This paper presents an analysis of the ionospheric response to geomagnetic storms in the European and North American regions. Ionospheric behavior during the major storms of the last solar maximum (2003-2005) has been analyzed using electron density profiles and foF2 and hmF2 characteristics obtained from the digisonde network. The storm of 24 August 2005 has been investigated most comprehensively. Our analysis provides experimental evidence in support of the ionospheric storm model based on the role of the neutral wind and thermospheric circulation. The main results showing the validity of this mechanism are observations of the ionospheric uplifting preceding both negative and positive storm effects at all locations. Using a latitudinal chain of digisonde stations in the European sector we clearly observed equatorward propagation of the storm disturbance that originated at high latitudes near the auroral oval. It is also shown that traveling atmospheric disturbances played a significant role in the development of the ionospheric storm of 24 August 2005. Attempts are made to determine the general pattern of the dependence of ionospheric storm effects on the local time. The results obtained also support the neutral wind based mechanism for ionospheric storms.