  An explicit expression for the temperature dependence of the specific heat of
clean anisotropic-gap superconductors is derived within the weak-coupling BCS
approximation. The specific heat is presented as a functional of the
superconducting gap on the Fermi surface. The obtained formula interpolates
between the correct low coupling jump at T_c and the low temperature behavior
for T<<T_c. For isotropic superconductors the formula gives a relation between
the specific heat and the superconducting gap. For anisotropic superconductors,
the interpolation formula incorporates averaging of powers of the gap
anisotropy function over the Fermi surface and provides a suitable set for
fitting model Hamiltonians to experimental data. The work of the interpolation
formula is illustrated by (i) the Pokrovsky formula for the specific heat jump,
(ii) Gor'kov and Melik-Barkhudarov formulas for the Ginzburg-Landau
coefficients, (iii) the Moskalenko two-band formula for the specific heat jump,
(iv) the temperature dependence of the specific heat for the two-band model,
applicable to MgB_2, (v) the two-dimensional d-wave model, applicable for
YBa_2Cu_3O_{7-delta}, and (vi) the Zhitomirsky and Rice triplet p-wave model
with horizontal line nodes for Sr_2RuO_4. The temperature dependence of the
penetration depth is illustrated by fitting the general theoretical formula to
the experimental data for MgB_2, YBa_2Cu_3O_{7-delta}, and the triplet
superconductor Sr_2RuO_4.
