Systematic time differences in the onsets of spread F events in the ionograms are observed between the magnetic equatorial station Fortaleza (4¿S, 38¿W, dip latitude 1.8¿S) and the low-latitude station Cachoeira Paulista (23¿S, 45¿W, dip latitude 14¿S), two stations in Brazil, located at close-by magnetic meridional planes (actually some 12¿ of magnetic longitude apart). On the assumption, justified from different experimental observations, that the spread F irregularities occur in strongly field-aligned plasma bubbles that extend several degrees on either side of the magnetic equator, and rise up in vertically elongated columns over the magnteic equator, we have related the observed time differences in the onsets of spread F events at the two stations to the plasma bubble vertical rise velocities of the plasma bubbles so determined are found to be well within the values measured by VHF radar and satellite techniques, and further show, at times, good correlations with the amplitude of the prereversal peak in the vertical drift velocities and the heights of the evening equatorial F layer. Possible implications of these results are discussed.