  The effect of Prandtl number on the linear stability of a plane thermal plume
is analyzed under quasi-parallel approximation. At large Prandtl numbers
($Pr>100$), we found that there is an additional unstable loop whose size
increases with increasing $Pr$. The origin of this new instability mode is
shown to be tied to the coupling of the momentum and thermal perturbation
equations. Analyses of the perturbation kinetic energy and thermal energy
suggest that the buoyancy force is the main source of perturbation energy at
high Prandtl numbers that drives this instability.
