Prediction equations for estimating evaporation from bodies of water, often require values of net radiation. Solar radiation data are usually more accurate and more readily available than measurements of net radiation. Measured solar radiation and the mean ratio of net radiation to solar radiation have been used to estimate the net radiation levels required to predict evaporation rates. However, although the average net radiation ratio is relatively stable from year to year the effect of daily variation in the net radiation ratio on evaporation estimates has not been investigated. An empirical energy balance and measurements of pan evaporation and meteorological factors are used to examine the effect of the average daily variation on evaporation rates computed with the mean net radiation ratio. The results indicate that the average daily variation from the mean net radiation ratio may produce an error of approximately 13 per cent in an estimate of daily evaporation when the mean net radiation ratio is used. However, daily variability in the net radiation ratio has little influence on the regeneration of a large number of evaporation estimates. Estimated values of the net radiation ratio and its variability are compared with observed values from polar climates.