The fire area, which is located at point A in the upper left hand panel is easily detected on all subsequent panels; however, note how the fire appears as black in the fog product images and then changes to bright white in the reflectivity product images. This change in appearance is related to how the two products are derived. The fog product is made by differencing satellite observed brightness temperatures at 3.9 and 10.7 microns, and is successful in helping to locate fog because the emissivity of water cloud at 3.9 microns is less than at 10,7 microns. Thus at night, to locate fog (and other water cloud) the temperature at 3.9 microns is subtracted from the temperature at 10.7 microns resulting in a positive difference in the fog/water cloud area (which is displayed as white). However, as is explained in the RAMM/CIRA 3.9 micron tutorial, fires will appear hotter in 3.9 micron imagery than in corresponding 10.7 micron imagery, thus at night in a fire region the fog product produces a negative difference which is displayed as black. 

During daytime, instead of the fog product, the reflectivity product is produced. As with the fog product, the reflectivity product is designed to locate water cloud but is based on the principle that water clouds are very reflective at 3.9 microns. The reflectivity product is actually a pseudo-reflectivity product since the temperature at 10.7 microns is used to eliminate the temperature contribution to the measured radiance at 3.9 microns (this is explained in the RAMM/CIRA 3.9 micron tutorial): this effectively leaves only radiance at 3.9 microns that is due to reflected solar radiation. The reflectivity product is displayed as is visible imagery, and the highly reflective water and fog clouds are white. However, in the reflectivity product fires will also appear as white because the adjusted radiance at 3.9 microns does not know where the apparent extra energy came from, water cloud or fire. Thus, the difference in appearance in fires between the nighttime fog product and the daytime reflectivity product. For a more in-depth discussion of the fog and reflectivity products, and the effect of fires on observations at 3.9 microns, see the RAMM/CIRA 3.9 um tutorial.

Jim Purdom 
 

We welcome your comments and discussion at ramsdis@comet.ucar.edu