Observing Constraints - Water Vapour Statistics

Mauna Kea

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:

[cum freq histogram]

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:

Period	CSO 225GHz tau
Period	20%-ile	50%-ile	80%-ile
1994-1995	0.047	0.078	0.130
1997Q1	0.048	0.085	0.150
1997Q2	0.052	0.092	0.160
1997Q3	0.068	0.100	0.160
1997Q4	0.041	0.063	0.120
1998Q1	0.037	0.047	0.072
1998Q2	0.042	0.078	0.143
adopted	0.05	0.08	0.15

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

Cerro Pachon

At present the conditions for Cerro Pachon are assumed to be the same as for Mauna Kea.

Last update March 1, 2001; Phil Puxley