This study investigates seasonal changes in the morphology of atmospheric-ionospheric disturbances. They are inferred from variations in the global distribution of negative storm effects. It is found that during summer, even moderate magnetic activity may lead to the development of significant negative storm effects extending from the higher latitudes down to the subtropics. In the winter hemisphere the same magnetic activity produces only small disturbance effects which furthermore are restricted to the higher-latitude region. It is suggested that future models of the disturbed atmosphere and ionosphere should allow for these seasonal variations.