  We consider the Casimir interaction energy between a plane and a sphere of
radius $R$ at finite temperature $T$ as a function of the distance of closest
approach $L$. Typical experimental conditions are such that the thermal
wavelength $\lambda_T=\hbar c/k_\mathrm{B}T$ satisfies the condition $L\ll
\lambda_T\ll R$. We derive the leading correction to the proximity-force
approximation valid for such intermediate temperatures by developing the
scattering formula in the plane-wave basis. Our analytical result captures the
joint effect of the spherical geometry and temperature and is written as a sum
of temperature-dependent logarithmic terms. Surprisingly, two of the
logarithmic terms arise from the Matsubara zero-frequency contribution.
