Reflection at 3.9 um is sensitive to cloud phase and is very sensitive to particle size, as is shown
in Fig. 3b. Notice how water droplets are more reflective than ice particles of the same size. In
clouds, water droplets are normally between 5 and 20 um in diameter, depending upon cloud
type, while ice crystals are usually an order of magnitude larger. Reflection that is detected by a
satellite is from multiple scattering (Fig. 3b represents a single scatter) and each reflection
further reduces the amount of energy returned to the satellite. For example, three reflections each, by ice at 100 um and water at 10 um, yield scattering values of 0.16 and 0.73 respectively.
During the daytime, clouds with small water droplets, such as cumulus, fog and stratus over land, are much brighter when viewed at 3.9 um than are ice clouds, which are very poorly reflective and hence, dark. Marine stratocumulus, with their larger water droplets, appear relatively dark when compared to cumulus or stratus over land.