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Ice Storms: Beauty Amid Destruction - Page 2

© Keith C. Heidorn

Now too heavy to be held aloft, the snowflakes fall into the air below. If the lower air temperature is below freezing (0^oC / 32^oF), the flakes remain frozen and continue their earthward descent as snow. However, if this air layer is above freezing and deep, the snowflakes will melt and continue their fall toward the surface as raindrops.

Whether or not freezing rain forms depends critically on the characteristics of the near surface air layer. If this layer remains above or very near freezing, the raindrops will hit the ground as a cold, but still liquid, rain. If the layer is well below freezing and thick, the rain will refreeze into ice pellets (sleet). (Often, freezing rain is mistakenly called sleet. Technically, however, sleet is composed of already frozen droplets -- ice pellets -- whereas freezing rain is still liquid until it hits a surface, freezing after impact.)

In order to reach the ground as freezing rain, the raindrops must be cooled to temperatures below freezing and yet not freeze (a condition known as supercooling) as they fall through a cold surface air layer before striking the ground or other surfaces. Often small differences in the surface-layer air temperature and raindrop size result in freezing rain mixed with sleet, snow and non-freezing rain.

Glaze ice generally forms when the near-surface air temperature is in the narrow range of -4^oC to 0^oC (25^oF to 32^oF). For thick, destructive glaze formation, the freezing rain should fall rapidly as large, slightly supercooled drops, and the rate of freezing on the surface must be slower than the rainfall rate. Once a supercooled water droplet strikes a surface, the impact triggers the transformation of the water from liquid to ice.

When falling drops are small and their freezing rapid, milky rime ice forms instead of glaze, its crystalline structure more like sugar than clear ice.

The sensitivity of freezing rain formation to the depth and temperature of the near-surface cold air layer makes precise forecasting of the formation, amount and ice accumulation rate difficult, as well as the exact location where it will fall. Often freezing rain episodes are hit and miss across an area, varying with elevation or degree of urbanization. Ice storms are more uniform and cover larger areas but still pose a challenge to forecasters.

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For more on freezing rain and its impacts on social and natural systems, see my Weather Doctor article: ICE STORMS: HAZARDOUS BEAUTY.

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