A study of the cross--polar cap potential estimated by the HF Super Dual Auroral Radar Network (SuperDARN) during periods of steady interplanetary magnetic field (IMF) and solar wind velocity is presented. The potential estimates were examined as time series along with time series of the IMF and solar wind. In addition, the data were examined statistically to obtain a best fit linear expression for the potential as a function of the solar wind and IMF, which was used for comparison with observed potential values. It was found that the observed potential was significantly more variable than would have been predicted by the observed solar wind and IMF. The variability was found to be on the order of 20 kV, while the predicted variability would have been only 5 kV. Implications of these findings with regard to magnetospheric physics, ionospheric simulations, and space weather are discussed.