Of primary concern for the DASI site is the level and stability of the atmospheric emission. Most high dry sites have excellent atmospheric transmission at 30 GHz. Atmospheric stability, especially on angular scales of order one degree, is a more stringent requirement for CMB observations.
The high altitude (2800 m) plateau at the South Pole is an exceptional site. The low atmospheric opacity and smooth continuous airflow are ideal conditions for large field imaging. Long periods of excellent transparency and low sky noise have been used to great advantage in past years by a number of different experiments (see, for example, Refs. 10-13). Atmospheric analysis of the 1996 - 1997 Austral summer Python experiment 40 GHz swept beam data indicates that the atmosphere is bimodal in nature with very good stability ~ 75% of the time, during which the experiment is instrument noise limited. The South Pole has negligible diurnal fluctuations; in addition the Sun is absent for 6 months of the year and is easily shielded during the Austral summer. From the South Pole, the largest dark region from the IRAS Sky Survey is accessible, and the DASI fields will overlap with those of Antarctic long duration balloon experiments as well as previously observed fields.