Wet surface conditions and light winds associated with a broad area of high pressure combined in the development of fog across a significant portion of the Midwest and Appalachian states in the early morning hours of 29 October, 2009. Although standard surface observations (METARS, ASOS) provide real-time observations of cloud ceiling and present weather, these observations are often limited to airports and cities. In the figure below (0843 UTC), observations of dense fog were present throughout much of Kentucky and Tennessee, the mountains of the Carolinas, and the Midwest.
The MODIS fog product uses a spectral difference between the 3.9 and 11 micron infrared channels to depict both low clouds and fog (yellows), and easily detected the bulk of the low clouds and fog across IL, IN, OH and WV. The value of the enhanced MODIS resolution is apparent, as the instrument clearly depicts the formation of valley fogs in the complex terrain of the Appalachians and their foothills.