Ionospheric storms of different intensities are examined based on the data of the meridional chain of ionospheric stations located in Eastern Siberia and China. It has been determined that afternoon troughs can be observed either during the main phases of strong and moderate storms if their starts fall on evening hours or during the recovery phase if they start at some other time. They are mainly observed in the equinox. Model calculations of variations of the electron density during the storm on April 3-6, 2004, are carried out. It is shown that the positions of the minimum and the polar wall of the afternoon trough coincide with the belt of the westward drift in the time sector 13-17 MLT at geomagnetic latitudes between 55 and 65 .