Near Infrared Channel - 3.9 microns - Emissivity Differences

http://www.meted.ucar.edu/satmet/goeschan/#

• Recall that emissivity is the fraction of radiation emitted compared to the amount emitted if the object were a blackbody.
• Emissivity is dependent on wavelength of the radiation (0.75 at 3.9 microns and 1.0 at 10.7 microns).
• Both channels absorb all longwave emitted by the earth.  
• Low clouds/thick stratus/fog  at night will look cooler in the 3.9 micron channel than at 10.7 microns because of the cloud top emissivity difference between the two channels.
• The brightness temperature difference T10.7-T3.9 is the fog/thick stratus product.