Almost all convection calculations require the use of temperature dependent (and sometimes, pressure dependent) fluid properties. Since fluid temperature gradients necessarily exist in convection, it's important use the appropriate temperature to find these properties. There are no absolute rules here, but there are general guidelines which are usually followed.

• For external forced convection use the film temperature, the average of the surface temperature and the fluid far from the surface:
  T_film = (T_s + T_infinity)/2
• For internal forced convection use the bulk mean fluid temperature, the average of the inlet and exit mean fluid temperatures: T_bulk = (T_m,in + T_m,out)/2
• For internal natural convection (enclosures) use the average temperature, the average of the temperatures of the two surfaces of interest: T_avg = (T_sur,1 + T_sur,2)/2