  We evaluate the effect that radome transparency has on atmospheric opacity
measurements performed by the skydip technique. We show that, except at rather
high opacities, it is not sufficient to ignore losses in the radome (or
"window") during the data analysis and then subtract them off from the derived
atmospheric opacity. Perhaps surprisingly, unless radome transparency is
correctly modelled, the atmosphere will appear to have a minimum opacity that
is many times greater that the radome losses. Our conclusion is that some
previous site studies may have significantly underestimated the quality of the
best submillimeter sites, and that the difference between these sites and
poorer sites may be much greater that currently believed. We also show that
most of the residual 857 GHz opacity at the best sites, currently ascribed to
"dry-air opacity", can in fact be just an artefact caused by not properly
modelling the radome during the data analysis.
