. . . The 3.9 um channel on the GOES imager senses both emitted and reflected radiation. At night, the reflected component disappears, and the signal consists solely of energy emitted in the wavelength range of 3.78 - 4.03 um. A characteristic of radiation from clouds at this wavelength makes the channel extremely useful for sensing water clouds or fog. Specifically, the emissivity for water clouds is less at 3.9 um than at 10.7 um, so by subtracting the brightness temperature at 3.9 um from that at 10.7 um, clouds with water droplets can be readily identified (at night). The dual-channel "difference images" for the 10 Dec, Lake Superior LES band reveals that the coldest cloud tops during the heavy snow periods were indeed mixtures of supercooled water and ice.
