Black Ice -- Hidden Hazard -- Cloudless Skies, Cold Air And Still Winds Can Make Roads Perilous

------------------------------------------------------------------ Atmospheric sciences. Motorists see it every winter, but black ice can be something of a mysterious phenomenon to many. A complex interplay of atmospheric conditions are necessary before this bane of traffic strikes. ------------------------------------------------------------------

University of Washington professor Cliff Mass reaches for a small stack of manila folders. Leafing through a couple of files, he stops at various legal papers. The county names change - as do the names of the victims and the photographs of sections of roadways - in each file.

The allegation of each lawsuit, however, is basically the same: A government body entrusted to keep roadways ice-free failed to meet its end of the bargain. Because of that alleged negligence, the victim's vehicle slipped, slid or, in some especially tragic cases, overturned on perilously icy roads.

Mass, a professor of atmospheric sciences, spends most of his time doing research and teaching weather forecasting and analysis.

From time to time, however, he steps into the courtroom and becomes an expert witness. There, he testifies about meteorology: How water droplets in the fog deposit themselves on chilled roadways, creating patches of black ice.

"It's almost like a broken record," he says of his unchanging testimony.

What for some is common knowledge, however, to others is elusive.

You need to look no further than Christmas Day. The morning began with thick fog blanketing roadways and ended with dozens of accidents when motorists skidded on black ice.

The Washington State Patrol called for the Department of Transportation to come with its sanding trucks at 15 accident scenes and relayed the same de-icing request at six other locations not yet the scenes of accidents.

Mass asserts that black ice is perhaps the single most perilous meteorological condition that local residents encounter. Motorists know that melted snow freezes into ice after sundown. But they're less clear about why ice forms on the road in the absence of rain or snow, he says.

With pencil and paper, Mass illustrates how black ice develops.

The phenomenon is associated with high-pressure systems, which bring chilly, cloudless nights to the area. A relatively clear night with winds in the 3- to 7-mile-per-hour range are "key pre-conditions," Mass says.

That's because the clouds slow the Earth's nighttime cooling. As the Earth sends heat skyward, in the form of infrared radiation, the clouds absorb some radiation and send it right back toward the Earth.

Clouds are "like a blanket, really," Mass says. "And a blanket keeps you warm, not because it heats you, but because it slows down the loss of heat from your body."

Without clouds, the Earth's surface cools more effectively and efficiently.

Those crisp, dry nights are prime breeding ground for fog - especially coupled with already moist ground. The fog is heavy with little water droplets; Mass fills a circular fog shape with dozens of dots, sort of meteorological pointillism.

The cold night has chilled the road's surface, perhaps already frosting some sections. When the fog moves over the road surface, its water droplets freeze there.

"You can develop quite a bit of ice very quickly. Within a half-hour or an hour, you can get an 1/8 inch," he says. "The longer the fog is in contact with (the road), the thicker the ice will be."

The worst fog-related ice forms on roads that are slightly elevated, near boggy areas, he says.

Many motorists head out on their travels, their optimism buoyed by weather reports of above-freezing temperatures. But those official weather readings aren't the total picture, Mass says.

The National Weather Service measures temperature about 5 feet off the ground; ground temperatures can be up to 5 degrees lower. And weather can vary by 5 to 10 degrees from the top of a hill to the bottom, depending on the elevation loss.

Also, temperatures tend to be warmer near large bodies of water but get gradually lower as you move into the hinterlands or move up in elevation.

To test that theory, Mass and several students set out before dawn a few years back, driving cars equipped with thermometers. They worked their way around the Puget Sound area, recording temperatures.

Seattle's downtown core, studded with buildings and covered with pavement, was a balmy 40-plus. Areas adjacent to Puget Sound and Lake Washington were relatively warm as well. But the farther they drove from major water bodies, the chillier their readings: 26.6 degrees in Maple Valley, 29 degrees in Carnation and temperatures in the mid-20s for Woodinville.

The historic trouble spots noted by city and county engineers back up these observations. Elevated roadways that are not warmed by the Earth or water freeze quickly, as do roads that cut through fog-populated valleys.

The city of Seattle has overnight crews that drive the traffic hot spots (or, rather, cold spots) where ice and frost are known to build up. They cruise the West Seattle bridge, the Magnolia Street Bridge, the Northeast 45th Street viaduct from the UW to University Village, among others.

Crews drive the roadways and sometimes step out of their vehicles to test the ground for ice and read outdoor temperature gauges, says Roxanne Thomas, the Seattle Engineering Department's manager of cleaning and drainage.

The scouting missions help better determine when sanding is needed.

The county has a top-four list of problem roads - two are valley roads, two are hillside roads; each is where fog and low clouds congregate.

"These four are the most troublesome: Cherry Valley Road (in the Duvall area) - that's the worst one," says Henry Kuga, King County's road maintenance superintendent. "These are in the order from worse to better: Carnation Farm Road is the second one. Kelly Road (in far East King County) and West Snoqualmie Valley," Kuga says.

The Department of Transportation (DOT), responsible for thousands of miles of roadway, operates a quasiweather system of its own on some roadways.

Thirty Roadway Weather Information Systems (RWIS) - which resemble little outhouses - are scattered throughout the state, collecting air temperature and the temperature 16 inches below the Earth's surface.

Inside the $25,000 outhouses are computers that relay the statistics to other computers. The readings are used to determine when the roads are likely to freeze. The DOT used to guess and send a truck filled with sand and de-icer.

The array of information - especially real-time surface temperature - is more useful than the air temperatures provided by weather forecasts.

"We get a printed forecast based on that (DOT) information that tells us when the pavement will reach 32 degrees. . . . It's probably as accurate as anything the weather people are able to do at this time," says Phil George, a DOT maintenance and operations superintendent whose area stretches roughly from Lynnwood to Southcenter and Interstate 5 to North Bend.

Most of the state's mountain passes, including White, Stevens and Snoqualmie, are equipped with such systems, says Dave Bowers, a DOT roadway maintenance engineer.

As early as the summer, the DOT begins building its collection of sand piles and mixing the sand with an agent that prevents it from freezing.

The DOT this season expects to use about half its 5,000 cubic yard stockpile in the greater Seattle-Bellevue region to combat frost and ice, George says.

No mere child's play, the sand piles are one of a number of tools the DOT uses to combat icy roads. It also uses a tiny amount of a liquid-based compound to prevent ice from even forming. The stiff price tag - $1,100 per metric ton for the liquid vs. $100 per metric ton for sand - precludes extensive use of the compound.

Whatever the method, the agencies might have plenty of opportunities to use them.

Back at the UW, Mass turns to a computer screen that show the extended weather forecast. Isobars of pressure swirl in yellow dotted lines, the `Hs' in the center signifying high-pressure systems projected to continue to move right over this area or just to the east.