A positive and negative ion chemical steady state model which combines about 400 ion-chemical and photochemical reactions with a two-dimensional neutral model for minor atmospheric constituents is presented in the height range 50--84 km, for latitudes -50¿ to +50¿ in January and September. The ion model yields concentrations of the main positive and negative ions and of electrons. Two important parameters for the description of the lower ionosphere were deduced, namely, the ratio &lgr; of electrons to negative ions and the f+ parameter, describing the ratio of the positive cluster ions to positive molecular ions. In addition, the parameterization of f+ indicates a strong dependency on the water vapor concentration. At the transition height (f+=1) at midlatitudes the water concentration can be expressed as a function of the electron density and the temperature: [H2O>=1.09¿10-6[p>0.6T-3[e>, with the units of the water vapor density [H2O>, the air density [&rgr;>, and the electron concentration [e> in cm-3 and T in degree kelvin.¿ 1997 American Geophysical Union