Understanding the necessary relationship between the radar reflectivity factor Z and the rain rate R is still important today and constitutes an interesting challenge for the scientific community. Despite this interest and the advances already obtained in this field, understanding the variability of this Z-R law is not yet trivial. This work therefore consists of studying the implication of rain spectra at low intensities R≤1mm/h in this variability. Thus, from a large historical database of raindrop size distribution (DSD) acquired in tropical Africa, we have shown that rain spectra R≤1mm/h lead to high values ​​of the coefficients A and b of the Z-R power relationship. However, the variability of pre-factor A is pronounced in agreement with those of the NT and D0/NT parameters of the DSD; while the exponent b remains quasi-constant in such rainy samples, independently of their percentage. Establishing the link between the observation parameters D0, NT and D0/NT of the DSD and the pre-factor A of the Z-R relationship made it possible to arrive at close power relationships D0 – A, NT – A and D0/NT – A corresponding respectively to the microphysical modes «D0 constant, NT variable», «D0 variable, NT constant» and «D0 variable, NT variable». The last microphysical condition seems to be more suited to the power relationship A – α where α, is the pre-factor of the power relationship D0/NT – R found in the literature.