Anatomy of a Storm: Freezing Rain

In addition, to the heavy snow that fell in Wisconsin last night and today there was a significant freezing rain event in Illinois. In Mercer County, there were reports of a 2 inch coating of ice on power lines. Add strong winds to the equation and it’s easy to see why 83,000 people lost their power.

What is freezing rain?
Freezing rain forms when a layer of warmer air aloft traps a shallow layer of colder air at the surface. As the rain or snow falls to the surface it melts in the warmer air, reaches the cold air and starts to supercool (cooling a liquid to temperature below it’s freezing point without the liquid becoming solid), then the precipitation freezes on contact with power lines, trees, and roads. The key is the depth of the cold air at the surface, if the layer is deep enough the rain will have time to freeze into relatively harmless sleet. Sleet is separate particles of moisture and can be moved around with a shovel or plow. Freezing rain on the other hand, forms a solid layer of ice because just before the liquid water freezes it molds to the power lines, pores in the road, and already formed freezing rain. One of the more notable freezing rain events took in place in 1998 when an area that included Montreal, Quebec and Northern New York was coated with a devastating 60 to 120 millimeters (3-4.5 inches) of freezing rain. Some people went without power for weeks and close to 30 people died due to the ice.

I couldn’t find a solid source for exact mechanism behind this particular freezing rain event, but my semi-educated guess is the counter-clockwise rotation of this storm system swept up warm air (via a warm front) from the south and this warm air was undercut by a strong and cold east wind that maintained freezing temperatures. This strong wind was due to the high pressure air mass of freezing temperatures that a particular strong jet stream rammed this storm system into.

For more about this storm system and the blizzard further north, click here.

The next article and stop on the tour of this storm will be the tornadoes that occurred to the south. Stay tuned for that.

——
Posted by Tim Roth, author of the political blog Think Anew and Act Anew

Sources:
1. “Midwest suffers through brutal snow system”, CNN.com article
2. “Freezing rain”, Wikipedia entry
3. “North American ice storm of 1998″, Wikipedia entry