Daytime astronomical observing conditions at South Pole

Extract

The Sun has been observed at South Pole nearly every austral summer since 1979. This experience shows that the duration of uninterrupted observations is limited by clouds to runs of at best ~150 hours. Impressively high duty cycles can be achieved over longer periods. Sky clarity is often superb but even in cloudless conditions, ice crystal precipitation storms can be a problem. Daytime observations of the Sun and bright stars show that visible seeing quality is limited to about 2-3 arc seconds at altitudes of 15-20 degrees. Nearer the zenith, the seeing quality approaches 1 arc second in light wind. Seeing quality appears to vary with wind speed and direction and also diurnally with changing solar illumination of the surface ridges (sastrugi). Seeing is degraded by turbulence in the exceptionally large temperature gradient in the first few hundred meters above the surface. There are suggestions that both daytime and, perhaps more so, nighttime cloudiness increased over the last decade. This may be related to increasing amounts of CO₂, CH₄ and CFCs in the polar atmosphere.