The large red contribution for sand in the troposphere depicted in the Dust RGB is due to the fact that the emissivity in 12.0 µm is larger than in 10.8 µm for thick dust clouds. As in 8.7 µm the emissivity is low for aerosols, like dust and ash, the green component is of medium intensity. The blue contribution is small for warm low clouds like dust. In the night the surface is cooler than during daytime; the result is that the blue component is larger during night. So, the night-time sand storm appears purple, whereas in daytime it would be lighter magenta.The graphic below explains the "inverse" behaviour of a dust layer compared to a thin Cirrus cloud for the brightness temperature difference (BTD) IR12.0 and IR10.8 micron.