Based on field measurement data on the atmospheric transmission spectra along the extended near-ground paths in an arid zone and in a region of the West Siberia, continuous absorption of radiation in the 0.44-3.97 mm wavelength region has been revealed, being linearly dependent on the absolute air humidity. By the magnitude of the absorption coefficient (about 0.02-0.03 g-1cm2) it noticeably exceeds the continuous absorption by water vapor; and the character of its spectral dependence points to its relation to the finely dispersed soot aerosol. The absorption coefficients obtained on the near-ground and slant paths are being compared. The comparison made shows quite a satisfactory agreement between them. The presence of the finely dispersed soot aerosol in the atmosphere is assumed to be one of the important factors causing the anomalous absorption of optical radiation by clouds.