  We present 220 GHz (1.36 mm) measurements of zenith optical depth obtained to
characterise the Indian Astronomical Observatory, Hanle (Ladakh, India) during
the period from late December 1999 to early May 2000 and early October 2000 to
September 2001. The data were sampled at an interval of 10 minutes. We describe
the automated 220 GHz tipping radiometer used, its basic principle, operation,
data acquisition method and data reduction scheme in detail. The 220 GHz
opacity is found to be less than 0.06 for a significant fraction (40%) of the
time during the winter months, indicating that Hanle is one of the good
observing sites for submillimeter-wave astronomy. We make a preliminary
correlation with the precipitable water vapour derived from surface relative
humidity and air temperature measurements made during the same period with a
weather station installed at the site. We also compare the Hanle site with
other high-altitude sites like Mauna Kea and Atacama desert.
