Abstract An ion model of the lower ionosphere is proposed. It consists of four positive ions: O 2 +, NO + and two cluster ions - a simpler CI 1 and a more complex CI 2. This model well explains the normal component of the winter anomaly (WA) in the D-region, which is recorded by absorption measurements on short radiowaves and rocket experiments at middle (40°N) and high (70°) latitudes. The higher values of the electron density during the winter appear as a result of the lower recombination because of smaller rates of cluster ion formation, i.e. the normal WA can be explained and modelled by the regular seasonal variations of composition, temperature and density.