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Ice storms are very bad for trees! NOAA image from here.

(I have no idea how blogger put the "U" on this post, nor any idea how to get rid of it....Grrrr....)

The CNN  headline today is "Forecast: Historic, crippling, catastrophic ice: Atlanta prepares for the worst." It is well known that freezing rain storms occur frequently in the southeastern part of the U.S. They are beautiful, but dangerous and costly.

And, they are not all that rare. Montreal, Quebec, typically receives freezing rain more than a dozen times a year. In 1998 the great North American ice storm of January 5-9 was one of the most damaging and costly ice storms in North American history, causing massive power outages on the east coast. Eastern Canada bore the brunt of the storm. Millions were without power for days to weeks to even months. 35 people died, a significant number from carbon monoxide poisoning from generators they used to try to keep themselves warm. The effort to reconstruct the power grid led to the biggest deployment of Canadian military personnel since the Korean War.
What makes an ice storm? The attached graphic from Gay and Davis summarizes the types of precipitation nicely and, when I read their paper, I learned a new word: "hydrometeor." It is "any water or ice particles that have formed in the atmosphere or at the Earth's surface as a result of condensation or sublimation." Examples are clouds, fog, rain, snow, hail, dew, rime, glaze, blowing snow and blowing spray.

Vertical temperature profiles in the atmosphere and
the kind of storms that they produce. From Gay and Davis,
1993 here.

The graphs shown here summarize the general conditions under which snow, sleet, freezing rain, and rain land on the ground within the context of the atmospheric temperature distribution.** Consider the situation when a warm front moves in.  If warm front isn't too strong, the atmosphere remains cold (below freezing) throughout, and precipitation falls in the form of snow. But, as a warm front moves in, an inversion layer develops with cold air near the surface under the warm air aloft. If snow starts falling aloft and encounters this warm air, the snowflakes melt. A mixture of frozen and unfrozen "hydrometeors" develops in the warm layer (left side of the Figure shown here). As these hydrometeors fall into the near-surface cold layer, they get supercooled. Any icy snowflakes that didn't melt as they traveled through the warm layer become efficient sites for refreezing, and a mixture of snow, ice, and some liquid falls to the ground, i.e., sleet. As the warm layer develops (gets warmer and thicker), all of the snowflakes melt as they travel through it. Without nearby ice particles to serve as nuclei, these become supercooled as they fall through the cold layer near the ground, i.e., they are supercooled liquid. When they land on cold ground, they freeze, producing freezing rain. If the liquid droplets formed in the warm layer reach ground that is above freezing temperature, the precipitation is cold rain.

As the warm front develops, it is common to see a sequence of precipitation progress from snow to sleet to freezing rain to rain.  The reverse situation occurs with cold front events in the southern Plain states.

A few factlets from Wiki: The thickest recorded ice accumulation from a single ice storm in the U.S. is 8 inches (northern Idaho, January 1961). In February 1994 a severe ice storm caused over $1 billion damage in the southeast.

**This discussion is from David Gay and Robert Davis, "Freezing rain and sleet climatology of the southeastern USA," Climate Research, vol. 3, 209-220, 1993. Notably, they comment that at the time this paper was written, relatively little was known about freezing rain and sleet climatology.