The viscosity of the Quark-Gluon Plasma (QGP) is usually described holographically by the entropy-normalized dynamic viscosity $��/s$. However, other measures of viscosity, such as the kinematic viscosity $��$ and the Reynolds number $Re$, are often useful, and they too should be investigated from a holographic point of view. We show that a simple model of this kind puts an upper bound on $Re$ for nearly central collisions at a given temperature; this upper bound is in very good agreement with the observational lower bound (from the RHIC facility). Furthermore, in a holographic approach using only Einstein gravity, $��/s$ does not respond to variations of other physical parameters, while $��$ and $Re$ can do so. In particular, it is known that the magnetic fields arising in peripheral heavy-ion collisions vary strongly with the impact parameter $b$, and we find that the holographic model predicts that $��$ and $Re$ can also be expected to vary substantially with the magnetic field and therefore with $b$.

Several improvements and clarifications; 24 pages, 4 figures; version to appear in Nuclear Physics B