04 September 1997 -- Radiational Cooling, Fog and Low Cloud
in the Central and Eastern U.S.
A broad area of high pressure at the surface covered much of the central and eastern U.S. on 04 September 1997. Clear skies and light winds allowed for strong radiational cooling from the central Plains into the Ohio River valley. The GOES-8 IR channel 4 image (left) shows the effects of this radiational cooling -- since land radiates thermal energy to space faster than water, the many rivers and lakes appear darker (warmer) than their surroundings. Urban "heat islands" are more apparent on GOES-8 IR channel 2 imagery, especially cities such as Indianapolis, Indiana (IND) and Dayton (DAY) and Columbus (CMH) in Ohio. Many locations across this region set record low temperatures for the date.
Upslope flow around the western side of the high pressure combined with the radiational cooling to produce an extensive area of low cloud across the central Plains. The GOES-8 fog product (right) more distinctly shows the low cloud extending into northern Kansas and central Nebraska, as well as other isolated low cloud features across northeastern Oklahoma, northwestern Arkansas, and portions of the southern Appalachians. Note how these areas of low cloud are not as evident (if at all) on the IR channel 4 product.
While the fog product is an excellent tool for determining the areal coverage of the low cloud features, one must use caution in labeling such features as "fog". In this case, surface observations over the central Plains indicated that the cloud ceiling heights were between 800 and 2600 feet over Kansas and Nebraska, and only one station actually reported fog. In addition, several locations across the upper Midwest and Ohio valley regions reported fog, but these were likely reports of shallow ground fog which lacked significant vertical depth or horizontal extent. As a result, no signature was apparent in the GOES-8 fog product over these regions.
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