Idaho—a land of rugged mountains and endless wilderness—doesn’t appear to be the best spot to pick for an eclipse of the Sun, but the state has a secret refuge from the clouds. That refuge is the Snake River Plain, a 160-km-wide, 600-km-long “smile” that stretches across the southern part of the state from Oregon to Wyoming (Figure 1), parts of which, fortuitously, fall under the track of the lunar shadow. In the up-and-down terrain of the Rocky Mountains, clouds form on the peaks and evaporate in the valleys. Nowhere is this more evident than in the valley that contains the Snake River (Figure 2).
The eclipse path first drops down from the Columbia Plateau onto the plain in eastern Oregon, near the community of Ontario. The weather statistics for Ontario, OR, (Table 1) and the satellite-based cloud graph below show that sunshine is nearly as abundant here as in the exceptionally cloud-free environment of the Columbia Basin near Madras and Mitchell. However, the track of the moonshadow is a little off, and when it first enters Idaho, taking a grazing path across the Snake River Plain. The southern half of the umbral shadow crosses the larger valley of the plain, while the northern part traverses the much narrower Long Valley of the Payette River. The wider Snake River Plain would be expected to have the more efficient cloud-eating ability. This is borne out by model-based cloud statistics, which show that the south side of the central line near Fruitland has two sunny days more in the month than the north side near Council (21.9 versus 19.8 days). The upshot of this is that the southern half of the eclipse track is likely to have better weather statistics than the already excellent values that appear in the graph of Figure 2.
The lunar shadow no sooner drops to lower elevations along the Oregon border, then it must climb again to pass over the rugged, 3,000 m peaks of the Sawtooth Range, a region of spectacular scenery that will be difficult for the seasoned photographer to resist in spite of its high cloudiness. The Sawtooth Range is true wilderness with few roads for access except for one enticing route: Highway 75, which travels northward into the Sawtooth Valley and the town of Stanley. The valley lies 1000 metres below the adjacent peaks of the range, a descent that will erode much of the cloud that might be present on the peaks. Though there are no cloud statistics for Stanley, it is noteworthy that August is the driest month of the year and the community is very close to the central line.
Once over the Sawtooth Range, the lunar shadow again drops down to the Snake River Plain, passing over Idaho Falls before reaching the Teton Range along the Wyoming border. The whole width of the track fits within this part of the Plain and there are many excellent eclipse-viewing sites. Idaho Falls sits close to the eastern edge of the valley, where nearby airport observations show that 2/3rds of August skies are clear or have only scattered cloud.
It’s not all clear sailing in this part of Idaho, however. Because the Snake River Plain provides an easy route for moisture from northern California to reach into the central Rockies, the cloud amounts reported at Idaho Falls are probably a bit higher than sites a little to the west or north. The Pacific moisture that manages to reach this area piles up against the Wyoming mountains and brings a little extra cloudiness to the region. August rainfall at Boise averages just 0.2 inches; that at Driggs, at the foot of the Tetons, averages 1.1 inch. While the effect on the cloud climatology is small, eclipse chasers looking for the best weather prospects should stay back from the Tetons, unless the forecast for the day is favourable.
Satellite observations displayed in Figure 2 show that the cloud cover on the Sawtooth Range has only a small increase from 10:30 a.m. to 1:30 p.m. as the slopes warm and convective clouds build. Cloud increases more dramatically later in the afternoon, but by then the eclipse shadow has passed well east and left the Earth. In the valley bottoms, there is almost no change in average cloudiness from mid-morning to early afternoon. Since the lunar shadow passes just before noon, any convective clouds that may form are likely to be small and overcome by eclipse cooling, but the cloud buildups in the mountains may not be so responsive.
The best appraisal of the prospects of seeing the eclipse is the direct measurement of sunshine itself, but the instrumentation to take such recordings isfound at few observing stations across the USA. In Idaho, there are two such stations along the eclipse track, at Boise and at Pocatello. Both indicate that August sunshine averages over 80 percent of the maximum possible, a number that is comparable to some of the sunniest locations on the globe. No comparably situated measurements are available for Oregon, but based on the cloud statistics, the percent of possible sunshine is similar and possibly even a bit larger. Because the sunshine measurements take place across the whole of the day and the eclipse is in the less-cloudy morning hours, the sunshine measurements may even be a little pessimistic.
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Updated December 2016