Modern digital ionosondes with capability of angle of arrival measurements provide information about tilts in the ionosphere. In the F region, most of those tilts are coupled with the temporal variation of this layer. Well-known examples for this behavior are the sunrise and sunset periods where the rapid temporal increase (decrease) of the electron density causes a strong horizontal gradient and therefore a significant deviation of the layer from horizontal stratification. Zenith angles close to 20° were recorded during the sunrise period in winter time. Less is known about the tilts produced by gravity waves, since periodicities of the order of a fraction of an hour are undersampled by the standard hourly recordings of ionograms. We found that such short period gravity waves are present very frequently and can produce local tilts comparable in magnitude with the sunrise/sunset tilts. Partial analysis of some observations during the winter 1980/1981 at Brighton, Colorado, will be presented and the consequences of F region tilts for the analysis of ionograms, especially the electron density calculations, will be discussed.