The relation between the integrated atmospheric transmittance P0 and the downward and near—horizontal atmospheric radiation I(0) and I(90) is presented in the form of the expression I(0)/I(90) = C0 (1 - P0), where C0 = 0.92 ... 0.93. It is based on the LOWTRAN—7 model calculations and on the analytical calculations using parametrizations for the vertical temperature profiles and atmospheric transmittance function. The absolute accuracy of determining P0 according to this relation is estimated to be 0.02. The practical accuracy of determining P0 from experimental values of I(0) and I(90) is equal to 0.035 ... 0.025 for P0 = 0.5 ... 0.85. The calculations of P0 are described on the basis of the experimental data obtained in the water area of the Atlantic Ocean in spectral regions between 11 and 12 mm. The dependence of P0 on the ambient humidity above the water surface is close to linear relation.