Weather Whys; Diamond Dust

This is weather wise. There's an old saying that goes something like, a January fog can freeze a hog. No doubt the author of that saying was referring to the penetrating chill that sometimes accompanies very moist air in January. That air appears as fog when it's cooled to its dew point. That's when water vapor condenses into tiny suspended droplets, essentially forming a cloud on the ground. But sometimes, if the air is especially cold, the vapor skips the liquid stage and forms minuscule crystals of ice. When that occurs, meteorologists call it an ice fog, but it's more picturesquely referred to as diamond dust. Ice fog can also form when the water droplets in an existing fog bank freeze. Sometimes the particles of ice are small enough to remain suspended in the air, but more often they drift to earth, looking like a light sprinkling of tiny diamonds on the ground. Since fog droplets are so small, it may seem that any temperature below freezing would

be enough to form an ice fog, but as strange as it may seem, liquid fog droplets actually require very cold temperatures before they can freeze. In fact, the highest possible air temperature at which the ice fog will form is about 23 degrees below zero on the Fahrenheit scale. Most water, like raindrops or tap water, isn't pure H2O. It contains all kinds of particles, which help it freeze faster. But pure water can stay liquid at temperatures well below what we normally call freezing. And fog droplets are often fairly pure. We'd like to know what you think of weather-wise. Please write us at Weatherwise, KGOU Radio, the University of Oklahoma, a Norman, Oklahoma 73019-0250. But weather-wise, I'm Drew Barlow.