Water aerosols, even when so finely divided as to be invisible, are capable of very strong absorption and emission in the infrared. This effect is pronounced in the 8–13-µ atmospheric window, owing to the 104 increase in the absorptivity of liquid water there over that for water vapor, and it contributes to the well known continuum in this spectral region. Water aerosol is found wherever suitable condensation nuclei exist and the relative humidity is above about 60%. Aerosol droplets increase in size and number with increasing relative humidity, affecting atmospheric radiance measurements accordingly. Trace quantities of aerosol can easily account for emission levels exceeding those of water vapor at 8–13 µ and are clearly indicated in cases where observed radiance levels cannot be accounted for by classical vapor band wing absorption theories. The aerosol emission mechanism is not associated with the formation or growth of the water droplets per se, but simply operates when droplets exist in the airborne state. Fog measurements are discussed and curves presented showing attenuation ratios between wavelengths in the visible and at 8–13 µ. Steam emission measurements leading to the formulation of an aerosol emission model are described briefly.
H. R. Carlon, "Infrared Emission by Fine Water Aerosols and Fogs," Appl. Opt. 9, 2000-2006 (1970)