When evaluating the Grashof number for a horizontal cylinder, at what temperature should the kinematic viscosity be evaluated (the surface, ambient air or some other value between)? In the same case,...

When evaluating the Grashof number for a horizontal cylinder, at what temperature should the kinematic viscosity be evaluated (the surface, ambient air or some other value between)? In the same case, at what temperature should the Prandtl number be evaluated?