Monthly averaged total ozone values measured at Halley station, Antarctica. All measurements are in Dobson Units. These monthly averages are a flat average of any daily average values that exist for each given month; the daily averages are a flat average of the measurements obtained during a particular 24-hour period (UTC). The number of observations may vary from day to day. The Dobson ozone observing season at Halley begins at the end of August and ends in mid April; however, very early and late season observations are made with the Sun at low elevation, and are less accurate than those made during the main observing period of September 6 to April 6. The values for 1956/57 (MacDowall, J., 1962) and 1957-1973 (Farman, J. C. and Hamilton, R. A., 1975) have been approximately corrected from the original using the WMO recommended guidance (Komhyr, W. D., Mateer C. L. and Hudson, R. D., 1993) for the Bass-Paur ozone absorption coefficients. Ozone values from 1973 onwards have been calculated using the Bass-Paur coefficients. The approximation of a US standard atmosphere, which will differ from the Antarctic atmosphere, has been used and the assumed temperature used for the absorption coefficients may be inaccurate.