Because the reflectance of solar radiation at 3.9 um differs between liquid water clouds and solid water (ice) clouds, images at this wavelength may be used to distinguish cloud phase. With the color enhancement used in this image, ice clouds are black or a dull turquoise. Liquid water clouds are difficult to distinguish from the ground by this channel during the summer months and are better resolved by using the multispectral reflectivity product (to be discussed later, or see it now). Examples of this are the liquid water clouds on the NE/KS border, which are not discernible in the 3.9 um image, but they are evident in the reflectivity product image.
For more information about the GOES 3.9 um channel and its applications, see the tutorial on the subject, The GOES 3.9 um Channel.