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ALMA Memo # 512


Otarola A., Holdaway M., Nyman L-E., Radford S.J.E., Butler B. J

2005 January

Abstract: Atmospheric conditions at Chajnantor have been monitored since April 1995. Starting then, the National Radio Astronomy Observatory (NRAO), joined by the European Southern Observatory (ESO), Onsala Space Observatory (OSO), and the Nobeyama Radio Observatory (NRO), have operated atmospheric and weather monitoring equipment to characterize the observing conditions for a large radio interferometer for astronomy at millimeter and sub-millimeter wavelengths. The data have demonstrated Chajnantor is an exceptionally good site.

In this work, we examine the atmospheric transparency at Chajnantor before direct measurements begun. We first show the surface water vapor pressure at Chajnantor is a good estimator of the daily-average atmospheric transparency by direct correlation with measurements of optical depth at 225 GHz. Next, we show the surface water vapor pressure at the Calama airport correlates reasonably well with the surface weather conditions at Chajnantor. These findings then allow us to examine the transparency at Chajnantor since 1973 through the analysis of the Calama surface weather data.

Besides, we also include here a determination of the strength of the so-called Bolivian winter (regional monsoon) based on a normalized index calculated out of surface water vapour pressure. This index helps us to get the annual regional humidity trend, and in turn can be useful to compare the atmospheric observing conditions from year to year.

View a pdf version of ALMA Memo #512.

Last modified: 2005-02-16