Honorable Mention, Places: 'Graveyard' by Julie Fletcher. I cannot describe the eerie feeling I had when I walked in on this scene. I followed a massive storm front several 100 kilometers hoping to capture something special but this blew my mind. The surreal milky green water is a natural phenomenon caused by electromagnetic activity from the lightning hitting the waters surface. There was no rain where I was and not much wind either but in the distance the sky was charged and angry subjecting its wrath over the graveyard of dead trees in this normally very dry lakebed. I was able to capture a series of unique images this being one of the best. ( Julie Fletcher/National Geographic Photo Contest

By Courtney Sherwood

PORTLAND, Ore. (Reuters) - Scientists from two U.S. Pacific Northwest laboratories plan to conduct tests of unusual precipitation that fell across the region over the weekend in hopes of pinpointing the origins of so-called "milky rain" that has mystified residents, officials said on Wednesday.

Officials at both the Pacific Northwest National Laboratory and the Benton Clean Air Agency, both in Washington state, said they had collected samples of the rain, which left a powdery residue on cars across a wide swath of the two states.

Scientists at the Richland lab said they believe the rain may have carried volcanic ash from an erupting volcano in Japan, while the clean air agency said its staffers believe dust from central Oregon was the culprit.

The National Weather Service has said it believes the powdery rain was most likely a byproduct of dust storms hundreds of miles away in Nevada, although it could not rule out volcanic ash from Japan as a possible culprit.

But the National Weather Service has also said it was not equipped to perform a chemical analysis of the rain that would be required to pinpoint its origins.

Wherever the milky precipitation came from, officials say they do not believe it poses any health risk. Air monitoring stations did not detect anything unusual while the rain was falling, said Robin Bresley Priddy, executive director of Benton Clean Air.

"We don't have any reason to think there's anything wrong, but there's no reason not to be cautious if you're concerned," she added. "You may want to wash it off your car with water, rather than with your hands, and avoid touching it and breathing it in."

(Editing by Cynthia Johnston and Eric Walsh)