  It has been suggested that the absence of macroturbulence in the atmosphere
of NGC 1624 - 2 is due its strong magnetic field (the strongest known for a
massive O star) suppressing convection in its outer layers, removing the
mechanism thought responsible for the observed macroturbulence in stars with
lower field strengths. Here, we develop and apply a criterion for a uniform
magnetic field to suppress convection in stellar envelopes in which radiation
pressure is a significant contributor to hydrostatic balance. We find upper
mass limits of ~55 Msun and ~30 Msun for magnetic suppression to be possible in
Zero Age Main Sequence and Terminal Age Main Sequence stars, respectively. For
evolved stars, magnetic suppression of convection can significantly alter the
stars' evolution. For NGC 1624 - 2, we find a polar dipole strength of 16.5 +/-
5.9 kG is required to suppress convection, in good agreement with the value ~20
kG measured by spectropolarimetry.
