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What will the weather be on eclipse day?

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Average August fractional cloud amount at 10:30 am local standard time. Map based on 17 years of satellite observations. See http://eclipsophile.com/overview
Photo: Based on Jay Anderson, eclipsophile.com

A total solar eclipse is a mind-blowing event that leaves a lasting impression on anyone who has the chance to see one. To make sure nothing is left to chance, some eclipse buffs prepare themselves months in advance. But in spite of all precautions, there is one element that can spoil your experience: the weather.

That’s why experienced eclipse chasers prefer to be mobile. In the days and hours leading to the eclipse, they’ll track the weather forecasts in every possible way. If need be, they’ll move to a site offering better odds of clear weather at the critical time. That way, they optimise their chances of observing the most important part of the eclipse: totality.

The next total eclipse occurs on August 21, 2017. Analysis of the cloud cover on that date over the last few years, shows that the western United States offer the best probabilities of favourable weather. In Oregon, Idaho, central Wyoming and western Nebraska, past years’ statistics based on satellite observations show that the fraction of clear sky at eclipse time is about 80% on average. In the eastern US, that proportion falls below 40%.

In Québec, the probabilities of having perfectly clear skies at eclipse time are quite low. However, because the eclipse will only be partial, this requirement is not as crucial as it would be for locations in the path of totality. Considering the duration of the phenomenon (more than two hours) and the absence of a specific, not-to-be-missed “critical moment,” even partly cloudy skies will let viewers catch glimpses of the eclipse and see how it evolves. In the Montreal area, the same satellite based statistics show that the fractional cloud cover is around 50% on average during August afternoons. The average cloud cover increases somewhat further north in Quebec, but unless you find yourself under completely overcast skies, the chances of seeing at least part of the phenomenon remain quite good.

To be clear: these data are only statistics. For true, specific weather forecasts, we’ll have to wait until a few days before August 21. Let’s just hope that Mother Nature decides to collaborate on that day and that the eclipse unfolds under perfectly clear skies!

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