A study of the effective emissivity from clear skies shows significant differences between night- and day-time behaviours. These differences seem to be related to the different conditions aloft for the day and night periods. For this reason, estimation formulae that rely only on surface parameters do not provide accurate estimates on an hourly basis. A deviation correction in terms of the hour of the day is proposed. This term takes into account the differences in the daily cycle of effective emissivity for different seasons, and provides estimates accurate to within the experimental errors.