Data logged continuously by the 225GHz receiver at the Caltech Submillimeter Observatory (CSO) over the periods 1994-1995 and 1997-1998 have been analysed. This equipment measures the zenith optical depth in the wing of an atmospheric water vapour line and may be used to derive the precipitable H₂O column density above the site (Holland, W. 1999, personal communication):

mm of H2O = 20 * tau(225GHz)
  

A cumulative frequency histogram of the optical depth was generated for each quarter in 1994 and 1995:

and compared with data for 1997 to mid-1998 available at CSO.

The following table shows the average optical depths and the values adopted for the Gemini observing condition constraints:

Thanks are due to Remo Tilanus for extracting the 1994-1995 data from the JCMT archive.

Last update March 1, 2001; Phil Puxley