On Aug. 21, the moon will totally block out the sun along a 60-mile-wide path from Oregon to South Carolina. The last time a total solar eclipse swept across the entire continental United States was 1979, and the next time it happens isn’t until 2045.
Given the rarity of the event, people have been making travel plans and booking hotels since last year. But before you buy the plane ticket, you might want to consider which destination has the best odds of clear skies in August.
The eclipse begins at 10:15 a.m. Pacific Time near Lincoln Beach, Ore., and races across the United States until 2:49 p.m. Eastern Time when it crosses the tip of Cape Romain, just east of McClellanville, S.C., as it heads out to the Atlantic Ocean.
Places north and south of the path of totality — including all of Alaska and Hawaii, Mexico, Central America and northern South America — will see a partial eclipse. In the Washington, D.C., area, for example, the moon will cover roughly 81 percent of the sun.
Unfortunately, while astronomers have been predicting the paths and timing of eclipses decades into the future, meteorologists can’t say more than a day or two in advance whether it will be cloudy enough to ruin the show.
Weather events operate on tiny scales from the molecular (latent heat released when water vapor condenses) to the global (jet stream patterns). This means small changes in conditions can lead to large differences in outcomes. Chaos plays a large role in weather forecasts — but not in eclipse predictions.
To decide where to go for the best chance of clouds not hiding the eclipse, we have to turn to the past. That is, look for places along its path with the best odds of clear — or at least partly sunny — skies in August.
“The big challenge with a summer eclipse is the risk of showers and thunderstorms, which are by nature short-lived and impossible to predict in a pinpoint fashion,” Weather Underground’s Bob Henson wrote. “Fortunately, the midday timing of the eclipse in the western and central U.S. should be just ahead of the typical formation time of the next day’s showers and thunderstorms.”
But Henson also notes that mid-August is also near the peak of hurricane season, which means there’s increased risk of cloudiness, especially for the Southeast.
Along the South Carolina coast, you could encounter afternoon “sea breeze” clouds. As the land heats up, warm air begins rising and air from over the cooler ocean flows inland, feeding humidity into the rising air to form cumulus clouds.
Since the clouds form over warm land, you are more likely to see the eclipse from the ocean beach than a few miles inland.
Coastal Oregon is the first place the eclipse will be seen, but the most likely to be clouded over. Nevertheless, if you head inland to the eastern slopes of the state’s north-south mountain ranges, you find good odds for clear weather.
Humid air flowing in from the Pacific feeds clouds and maybe precipitation on the mountains’ western slopes, which usually removes much of the humidity from the air by the time it crosses the mountains. Most of the time this makes places on the eastern sides of the West’s mountain ranges prime viewing locations. This isn’t true of the southern Appalachians where it’s much more humid. In this region, moist air flows in from the Atlantic Ocean or Gulf of Mexico.
If you want to get really specific, Jay Anderson and Jennifer West developed eclipsophile.org, which is a state-by-state guide to typical weather along the path of totality.