  We consider magnetohydrodynamics with an external magnetic field. We find
that in general one must allow for a non-zero incoherent Hall conductivity to
correctly describe the DC longitudinal and Hall thermal conductivities beyond
order zero in the magnetic field expansion. We apply our result to the dyonic
black hole, determining the incoherent Hall conductivity in that case, and
additionally prove that the existence of this transport coefficient leads to a
significantly better match between the hydrodynamic and AC thermo-electric
correlators.
