So, with the moon now passing into the waning crescent phase, the Day-Night Band imagery is less operationally useful, at least for the detection of fog and other lower level cloud types. That is, at least until the moon is back into the waxing gibbous phase. Nevertheless, when cirrus clouds aren’t present, the Nighttime Microphysics RGB has proven to be a very valuable tool for the detection of fog and other low-level clouds. Just this morning a forecaster at the Huntsville, AL WFO was able to use the imagery not only for the detection of fog, but also to aid in the issuance of a special weather statement about the fog. The image below valid at ~724 UTC (0224 am CDT) 17 Oct shows the fog (whitish-aqua colors) lying across the valley areas of NE Alabama and adjacent areas of southern Tennessee and NW Georgia.
Around the time of this image, the visibility in the foggy locations had decreased to ~1/4 – 1/2 SM or less. Notice the fog in the DeKalb Valley is fainter than the fog in areas to the north and west. Not only is the DeKalb Valley more narrow, but the fog was likely more shallow. This feature of the imagery can also help to guide forecasters in assessing the longevity of the fog once sunrise breaks. Over time, forecasters can develop a sense of pattern recognition with the varying degrees of color shading and tailor forecasts to better match the time of dissipation. In this case, the fog in the DeKalb Valley began to dissipate significantly by about 1430 UTC, while the deeper and more expansive fog to the north and west lasted about an hour longer.