Weathercatch: Solving riddle of the milky rain that fell seven years ago this month

The Inland Northwest is used to precipitation in February, but not the kind that fell on Feb. 6, 2015.

That Friday morning, a mysterious milky rain began to fall in parts of Eastern Washington, including here in Spokane, and northeast Oregon. It left a chalky sheen on cars and windshields, people’s coats and along roadside curbs.

The odd-colored rain made big news from Portland to London. Where it came from was “a good old-fashioned weather mystery,” CNN reported.

And it was a mystery that took weeks to solve. Theories ranged from ash blown by a volcanic eruption in Russia and another in Mexico, a Nevada dust storm and ashy particles from burn scars left by Pacific Northwest wildfires that summer.

Ultimately, all of those sources were ruled out, as were aliens and chemtrails. Working with scientists at Washington State University, including a hydrochemist, two geologists and an atmospheric scientist, our group concluded that the source of the milky rain was an ancient saline lake bed in remote Oregon, nearly 500 miles away.

How did we figure it out? By examining the chemical composition of the milky substance and analyzing recent wind patterns and the overall weather.

The liquid samples contained high-sodium dust particles, suggesting they had originated from a dry alkali lake bed. Analyzing weather conditions from the night before the downpour, we discovered a severe wind storm had whipped up copious amounts of dust and debris at Summer Lake in southcentral Oregon. As our geologists knew, a historic drought the summer before had left the lake mostly dry and its bed exposed. We concluded that on the night of Feb. 5, 60-mph wind gusts kicked up loads of saline-saturated dust into the air from the lake bottom. Then, strong southerly winds lifted the dust plume high into the atmosphere where it was carried hundreds of miles northward. Hours later, a rainstorm moving through parts of Washington and Oregon dragged the residue downward.

The whitish raindrops that battered the ground and dirtied people’s cars and clothes drew reports within a 200-mile stretch that included Spokane, Coeur d’Alene, Moses Lake, the Tri-Cities and Pendleton, Oregon.

The phenomenon marked the beginning of several days of rainfall across the Pacific Northwest. The droplets turned clear within an hour of landing. Even so, they left plenty of white residue and bewildered commuters and meteorologists in their path.