  For a systematic assessment of submillimeter observing conditions at
different sites, we constructed tipping radiometers to measure the broad band
atmospheric transparency in the window around 350 $\mu$m wavelength. The
tippers were deployed on Maunakea, Hawaii, at the South Pole, and in the
vicinity of Cerro Chajnantor in northern Chile. Identical instruments permit
direct comparison of these sites. Observing conditions at the South Pole and in
the Chajnantor area are better than on Maunakea. Simultaneous measurements with
two tippers demonstrate conditions at the summit of Cerro Chajnantor are
significantly better than on the Chajnantor plateau.
