Reading the clouds

In addition to explaining the forces behind Northwest weather patterns, providing a primer on forecasting, and a look at what climate change will bring to the region, Cliff Mass' new book offers tips on how to read the skies for signs of impending rain or snow (photos courtesy of Cliff Mass):

• The first hint of an approaching storm system is often high, wispy cirrus clouds (right) caused by the rising air motion.

• When the wispy clouds spread and thicken into a layer of cirrostratus clouds, often producing a halo or ring around the sun, bad weather can be from six to 18 hours away. The storm system may also pass to the north or south.

• The cap clouds and lens-shaped lenticular clouds (above) that form on Mount Rainier and other mountain peaks mean winds aloft are accelerating and the atmosphere is close to saturation, which can also presage an approaching weather system.

• As high cirrus clouds thicken and lower into altostratus clouds — the type that leave only a dim, watery hint of the sun or moon — the weather threat is serious and rain or snow is perhaps three to six hours away.

• Pull out your umbrella if the clouds descend and thicken farther into a nimbostratus layer (above) that completely obscures the sun or moon, and the temperature drops suddenly.

• After a front moves through, rain stops and the sky brightens a bit. But often, the unstable air leads to formation of towering cumulus clouds and a postfrontal pattern of showers and sunbreaks.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com