Using data from 75 ionosonde stations and 43 storms, and based on the knowledge gained from simulations from a physically based model, we have developed an empirical ionospheric storm-time correction model. The model is designed to scale the quiet time F region critical frequency (foF2) to account for storm-time changes in the ionosphere. The model is driven by a new index based on the integral of the ap index over the previous 33 hours weighted by a filter obtained by the method of singular value decomposition. Ionospheric data were sorted as a function of season and latitude and by the intensity of the storm, to obtain the corresponding dependencies. The good fit to the data at midlatitudes for storms during summer and equinox enable a reliable correction, but during winter and near the equator, the model does not improve significantly on the quiet time International Reference Ionosphere predictions. This model is now included in the international recommended standard IRI2000 [Bilitza, 2001] as a correction factor for perturbed conditions.