Sky Transparency Conditions at Paranal
The sky transparency is assessed at various times per night, both visually and by observations of photometric standard stars with the Omegacam instrument (VST) or FORS2 (UT1).
Definitions:
- Photometric: no visible clouds above 30 degrees from horizon, transparency variations across the sky less than +- 2%
- Clear: no visible clouds above 30 degrees from horizon, transparency variations across the sky less than 10%
- Thin clouds: thin cirrus with transparency variations >10%
The following plots show:
- The monthly percentage of Photometric + Clear sky condition, and
- The monthly percentage of usable observing time, i.e. Photometric + Clear + Thin cirrus sky conditions
The long-term (2014-2024) of the Photometric + Clear fraction at the Paranal Observatory is 77%. And the usable time fraction is 90%.
The long-term average of the fraction of clear skies is 75% for the winter semester April-September, and 80% for the summer semester October-March.
