Cloud cover forecasts increase the concern of solar eclipse observers

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

“Sometimes the weather gods like to laugh at you,” said Epstein, 58, whose mood has lifted this week thanks to a modest improvement in the still-dubious Dallas forecast since Monday.

Nationally, eclipse forecasts are rather… Cloudy – As in unclear, murky, mysterious, but also as in literally filled with obnoxious clouds that could obscure this big scene.

A total eclipse is astronomically predictable and meteorologically unpredictable. Experts know exactly when the Moon will completely cover the Sun. They cannot predict whether humans on Earth will be able to see it happen.

While the Moon needs roughly three hours to eclipse the Sun, this wonderfully strange period of totality — when the Sun is completely obscured except by its magical atmosphere, and bright stars and planets stand out in the dark sky -It only lasts a few minutes. People in the contiguous United States would not have another opportunity to see such a thing for twenty years.

With less than a week before the April solar eclipse, New England appears to have the best chance for perfect weather. Mexico also sits beautifully. But these are anxious times for eclipse enthusiasts 2,000 miles apart.

“I wish I could express my appreciation,” said astrophysicist Adam Frank of the University of Rochester, noting that his city in upstate New York experiences lake-effect weather, often cloudy in the spring. He will remain in Rochester no matter what, because he is committed to providing televised commentary on the eclipse.

“I have high hopes and low expectations,” he added.

The hard work of forecasting clouds

Cloud forecasts are full of ambiguities, uncertainties, and probabilities that are difficult to understand. It's fair to ask: What exactly does “cloudy” mean?

Clouds form when the air rises and there is enough moisture in the air. Low pressure, which allows air to rise more easily, often generates drag. High pressure, which prevents air from rising, tends to promote brighter skies.

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

What people really want to know is whether it will be cloudy over their specific location during the minutes and hours of Monday's eclipse. However, models cannot accurately predict clouds with this kind of accuracy so early. Instead, they predicted what proportion of the sky might be covered by clouds at three-hourly intervals.

With that in mind, eclipse astronauts in the path of totality should probably be concerned about any forecasts of cloud coverage of more than 60 percent, and cautiously optimistic about any forecasts of less than 30 percent. In the middle, the situation is very ambiguous.

The type of clouds is also important. High clouds are composed of ice crystals, while lower clouds are composed of water droplets. High, thin clouds won't completely obscure the eclipse, but low, dense, dark clouds blocking the sun can spoil the show.

Adding concern is that spring is a particularly difficult time of year to predict cloud cover.

For example, persistent cold from winter can result in cold, moist air that creates clouds overnight, while daytime sun and warmth are not yet strong enough to dissipate clouds as quickly as forecast models predict. The jet stream tends to move weather systems more slowly in the spring than in the winter. This could also cause cloud cover to clear more slowly than expected.

Another variable is the direct effect of the eclipse. The air temperature drops dramatically as the sun blocks out and stops heating the Earth, causing the air to stop rising. One possible effect, noted by many eclipse astronauts, is the creation of an “eclipse hole” in the cloud cover.

But this does not happen for all types of cloud. Low-level cumulonimbus clouds — those puffy cotton balls — are likely to dissipate during the eclipse, according to paper It was published earlier this year in Communications Earth & Environment.

Where the forecast stands for Monday

The models are currently in very good agreement for April 8, showing lower pressure and a cold front from Texas to Arkansas, then higher pressure moving to the northeast. So this is most encouraging for New York, Vermont, and Maine, and least encouraging for Texas and Arkansas.

There are two caveats, though. First, we are five days away. At this range, things can still change, no matter how confident the forecast is now. Thursday or Friday is when people should start taking cloud forecasts seriously. However, cloud forecasts can sometimes be a challenge even on the same day.

Second, just because the models may be right about the overall weather pattern, doesn't mean they're right about the timing. At this range, patterns can remain off for 12 to 24 hours in either direction. If so, it's not impossible that cloud forecasts could change dramatically for better or worse, depending on location.

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

“I know I'm not responsible for the weather, but still, a lot of people put their faith in the idea that this will be a great event,” he said. “To think that it was all for nothing was very upsetting.”

At the Dallas Arboretum, the eclipse will be celebrated with three days of events and organizers We expect 10,000 people too As did NASA scientists and the national media on Monday. But the nursery's vice president of marketing, Terri Leyendecker, said Tuesday she's not worried about the weather.

“They're predicting a 30 percent chance of rain. In Texas, that doesn't really mean anything. It's changing so quickly all the time,” Leyendecker said. “While we monitor the weather, mainly for safety reasons for our guests, the show must go on.” “When you're outdoors.”

“It will be a beautiful day in the park, regardless,” she added.