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Saturday might have been the Northern Hemisphere's shortest day of the year. But in Seattle, Friday proved to be its darkest day in (recent) history.

Technically, it was the darkest day ever recorded by three devices that the University of Washington installed on the roof of its atmospheric sciences building in 1996. The devices, known as pyranometers, measure the amount of solar radiation—or energy—that reaches the surface of the earth each day. The approaching winter solstice already would have limited the amount of solar radiation reaching the atmosphere Friday. But, mixed with the record-setting rainfall and heavy clouds that parked above the Seattle area late last week, "everything came together to give us this record," said Cliff Mass, a meteorologist and professor of atmospheric sciences at the UW.

On his weather and climate blog, Mass wrote Saturday that the pyranometers on campus recorded just 0.37 megajoules of solar radiation over a square meter surface. By comparison, the levels of solar radiation were nearly 73 times that of Friday during several days in July. The previous record low was set in Dec. 2006, at 0.39 megajoules of solar radiation. "The infernal darkness on Friday was extraordinary, even by Seattle standards, and to a lesser degree continued into Saturday," Mass wrote. "But sunglass owners can rejoice: More light is on the way."

Citing local weather forecasts, he expected a "mixed bag" Monday, with clouds delivering darker conditions along the western side of the Cascades. "But some locations in Western (Washington) will be brighter, and Southern Oregon will be quite luminous," he predicted. Mass said more than 100,000 people read his darkest-day-in-history post Friday, but he wasn't necessarily surprised by the traffic. "Psychologically, this has a big impact on some people," he said, referring to seasonal depression. "That's one reason to want to understand this ... but especially when you have severe weather, it's a force that's greater than ourselves, and people can sense that."