It is shown within the weak-coupling model that the macroscopic
superconducting anisotropy for materials with the gap varying on the Fermi
surface cannot be characterized by a single number, unlike the case of clean
materials with isotropic gaps. For clean uniaxial materials, the anisotropy
parameter γ(T) defined as the ratio of London penetration depths,
λc/λab, is evaluated for all T's. Within the two-gap model
of MgB2, γ(T) is an increasing function of T.Comment: 4 pages, 2 figure