A qualitative model of the negative phase of ionospheric storms is presented. Only stations located within an atmospheric disturbance zone of a low O/N2 ratio will observe a depletion of ionization. The extent of this disturbance zone is determined by geomagnetic coordinates. Thus stations located in the North American and Australian sectors are more liable to observe negative storm effects. On the other hand it is determined by the asymmetric energy injection along the auroral oval. It follows that stations located in the early morning sector during enhanced substorm activity have a greater chance of observing negative storm effects than those situated in the daytime sector. Seasonal and magnetic storm induced changes in the O/N2 ratio are in phase during summer and out of phase during winter, explaining the seasonal variation of storm effects.