A cloudy forecast will add to the worry for solar eclipse watchers

When Adam Epstein saw the forecast for Dallas a few days ago on April 8, he felt sick to his stomach. Clouds!

A New York real estate developer was so awestruck by the 2017 total solar eclipse he saw in the perfect conditions in the Oregon desert that he told his friends they had to see the next one. They believed him. Epstein organized a trip this year to see “Totally,” and at last count he had 82 people in his group.

He studied climate maps and chose Dallas as their destination because it had the best chances for historically clear skies. In early April.

“Sometimes the weather gods want to laugh at you,” said Epstein, 58, whose mood has been upbeat this week thanks to modest improvements in the Dallas forecast since Monday.

Across the country, the eclipse forecast is the opposite Cloudy – vague, vague, dark, but full of hideous clouds that can obscure this great scene.

A total eclipse is astronomically predictable and meteorologically unpredictable. Experts know when the moon will completely cover the sun. Humans on earth cannot predict whether it will happen.

It takes almost three hours for the moon to cover the sun, when the sun is completely hidden but for its absorbing atmosphere, and bright stars and planets appear in the dark sky. – Lasts only a few minutes. For the next 20 years people in the United States would not have another chance to see something like this.

With less than a week until the April eclipse, New England looks like its best chance for perfect weather. Mexico is also sitting pretty. But these are anxious times for eclipse lovers 2,000 miles away.

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“I'm going to keep my fingers crossed,” said Adam Frank, an astrophysicist at the University of Rochester, who noted that his city in upstate New York experiences lake-effect weather and is mostly cloudy in the spring. He'll be in Rochester no matter what, because he's committed to providing eclipse commentary on television.

“I have high hopes, low expectations,” he said.

Predicting clouds is tricky work

Cloud forecasts are shot through with ambiguities, uncertainties and hard probabilities. It is fair to ask: What is “the cloud”?

Clouds form when the air rises and there is enough moisture in the air. Low pressure allows air to rise more easily, often forming clouds. High pressure prevents air from rising and promotes sunlight.

Some weather systems create large areas of rising, moist air that lead to large areas of solid cloud cover. Other systems only create pockets of rising air here and there, some pockets moist enough to form clouds, others not. These clouds – both their location and timing – are very difficult to predict, especially a day or two in advance.

What people really want to know is whether it will be cloudy in their exact location during the exact minutes and hours of Monday's eclipse. However, models cannot accurately predict clouds with that kind of accuracy. Instead, they predicted the percentage of the sky covered by clouds over a three-hour period.

With that in mind, eclipse watchers in the path of totality should be concerned about forecasts of more than 60 percent cloudiness, and cautiously optimistic about any forecast of less than 30 percent. In between, the situation is very confusing.

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The type of clouds is also important. High clouds are made up of ice, while low clouds are made up of water droplets. High, wispy clouds won't completely cover the eclipse, but low, thick, dark sun-obscuring clouds can spoil the show.

Adding to the anxiety, spring is a very difficult time to predict cloud cover.

For one thing, the lingering cold from winter will lead to cool, moist air that creates clouds at night, while the daytime sun and heat aren't strong enough to disperse clouds as quickly as forecast models expect. The jet stream tends to move weather systems more slowly in spring than in winter. It can also lead to slower-than-predicted cloud cover.

Another variable is the direct effect of the eclipse. As the sun is obscured the air temperature drops dramatically and it stops heating the ground, causing the air to rise. One potential effect noted by many eclipse goers is the formation of an “eclipse hole” in the cloud cover.

However, this is not the case for all types of clouds. Low-level cumulus clouds — wispy, puffy cotton balls — are often dispersed during an eclipse. A sheet Published earlier this year in the journal Communications Earth & Environment.

The forecast stands on Monday

Models are currently in good agreement for April 8th, showing low pressure and cold from Texas to Arkansas, then high pressure moving northeast. So it's most encouraging for New York, Vermont and Maine, and least encouraging for Texas and Arkansas.

However, there are two caveats. First, we are five days away. At that range, however optimistic the forecast may seem right now, things can still change. Thursday or Friday people should take the cloud forecast more seriously. Cloud forecasts can sometimes be challenging even on a single day.

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Second, just because the models are correct in the overall weather system does not mean they are correct in time. In this range, models can be disabled for 12 to 24 hours in both directions. If so, depending on the location, it is unlikely that the cloud forecast will change for better or worse.

Epstein, a real estate developer, said his friends assured him they'd have a good time even if the skies didn't cooperate over Dallas. However, eight days before the eclipse, when the forecast was particularly bleak, he felt miserable.

“I know I'm not responsible for the weather, but a lot of people have faith in the idea that it's going to be a great event,” he said. “It was so sad to think it was all going to waste.”

At the Dallas Arboretum, the eclipse will be celebrated with three days of events and organizers Expect 10,000 people NASA scientists and the national news media on Monday. But Terry Lendecker, the Arboretum's vice president of marketing, said Tuesday he wasn't worried about the weather.

“They have predicted a 30 percent chance of rain. In Texas, that doesn't really mean anything. It changes so fast all the time,” Lendecker said. “When we look at the weather, the show has to go on when you're an outdoor venue, mainly for safety reasons for our guests.”

And, “It's going to be a beautiful day in the garden, regardless.”

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