Experimental data on the width and depth of the valley above the ionospheric E layer are available for only a limited range of conditions. Theoretical models are reasonably well deΓΏned in this region, where time constants are short, and they now provide a good match for most observations. Calculations can therefore be used to study the changes that will occur under di erent conditions, due to known changes in atmospheric composition, EUV radiation and the solar zenith angle . Results are given for the shape of the E layer, and the E-F valley region, throughout the day and for di erent geophysical conditions. Valleys are generally 10 -15 km wide near noon, with a depth of 4 -7%. Both width and depth increase at larger zenith angles, varying approximately as (sec ) 0:6 . At sunset the day production peak rises until it disappears near 150 km, while the stable night peak becomes visible at 105 km. This di ers appreciably from the variations assumed in the International Reference Ionosphere. A numerical model (available as a FORTRAN programme) is derived to give a close approximation to the full theoretical calculations under all conditions. The model begins at 80 km, to include some realistic D region ionisation, and extends well into the F1 region giving smooth, physically based variations as a function of height, local time, latitude, season and solar ux.