[en] Simple daytime approximations to the diurnal, seasonal, and solar activity dependence of E-region critical frequencies have been extensively used to predict the propagation parameters of the E region. Frequencies, as observed by the vertical incidence ionosondes, have been merged with recent work on the nighttime foE by Wakai (1971) to develop a numerical representation of foE continuous over the entire 24 hour period for each month at minimum and maximum solar activity levels. The use of this numerical representation is recommended whenever predictions of foE are required. Sample contour maps derived from this representation are presented