  Small (compared to coherence length) weak superconducting defects when
located at the surface, combined with the proximity and percolation effects,
are claimed responsible for various observations with superconducting rf
accelerating cavities with non-constant Q-value, such as "Q-slope" and
"Q-drop", the role of temperature, the result of "nitrogen doping" and its
relation to the free path, and the influence of the static external magnetic
field. The Ginzburg-Landau equations are used to confirm the results.
