P.G. Ananthasubramanian¹, Satoshi Yamamoto², T.P. Prabhu³ and Dorje Angchuk^4
1Astrophysics group, Cavendish lab., University of Cambridge, Cambridge CB3 0HE, UK &
Raman Research Institute, Bangalore 560080, India.
²Dept. of Physics, University of Tokyo, Tokyo-113, Japan.
³Indian Institute of Astrophysics, Koramangala, Bangalore 560034, India.
⁴Indian Astronomical Observatory, Indian Institute of Astrophysics, Skara, Leh 194101, (J & K), India.

Abstract: A 220 GHz tipping radiometer was installed at the Indian Astronomical Observatory (IAO), Hanle (Latitude 32^o46'46" N; Longitude 78^o57'51" E; Altitude 4500 m, above msl) towards the end of December 1999. The system is in continuous operation since early October 2000. A sky scan, i.e., a forward and a reverse scan is taken once every 10 minutes and these data are averaged to fit for zenith opacity. We present in this report detailed results of zenith opacity measurements for a continuous 3-year period. The system was up and running to make these measurements for nearly 90% of the time during this period. In particular, we cover the opacity trends in three consecutive 6-month periods of `winter' months (October to March, 2000-01, 2001-02 & 2002-03) and three 6-month periods of `summer' months (April to September, 2001, 2002 & 2003). The fractional time for opacities less than 0.06 in the three winter periods mentioned are above 30% and the corresponding fractional time for opacities less than 0.1 are above 70%. The opacities show seasonal variations as normally expected and in general one month in summer (July or August) peaks in opacity values. Diurnal variations are not easily noticeable. These results indicate that Hanle is a very promising site for sub-mm and infrared astronomy during most of the year excluding the months of July-August.

Keywords: astronomical site characterisation, atmospheric water vapour, sub- mm astronomy.

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