In Fig. 4a, satellite-measured radiance temperatures at 3.9 and at 10.7 um are compared at night-time, when there is no reflected radiation at 3.9 um, for a FOV where the ground at 300 K becomes increasingly covered with clouds at 260 K. Also, in this case, surface emissivities are assumed to be equal to 1.0 and atmospheric absorption is assumed to be negligible.
Notice that when the FOV is either totally clear or totally cloud covered, the radiance temperatures at 3.9 and 10.7 um are the same. However, as fractional cloud cover increases, the radiance temperature at 3.9 um becomes greater than the radiance temperature at 10.7 um due to the stronger response at 3.9 um to the warm portion of the partially filled FOV.