Radiation from outer regions of hot-star winds (r > 100 R_O) is detectable mainly at radio wavelengths. Thermal radio (free-free) emission is proportional to the square of the density and is very sensitive to density contrasts in the wind. The multi-wavelength observations in the radio range can in principal be used for scanning the inhomogeneity of farout wind regions. To extract this information, we need to know the thermal, ionization and kinematic structure of the wind. In the present paper we investigated the problems connected with the determination of thermal and ionization state in the outer winds of hot stars (OB, WR and LBVs).