Rainier's 'cap cloud' tells when rain is coming

Mount Rainier is anyone's quickest weather report, deciphered at a glance. See cap clouds hovering over the mountain and, more than likely, we'll soon be in for it.

Sure enough recently, a tight toque of clouds over the mountain ushered in a frothing blast from the southwest, a true November soaker that rattled the roof.

So it goes as our Northwest moisture-therapy program gets under way, with regular appointments from now through spring.

The mountain sits there, blocking the path of stable, moist air, forcing it up and over the mountain in its way. The air rises smoothly on the windward side of Rainier in an unbroken wave up and over to the lee side.

The leading edge of the moist air cools as it rises, and water droplets form, creating a smooth, lens-shaped cloud over or near the mountaintop.

Each droplet persists for only about 10 minutes as it cruises over the mountain, estimates Dale Durran, chairman of the Department of Atmospheric Sciences at the University of Washington. The air warms as it descends into the lowlands. And poof, the cloud dissipates on the leeward side of the flow.

The cap is continuously rewoven aloft, as more moist air continues to flow up and over the peak. That is what creates the appearance of a cloud snugged over the ears of the mountain, unmoving — until the weather pattern breaks.

More saucerlike clouds can also persist, sometimes neatly stacked, for many miles in the lee of the mountain.

Also called lenticular clouds, or mountain-wave clouds, these spectral visions, sometimes glowing with sunset or sunrise hues, have enlivened Northwest lore every bit as much as Sasquatch. UW meteorologist Cliff Mass reports in his book "The Weather of the Pacific Northwest" that the UFO craze started right here, with a sighting of mountain-wave clouds from an airplane in 1947.

Kenneth Arnold, a businessman from Boise, Idaho, was sure he was seeing flying saucers as he looked down from an airplane at nine lenticular clouds over the Cascades, near Mount Rainier. He told The Associated Press of brilliant "saucerlike objects," igniting a national UFO obsession that has yet to abate, Mass writes.

But really, who hasn't wondered just what those clouds are, suspended so perfectly over Rainier's snowy visage? Nothing else looks quite like them. Their otherworldly look is part of the fun — a bit of a payoff for the weather to come.

The clouds can form at any time of year. But because moisture is part of the recipe, they are more common in fall and winter as our dreamtime sets in, a long winter's nap in blankets of mist, fog and clouds.

Lynda V. Mapes: 206-464-2736 or lmapes@seattletimes.com