On the basis of an ionospheric definition of disturbed conditions independent of any causative mechanism, a feature-guided pattern recognition method reveals the dominant morphology of long-duration negative f o F2 disturbances. A catalogue of negative disturbances lasting more than 24 hours is compiled from hourly f o F2 data from 75 ionosonde stations and three solar cycles. Disturbances in each month and station are handled separately, and four local time intervals of disturbance commencement are considered. A median disturbance profile is produced only when a minimum occurrence probability holds. The time window under morphological investigation is selected such that nonsystematic features, precursor phenomena, and poststorm effects are not included in analysis. The disturbance patterns, first grouped according to major characteristic features and then fitted with simple mathematic functions, are described by a range of the normalized deviation of hourly f o F2 to its corresponding monthly median and are provided to radio users along with their distribution in space and time. The present model is a nonconditional stand-alone model which may, in the event of an ionospheric disturbance at a certain location, predict its further development.