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Ice Storms: Beauty Amid Destruction

© Keith C. Heidorn

If the blizzard is the king of winter storms, its queen must surely be the ice storm. Often within the realm of a single large winter storm, the blizzard king will sit on the cold seat of power, blustering over the subjects to the west. To the blizzard king's right, its ice-storm queen sits, dispensing destruction encased in great beauty, a dazzling crystal sheath fallen over the lands to the east.

Ice storms differ from short freezing rain episodes in their duration, total ice accumulation and areal extent. While freezing rain may fall in any winter storm, significant ice storms usually develop in the region stretching from northern Texas to Newfoundland any time between late October and early May. Ice storms generally last 12 hours or more, accumulate ice several centimetres thick, and affect an area ranging from a few square kilometres to regions as large as several states/provinces. The mean ice storm swath is typically 50 km (30 mi) wide and 500 km (300 mi) long. While residents of eastern Canada and the United States may experience some freezing rain each winter, ice storms that cause severe damage generally warrant major headlines only one year in three.

North America's worst ice storms are commonly associated with slow-moving low-pressure systems having very large temperature differences between colliding warm, moist Gulf air and very cold Arctic air in their northeastern sector. When these storm systems stall for an extended period over one region, heavy, accumulations of ice may blanket a region, causing much destruction. The great ice storm that hit New England and Quebec in 1998 is an extreme example of a stalled storm.

Ice storms may produce either of two forms of icing: glaze ice or rime ice. Meteorologists classify transparent and homogeneous ice similar to ice cubes, which forms on vertical and horizontal surfaces, as glaze. Its amorphous, dense structure clings tenaciously to any surface on which it forms. In contrast, milky and crystalline ice is termed rime. Rime lacks the transparency of glaze, is less dense, and does not cling as tenaciously as glaze. Therefore, glaze ice generally inflicts much greater damage than rime ice.

Freezing Rain Formation

Ice storms typically begin with a period of snow and strong easterly winds ahead of the approaching warm front. Advancing warm, moist air from the south overruns the surface cold air at the warm front and produces the characteristic vertical temperature layering (warmer air above, colder below) of the pre-warm front atmosphere. As the warm, moist air is forced upward, it cools. By the time it reaches the upper cloud levels, its temperature has dropped below freezing, and its liquid moisture forms ice crystals. With additional moisture constantly condensing, the crystals quickly combine into snowflakes.