  The lack of observed transition discs with inner gas holes of radii greater
than ~50AU implies that protoplanetary discs dispersed from the inside out must
remove gas from the outer regions rapidly. We investigate the role of
photoevaporation in the final clearing of gas from low mass discs with inner
holes. In particular, we study the so-called "thermal sweeping" mechanism which
results in rapid clearing of the disc. Thermal sweeping was originally thought
to arise when the radial and vertical pressure scale lengths at the X-ray
heated inner edge of the disc match. We demonstrate that this criterion is not
fundamental. Rather, thermal sweeping occurs when the pressure maximum at the
inner edge of the dust heated disc falls below the maximum possible pressure of
X-ray heated gas (which depends on the local X-ray flux). We derive new
critical peak volume and surface density estimates for rapid radiative clearing
which, in general, result in rapid dispersal happening less readily than in
previous estimates. This less efficient clearing of discs by X-ray driven
thermal sweeping leaves open the issue of what mechanism can clear gas from the
outer disc sufficiently quickly to explain the non-detection of cold gas around
weak line T Tauri stars.
