and to correct empirical mass loss rates. This usually leads to a conclusion that the stellar wind is clumped. To understand how clumping may influence theoretical predictions of mass-loss rates, different clumping properties have to be taken into account. If clumping appears already below the critical point, the mass-loss rates is changed.

Comment: in Four Decades of Research on Massive Stars, C. Robert, N. St-Louis & L. Drissen eds., Astron. Soc. Pacific. Conf. Ser., in press