As the eclipse track crosses the northeast tip of Georgia and passes into South Carolina, it makes a steady descent from the 1700 m heights of the Blue Ridge Mountains to sea level at the coast (Figure 1). In contrast to the Rockies, the descending flow off of the Appalachian Mountains does not automatically give South Carolina a dry climate, as the moisture supply that humidifies the region comes from both the Atlantic in the east and the Gulf of Mexico in the southwest and so it is maintained by winds from either direction.
However, the Appalachians do behave like any other mountain chain when it comes to larger-scale weather systems. They can dam the moisture from such systems and hold them up over Tennessee until the system can slide around the Appalachians through Georgia and Florida. They also dry out mid-level flows that descend from the mountain peaks. All-in-all, the Appalachians do not affect local cloud development in South Carolina (and probably promote it), but do impede weather systems moving from the west in the immediate vicinity of the mountains. Figure 2 shows that the Piedmont Plateau, east of Anderson, is the major cloud producer. This is not surprising in view of its height and position, but there is also a small 5 percent decrease in average cloudiness west of Anderson that is due to descent from the Blue Mountains.
A more interesting feature revealed by the cloud graph is the sharp drop in cloudiness over and surrounding Lake Marion and likely also Lake Moultrie, features that are obvious in Figure 4. Much of the August cloudiness across South Carolina comes from convective buildups and the waters of the lakes, being cooler than the land, are much less inclined to promote clouds. Eclipse seekers should locate close to the lake and on the downwind side on August 21, perhaps at Santee, where quick connections can be made to Interstate 26 and Interstate 95 if movement is needed.
An comparable location would be to set up directly on the Atlantic coast. Cool ocean currents suppress the formation of convective clouds offshore and sea breeze winds carry this cool cloud-suppressing air inland as temperatures build over the land. Based on the afternoon cloud graph in Figure 2, an ocean-front location could offer as much as a 20 percent decrease in average August cloud cover. The sea-breeze effect may be a short-range one, extending only a few kilometres inland, and so eclipse expeditions should plan on being right at the coast or very close to it. A careful watch has to be kept on the sky, as the boundary between the sea breeze air and that inland can focus the forces that cause the development of thunderstorms. Sea breezes have a Jekyll-and-Hyde personality, often clearing the air, but sometimes bringing heavy storms.
Observations taken at airports in Georgia and the Carolinas (Table 1) largely mimic the pattern displayed by the graphs of Figure 2, but do not show the impact of the lakes or the ocean sea breezes. Of interest is the cloudiness calculated for Anderson, which is more than 10 percent lower than any of the other airport weather stations in Table 1. This benefit comes from air descending from the Blue Ridge Mountains, which the cloud graphs show is most likely a bit to the west of the city.
Missing from the climate data in Table 1 is evidence of the sea breeze effect at Charleston International Airport and Beaufort MCAS, both airports near the coast. Sea breezes typically have an influence for only a short distance from the coast, and both airports lie more than 22 km inland, too far for the cooler Atlantic air to reach by early afternoon. In addition, sea breeze clearing tends to be preceded by a band of heavier cloud as the incoming ocean circulation collects the moist land-heated air and piles it up into bigger and more extensive convective build-ups (Figure 4).
The shadow passes in mid afternoon along the Atlantic States, well into the time of convective cloud buildups. Even so, the cooling that comes with the eclipse shadow should have a dramatic effect in dissipating the cloudiness provided the buildups haven’t reached the rain-producing stage. Past experience at other eclipses promises that cumulus clouds at least will evaporate quickly about half-way between first and second contact. The large amount of cumulus cloud in Figure 4 suggests that much of South Carolina would be essentially free of cloud were the eclipse to take place at the time of the image. Eclipse-induced cooling is more than capable of eliminating all but the largest and best-organized patches of these convective clouds. The question then becomes: how common is the cumulus environment over South Carolina and what will it do to cloud prospects for eclipse day?
Examination of high resolution satellite images from other days and years shows that cumulus clouds are very common in the afternoon over South Carolina—more so than in any other state along the eclipse track. This implies that the graph in Figure 2 greatly overemphasizes the cloudiness that would be likely on eclipse day once cooling begins with the approach of the lunar shadow. The cloudiness graph for South Carolina in Figure 2 in not wrong (the cloud is there), but it is not representative of what might happen on August 21.
Based on an examination of imagery from other days and years, it would seem that a reduction of 20 to 40% in the average cloud amount would be a reasonable number. This would make South Carolina the best state in the southeast to watch the eclipse. Of course other states will also have a reduction in cloud because of eclipse cooling, but none of them have the very prominent cumulus climatology that marks the east side of the Appalachians.
The evidence is in the satellite record. The graph in Figure 2 shows a sharp decline in average cloud cover where the centre line passes over Lake Marion, even though the resolution of the data is not sufficient to completely reveal the impact of the cooler lake water. A much larger reduction can be found over the nearshore Atlantic, where the cloud cover declines by more than 25%. Without making any correction for eclipse cooling, it is quite clear that the best site for observing the eclipse is on the windward coast or on the leeward side of Lake Marion or Lake Moultrie. Except for the fact that it is off to the edge of the eclipse track, Charleston’s waterfront is likely to be a very favourable site, as are any others to the north. One very attractive spot is Buck Hall Recreation Area, about 4 km from the central axis of the eclipse and 45 km north of Charleston. Buck Hall is a small park however and cannot accommodate a large crowd. Though access to the waterfront is limited by marshy offshore islands and private homes along the coast, it should be sufficient to be within a kilometre or two of the Atlantic to take advantage of the cloud-suppressing sea breezes, if they are present on the day of the eclipse.
The last word is left for a discussion of hurricanes. Storm statistics for the Carolina coast reveal that the eclipse comes at a time when the frequency of tropical storms is on the rise toward its mid-September peak. Since 1867, 43 hurricanes have made landfall over South Carolina. Of these, 5 occurred within the four-week period centered on the date of the eclipse. Talk about hurricanes with any long-term South Carolina resident and the names Hugo (1989), Charley (2004), and perhaps Gracie (1959) and Hazel (1954) will come quickly into the conversation. Of these, Hugo, at category 5, was the strongest, bringing wind speeds of 140 mph when it made landfall right over the eclipse track on September 22. At worst, hurricanes are very unlikely, but not entirely impossible.
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Updated January 2017