Measured surface effects for non-adiabatic (filled circles) and adiabatic (open circles) frequencies of the most probable solar model from evaluating the BiSON frequencies using approach M2c. The uncertainties of the differences are smaller than the symbols. Least-squares power-law fits (see Equation (31)) to the surface effects for the adiabatic (solid line) and non-adiabatic (dashed line) frequencies are also shown.