There will be a total solar eclipse on August 21, 2017, the first such eclipse in the United States since 1979, and the last until 2024. If you are anything like me, you are now thinking, “Where do I watch this event?” Although there are several maps of the narrow path out there, there is a different question that this post will answer with analytics: “Where is the best place (the view least likely to be obscured by clouds) to watch this event?”
This analysis used historical cloud cover data from the Global Data Assimilation System (GDAS) archive, which comes in the form of the cloud cover percentage every three hours in a grid of latitudes and longitudes. I pulled all of the U.S. data available within one week of August 21st for every year from 1979 to 2010. Data at each grid point was specific to 18:00:00 UTC, the closest to the time of the eclipse.
Plotting the average cloud cover at each grid point, with the lowest average that falls within the eclipse path, seemed to hint at the “best” location. However, that approach involves the assumption that the average is a good descriptor of the historical cloud cover distribution. Put another way, it assumes the cloud cover over a given grid point follows a roughly Gaussian distribution, or at least a symmetric and single-peaked one. Figure 1 shows the contrary:
The cloud cover distribution tends to peak at 0 (clear skies) and 100 (overcast). There are therefore two statistics to plot to decide where clouds are least likely to hinder the eclipse viewing experience:
1) How likely is it to be clear (or the fraction of historical data with cloud cover under 25%)? (See Figure 2)
2) How likely is it to be overcast (or the fraction of historical data with cloud cover over 75%)? (See Figure 3)
WHERE SHOULD YOU WATCH THE ECLIPSE?
The two plots above show the likelihood of clear/cloudy skies in the US during the eclipse. They are color coordinated: yellow=“Sunny and good to go!” and dark blue=“Cloudy, so don’t watch here!” The statistical uncertainty on the frequencies is about 5% and labeled with a binned color scale, such that a different color roughly indicates a statistically different value. The eclipse path is plotted with a gray curve. If located in this path, you will get to see the total solar eclipse, lasting longer the closer to the center you get.
The most striking feature of both plots is sunny California, which is very likely to be clear (˜80% chance) and not overcast (<10%). Unfortunately for Californians, the trajectory of the eclipse passes over them! The best parts of the country for viewing the eclipse are Oregon and Idaho. The Payette National Forest places itself as the best wide-open view, with a 66% chance of clear skies and an only 20% chance of overcast skies.
The Midwest states (Nebraska, Kansas, and Missouri) are equally good for witnessing the eclipse with the probability of overcast skies at about 25% throughout. Moving into the Southern states (Kentucky, Tennessee, and South Carolina), that chance goes up to about 30% while the chances of a clear sky fall below 50%.
May this analysis help you decide where to watch the total solar eclipse this upcoming August. Since the eclipse falls on a Monday, you will likely have to take the day off and drive or camp to see it, unless you are lucky enough to already fall in the path of totality. For any readers in Kansas City, I sure hope you take a moment to go outside, look up (with protective glasses, of course), and experience the astronomical event of a total eclipse.
Last modified: November 22, 2017