Kentucky and Tennessee

Topographic map of Tennessee and Kentucky along the path of the eclipse.
Figure 1: Topographic map of Tennessee and Kentucky along the path of the eclipse.

Beyond Illinois, the landscape beneath the umbral track begins a slow upward climb—gentle at first in Kentucky and western Tennessee, but then accelerating in eastern Tennessee as it reaches the ramparts of the Appalachian Mountains. Within this general trend are small undulations in the terrain that give the states a rolling, hilly character with elevations that change by only a hundred metres or less from place to place along the eclipse centreline. It’s an attractive landscape, especially off of the Interstates.

Beyond Nashville, the topography begins a more aggressive ascent, eventually culminating at nearly 1700 m on the peaks of the Blue Ridge Mountains along the border with North Carolina. The ascent to the  peaks is punctuated by plateaus, ridges, and valleys that make this neighbourhood one of the more interesting topographical regions in the United States (Figure 1). From Nashville, there is first a rise to the Cumberland Plateau, then a descent into the Sequatchie Valley, up again to drop into the Appalachian Valley with its own series of undulations, and then  the final climb into the Blue Ridge Mountains.

There is no disagreement between the various measures of cloud cover through these states, as all show a steady increase in August cloudiness as the eclipse track heads to the southeast. It is early afternoon—around 1:30 in this time zone—when the Moon’s shadow approaches and the convective clouds, if there are any, will be building toward their daily maximum, but the graph of satellite-derived cloud cover (Figure 2) shows little difference between 10:20 a.m. and 1:30 p.m. cloudiness. Morning and afternoon graphs show a dramatic drop in cloudiness in the Appalachian Valley east of Nashville (near Athens), not unusual in view of the more than 300 m decline in elevation from the Cumberland Plateau. This is evident in Figure 4, where both the Sequatchie Valley and the Appalachian Valley have clear skies while the surrounding higher landscape is dotted with cumulus clouds.

Figure 2: Graph of average cloud cover along the central axis of the eclipse based on observations from satellites over a 15-17 year period. Data NASA/GSFC.

Figure 2 shows average cloudiness rises abruptly from around 53 percent at the Illinois-Kentucky border to nearly 70 percent at Nashville and to 80 percent near Athens. The peak near Athens is a bit of a mystery, as there is no terrain in the area to promote so much cloud generation.  A similar-sized peak just to the east in the afternoon graph is easily explained by the Blue Ridge Mountains.  Nevertheless, the general trend toward heavier cloud is confirmed by the data in Table 1, which shows that cloudiness measured from the ground averages over 50 percent for the most part, save for the 46 percent at Chattanooga, which lies in the Appalachian Valley.

Table 6 KY-TN

Table 1: Climate data for selected stations along the eclipse track in Kentucky and Tennessee. Data: NCDC.

One of the more sobering statistics that can be extracted from Table 1 is the increasing frequency of overcast skies along the shadow path as it proceeds across Tennessee and the Carolinas. The table shows that the overcast conditions, which bring almost no chance of seeing the eclipse, hovers at just under 20 percent for much of Kentucky and Tennessee, except for an out-of-character 6 percent at Chattanooga. Compare this to a frequency of 10 to 15 percent in Wyoming and to just 5 to 10 percent on the Columbia Plateau in Oregon.

The point of maximum eclipse lies within the sorghum field to the left.
Figure 3: The point of maximum eclipse lies within the sorghum field to the left.

The point of maximum eclipse lies within Kentucky, close to the town of Hopkinsville, where the average cloudiness hovers around 55 percent in the airport observations and about 58 percent in the satellite record. Both of these seem pessimistic in view of the measured values for actual sunshine shown in Table 1, which reaches about 70 percent. Every measure of cloud cover has unique biases – there is no way to measure a “true” cloudiness, since it comes in thick and thin forms, in bands and blobs, and several different levels. Instead, the cloud measurements could better be described as “estimates.” Two human observers will usually have a very different idea about the amount of cloud in the sky (unless clear or overcast). Satellite observations are at least consistent from point to point, though recording higher amounts than a human might measure.

Figure 4: Satellite image of the cloud cover across Tennessee on August 21, 2014.
Figure 4: Satellite image of the cloud cover across Tennessee on August 21, 2014. Most of the cloud over Tennessee is cumulus, most of which would dissipate as the lunar shadow approaches. Note the absence of cloud in the Sequatchie Valley and the Appalachian Valley. Deeper convective clouds can be seen on the slopes of the Blue Ridge Mountains; most of this would not dissipate. Cumulus cloud is very sensitive to the underlying terrain. Image: SSEC.

On the border with Georgia and the Carolinas, the track rises abruptly to Blue Ridge Mountains, rising, in one place, over 1300 m in a distance of only 32 km. This region has the highest cloud cover along the track, reaching over 70 percent in the afternoon satellite measurements. It’s a rugged terrain, with twisting, slow-moving roads, so eclipse watchers who want the striking vista of the Blue Ridge Mountains in their photographs should pay close attention to the weather forecast ahead of eclipse day.

Summary

In the Moon’s course across Kentucky and Tennessee, the percent of possible sunshine declines from a tolerable 71 percent at Paducah to 63 percent at Nashville, and presumably a fair bit lower over the Appalachians, though there are no measurements to show that. The Tennessee Valley would seem to be the destination of choice for long-range planning, though the weather on eclipse day should dictate your eventual destination.

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Updated December 2016

Climatology and weather for celestial events