Highway Glare Ice, Part II
Recently this column discussed why winter highways often wear a thin layer of ice when there has been no rain or snow for a long time.
Most experts polled believed that the temperature differential between the air and the pavement caused the icing. When warm air overlies a cold surface, such as a paved road, moisture in the air may precipitate out directly onto the asphalt and freeze. This type of ice is most common when warm days alternate with cold nights, or when the weather warms after a long cold spell has chilled the ground. Automobile exhausts were another reason cited, as was the obvious one of the ice simply being the compressed remnants of a snowfall.
That column prompted many readers to write or call in with their own theories on the subject. Vern Seifert of Anchorage, for example, feels that the Bernoulli effect may be partially responsible. When a fluid (or gas, like air) is forced to flow faster by passing through a constriction or around an obstacle, its pressure is lowered. The Bernoulli effect is what keeps airplanes flying, because the lower pressure of the air flowing over the rounded top of the wing provides lift. Seifert suggests that when air near its saturation point is channeled beneath a moving vehicle, this effect may cause it to lose enough pressure to drop part of its suspended moisture.
Salt was the preferred villain to several callers. The State Department of Transportation (DOT & PF) mixes either common salt--sodium chloride--or calcium chloride in a ratio of 1 to 10 in its sand piles during the winter to keep them from freezing solid. Straight salt is sometimes used to melt ice at critical locations like intersections. Some readers felt strongly that the salt mixed with gravel used on the roads during the winter is responsible for the ice layer, although different reasons were given.
Glen Shier of Anchorage believes that when salt lies on the highway, it can draw moisture out of the porous pavement below. The moisture then freezes on the surface. I checked with Billy Connor at the DOT & PF Research Section about this, and found that salt is indeed hydroscopic--that is, it absorbs and retains moisture. In fact, it is for this very characteristic that salt is sometimes used to hold down dust on gravel roads during the summer.
Gary Demientieff of Nenana also believes that salt is largely responsible for road icing, but mainly because it causes repeated freezing and thawing cycles by depressing and spreading out the freezing point. Billy Connor confirmed this, saying that, while pure water will freeze almost immediately on reaching 32 degrees F, salt water exhibits a larger range of freezing temperatures, centered on the 25 degree F point, including slushy states.
Demientieff, who is an equipment operator for DOT & PF, further states that road icing is almost always worse just beyond bumps in the roadbed. He attributes this to salt on the underside of vehicles being jarred loose and falling onto the road.
Both Shier and Demientieff are of the opinion that using salt on the highways during winter creates more problems than it solves, particularly when corrosive damage to vehicles is taken into account. This is a matter that has been debated for years, and just as with the question of how the ice forms, one that is not likely to be resolved to everybody's satisfaction.
