Recent rainfall in the U.S. Pacific Northwest has left a white, ashlike residue on cars left outside overnight. Reuters/Dinuka Liyanawatte

Meteorologists are stumped after rainfall in Oregon, Washington and neighboring areas in which the precipitation appeared to be dirty, or milky, in texture. Photographs posted by the U.S. National Weather Service indicate that, unlike typically clear rain, this liquid was infused with something else. But what?

The so-called milky rain, pelting the Pacific Northwest Friday morning, left cars and other outdoor fixtures covered with streaks of white, dusty material.

A number of theories abound, with meteorologists telling BuzzFeed the likeliest cause is dirt and dust from the Northwest landscape being circulated within the weather system.

“The ash is more than likely from Volcano Shiveluch in Kamchatka Krai, Russia, which spewed an ash plume to about the 22,000-foot level in late January,” the Walla Walla County Emergency Management agency in Walla Walla, Washington said in a Facebook post. It added there are a number of active volcanoes that could be responsible. “It has been deposited in a wide spread area, including Washington and Oregon,” the agency said.

The right-leaning media outlet the Blaze suggested the cloudiness of the rainwater could be caused by dust from recent eruptions of Japan’s Sakurajima volcano. Commenters beneath the article took on a decidedly more conspiratorial bent, accusing the liberal media of being too quick to blame climate change.

The NWS plans to re-examine old satellite data with the hope of finding out exactly what happened. This type of activity isn’t totally unprecedented in Oregon, although ashlike rain typically falls toward the end of a long hot summer.

“Oregon has strong winds,” NWS meteorologist Mark Tuner told BuzzFeed. “In some summer dry spells, we’ll get big dust storms, and then if it rains, we can see a similar thing. ... It’s a mystery at this point.”