Emissivity is a function of both wavelength and surface type. Fig. 3a is a plot of emissivity vs.
wavelength for four different surfaces. Notice that the emissivity for soil types is more variable
near 3.9 um than 10.7 um, e.g., dry sand has an emissivity near 0.8 at 3.9 um and 0.95 at 10.7
um. Imagery of sandy areas appears cooler at 3.9 um than at 10.7 um when there is no
reflected solar radiation (e.g., at nightime, in a dry atmosphere). This "apparent" temperature
difference affects the interpretation of derived image products
Referring again to Fig. 3a, strong absorbers at a particular wavelength are also strong emitters at that wavelength. Furthermore, during the daytime, radiation that is not absorbed is reflected by the surface. Because of this, in the 3.9 um imagery, land surfaces will appear different, depending on their composition. This is especially apparent over regions like White Sands, NM, where the soil type is mainly sand (g = 0.75 to 0.85) and across north-central GA, AL and MS, where more reflective soils are found. The imagery shown in the "fire detection" section also demonstrates this characteristic.