  We study the convection zones in the outer envelope of hot massive stars
which are caused by opacity peaks associated with iron and helium ionization.
We determine the occurrence and properties of these convection zones as
function of the stellar parameters. We then confront our results with
observations of OB stars. A stellar evolution code is used to compute a grid of
massive star models at different metallicities. In these models, the mixing
length theory is used to characterize the envelope convection zones. We find
the iron convection zone (FeCZ) to be more prominent for lower surface gravity,
higher luminosity and higher initial metallicity. It is absent for luminosities
below about $10^{3.2}\Lsun$, $10^{3.9}\Lsun$, and $10^{4.2}$\Lsun$ for the
Galaxy, LMC and SMC, respectively. We map the strength of the FeCZ on the
Hertzsprung-Russell diagram for three metallicities, and compare this with the
occurrence of observational phenomena in O stars: microturbulence, non-radial
pulsations, wind clumping, and line profile variability. The confirmation of
all three trends for the FeCZ as function of stellar parameters by empirical
microturbulent velocities argues for a physical connection between
sub-photospheric convective motions and small scale stochastic velocities in
the photosphere of O- and B-type stars. We further suggest that clumping in the
inner parts of the winds of OB stars could be caused by the same mechanism, and
that magnetic fields produced in the FeCZ could appear at the surface of OB
stars as diagnosed by discrete absorption components in ultraviolet absorption
lines.
