A collation of data on the resistivity of aluminium foils is presented. The T² term caused by the surface, which is now well established, is examined and found to exist over a large range in sample thickness and observational temperature. Contrary to previous assertions, the magnitude of this T² term does not appear to vary simply as the inverse of the sample thickness. It is suggested that there are two ways in which a surface-caused T²-like term may arise. In metals with simple Fermi surfaces conventional surface scattering theories yield such a term from a combination of a bulk T⁵ term and surface scattering. By contrast, for a metal such as aluminium with a disconnected Fermi surface an extra T² term arising directly from surface scattering processes may appear.