  The conditions for disk evaporation by electron thermal conduction are
examined, using a simplified semi--analytical 1-D model. The model is based on
the mechanism proposed by Meyer & Meyer-Hofmeister (1994) in which an advection
dominated accretion flow evaporates the top layers from the underlying disk by
thermal conduction. The evaporation rate is calculated as a function of the
density of the advective flow, and an analysis is made of the time scales and
length scales of the dynamics of the advective flow. It is shown that
evaporation can only completely destroy the disk if the conductive length scale
is of the order of the radius. This implies that radial conduction is an
essential factor in the evaporation process. The heat required for evaporation
is in fact produced at small radii and transported radially towards the
evaporation region.
