The long nights of winter can be ideal for the formation of fog. Clear, calm conditions are conducive to strong radiational cooling, and if the cooling occurs over a moist surface, cooling to the dewpoint is likely and fog may be a result. Once fog has formed over a location, when will it lift? In large part that depends on the distance to the fog edge, as radiation fog will frequently erode from the outside in. Consider this example from December 15.

Visibilities are plotted and mean sea level pressure values (both from 1300 UTC) are contoured on this visible image from 1332 UTC on 15 December 2006. Very low visibilities (in statute miles) correspond to the region of fog and low clouds in central North Carolina. The sounding from Greensboro from 1200 UTC that morning (click on the thumbnail below) confirms that the saturated layer is confined to the surface and was likely a result of strong raditional cooling in the absence of any strong winds (as evidenced by the weak pressure gradient and the surface winds in the Greensboro sounding). The region of fog is centered on the Pee Dee River valley east of Charlotte.

What happens during the day? A thin layer of fog will dissipate as it mixes with the dryer air above it. For that to happen, mixing in the vertical that is driven by insolation must begin. Insolation underneath a dense fog is very minimal, but as you approach the edge of a region of fog, the fog thins and insolation increases, allowing more vertical mixing that entrains dry air above the fog into the fogbank, hastening evaporation. Because of that, isolated radiation fog banks such as this erode from the outside in.

Note in the visible image loop from 1332 UTC to 1815 UTC below (at 15-minute intervals, with some gaps) that this fog bank initially expands to the east. It is likely that regions east of the fog bank continued cooling to their dewpoint after sunrise. Cooling to the dewpoint at night was inhibited by the presence of upper level cirrus associated with a jet streak propagating up the East Coast.

The development of this fog at night was detectable using the method described here.

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On 12 December 2006 (Day 6 of the GOES-13 post-launch NOAA Science Test), the GOES-13 satellite was placed into Super Rapid Scan Operations (SRSO) mode, providing images at 30-second intervals for the entire day. A 200-image QuickTime animation of visible imagery (above) shows the development of convection along an advancing cold frontal boundary during the late morning into the early afternoon hours; isolated severe thunderstorm warnings were issued for counties in eastern Mississippi (however, no reports of severe weather were received from these particular storms). Previous GOES satellites have provided 30-second interval imagery during special test periods (for example, GOES-8 in 1996), but such SRSO test periods were much shorter (about 10 minutes total duration).

A QuickTime animation of GOES-13 10.7µm IR images (below) reveals fairly cold cloud top temperatures (around -55 to -60 C, orange to red enhancement), but no “enhanced-v” signature was observed. A few cloud to ground (CG) lightning strikes were seen in the vicinity of the strongest convection, but flash rates were quite low (GOES-12, MODIS IR images with CG strikes).

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As a part of the GOES-13 post-launch NOAA Science Test, the satellite provided continuous “full disk” images every 30 minutes for 2 consecutive days (09-10 December 2006); the current operational GOES full disk imaging interval is only once every 3 hours, but the Advanced Baseline Imager on GOES-R will provide full disk scans every 15 minutes. GOES-13 6.5µm “water vapor channel” imagery (above) showed cloudiness as well as non-cloudy water vapor circulations within the middle troposphere (Java animation), while the visible channel (below) provided a view of cloud features during the daylight hours (Java animation).

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On 08 December 2006 (Day 2 of the GOES-13 post-launch NOAA Science Test), 3.9µm shortwave IR images from GOES-13 and GOES-12 (above) revealed several “hot spots” (black enhancement) due to fire activity (NOAA HMS) across parts of Arkansas (Java animation). The performance of the GOES-13 3.9µm IR channel was comparable to that of GOES-12 for this particular group of relatively small and short-lived fires — a plot of the GOES-13 vs GOES-12 shortwave IR brightness temperatures (below) for the fire that was located between Russellville (KRUE) and Hot Springs (KHOT) Arkansas showed similar values as that particular fire was reaching maximum size and intensity.

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