We present a qualitative study of the seasonal behaviour of the D region. Systematic examinations of continuous recordings of diurnal analogical data (1998, 1999 and 2000) in amplitude signals at LF (MSF of 60 kHz) and VLF (GBR of 16 kHz) were carried out. In order to decipher the dynamical signature of the D layer on these signals, we have worked out an original method basing itself on the study of the seasonal behaviour of various morphologies. Our results show that various ionospheric or atmospheric phenomena affect the transmission of these signals.