|10.7 um Image|
Outflow clouds can often be seen at night using a multispectral image product, made by subtracting the brightness temperatures at 3.9 um from those at 10.7 um. The result, known as the fog product, relies on the fact that liquid water clouds (which include most clouds associated with low-level thunderstorm outflow) have an emissivity at 3.9 um that is less than that at 10.7 um, thereby making the fog product show a positive difference (colored white in this enhancement).
Although this product's main use is in the detection of low-level, liquid water clouds at night, it is also useful for discriminating between thin and thick cirrus. The 3.9 um channel is more sensitive to radiation passing through thin cirrus than the 10.7 um channel. Here the fog product indicates a negative difference between 10.7 and 3.9 um. The color table used shows these negative differences as dark. For very cold clouds, such as the thunderstorm tops in NE and SD, the signal at 3.9 um becomes very noisy, with the multispectral image product dominated by that noise. In this case, the fog product takes on a black and white, speckled appearance.
As the MCS moved east, it left behind low-level cloudiness associated with its outflow over northern KS, and set up a boundary which played a role in the initiation of the severe weather in KS later on May 31.
For more information on the fog product, see the RAMMTeam's tutorial on the GOES 3.9 um channel at: http://www.cira.colostate.edu/ramm/goes39/cover.htm