SPOKANE - Scientists at Washington State University said Tuesday they have solved the mystery of the dirty rain that fell on portions of the Pacific Northwest in February.

The milky rain on Feb. 6 was the result of a rare weather pattern that began nearly 500 miles away near a dry Oregon lake bed, meteorologist Nic Loyd said. A meshing of weather systems ultimately caused dirty-white-colored raindrops to fall in parts of Washington, Oregon and Idaho.

The high sodium content of the rain, combined with an analysis of wind patterns during that time, point to a severe dust storm that whipped up sand and soil at Oregon's remote Summer Lake, the scientists said.

The particles were carried north by strong winds and then dragged down by a rainstorm over parts of the three states.

"A lot of sodium was in that milky rain. The chemistry is consistent with a saline source from a dry lake bed," said WSU hydrochemist Kent Keller, who studied samples taken in Kennewick, Wash.

Scientists at WSU initially focused on Summer Lake shortly after the rainfall. But they also considered other theories, such as distant volcanic eruptions, ashy debris from last year's giant wildfires and fallout from a Nevada dust storm.

"At first, we suspected it was related to wind erosion of landscapes that had previously burned, but the wind trajectory analyses didn't add up," said Brian Lamb, who runs the university's Laboratory for Atmospheric Research.

Nor did the wind trajectories from volcanic eruptions that occurred the same week in Russia and Mexico, Loyd said.

Evidence instead pointed to Summer Lake where, the night before the milky downpour, a dust storm clocking 60 mph winds struck.

"That would have been powerful enough to lift a good-size dust plume," Loyd said.

The dirty rain left chalky residue coating windows and vehicles in at least 15 cities, ranging from Hermiston, Ore., to Spokane and North Idaho.