Scientists say they've determined the cause of the strange "milky" rain that fell on parts of the U.S. Pacific Northwest in February, putting it down to extreme weather and a dry lake bed 500 miles away.
A rare weather pattern near Oregon's remote Summer Lake whipped up particles of sand and soil from the dry lake, which were carried north before a rainstorm deposited them in dirty, milky-white raindrops over parts of Oregon, Washington and Idaho, researchers at Washington State University in Spokane say.
A clue to the rain's origin was a high sodium content detected in collected samples, explains WSU hydrochemist Kent Keller, who studied rainwater gathered near Kennewick, Washington.
"A lot of sodium was in that milky rain," he says. "The chemistry is consistent with a saline source from a dry lake bed."
The analysis ruled out other theories that had been put forward about the unusual phenomenon, such as ash from distant volcanic eruptions in Russia or Mexico, or wildfires or a Nevada dust storm that occurred around the same time.
An examination of global and regional wind patterns ruled out the volcanoes.
"The strongest upper level winds didn't reach this region on a direct line from Russia or Mexico. Instead, the air flow was locally from the south," says WSU meteorologist Nic Loyd.
That also ruled out ashy debris from last year's giant wildfires, the researchers said.
Instead, the wind direction and patterns pointed directly at Summer Lake in southwest Oregon, where a dust storm with winds of 60 mph had struck the night before the fall of the distinctive rain.
"That would have been powerful enough to lift a good-size dust plume," says Loyd.
The plume of light-colored particles was carried north to where a meshing of three weather systems created a rainstorm on Feb. 6 that left a chalky residue streaking windows of buildings and cars in some 15 cities in the three states, the researchers say.
The scientist noted a similar weather mystery that occurred in southern New Mexico seven years ago, in which sodium-rich rain was eventually traced to a dry salt lake bed 120 miles from the rainstorm.
