A computer technique is described for determining the quiet day curve (QDC) from riometer measurements of cosmic radio noise. In this technique, the QDC value for a given sidereal time interval is taken to be the signal level corresponding to the inflection point on the high-signal side of the peak of the distribution of the measured cosmic radio noise power for that interval. A comparison is made with earlier methods of calculating the QDC and the superiority of the present method is demonstrated. This method has been applied to 30 MHz riometer data from Siple station, Antarctica (75°36'S, 83°36'W) for 1975 and 1980. Despite the presence of high levels of propagated interference during some months in 1980, the technique is able to generate useable QDC's for each month. The uncertainty in the determination of the QDC is estimated to be about 0.1 dB when interference is present, but much lower at other times. Changes in the QDC levels from month to month have been found which are qualitatively consistent with expected seasonal (i.e., solar zenith angle) effects. The technique described here provides an objective operational definition of the QDC which is relatively easy to adapt to computer calculation, and it appears to be a promising tool for the detection and monitoring of long-term variations in background radio-wave absorption by the ionosphere.