Time-dependent hydrodynamical models predict that the stellar winds of early-type stars are clumped, due to the sweeping up of material into dense shells. In this paper we investigate whether these shells can explain the long wavelength (>=10 micron) continuum fluxes of O and early B stars. We had previously found that, for some stars, smooth wind models failed to explain the infrared and millimetre fluxes. To calculate the continuum flux, we model the clumping by a single shell. This single shell can represent the joint effect of a number of shells and we discuss how multiple shells can be combined into a single shell. The shell strength parameter is introduced, which combines density contrast and width of the shell. From the 12, 25 and 60 micron IRAS observations of zeta Pup, we derive a shell strength and position. We find that the clumping is less extreme than predicted by the hydrodynamical models. This means that the strength of the shells is less than the models predict, that there are not as many of them or that they do not fill a complete solid angle. Considering such partial (i.e. filling a solid angle of