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| Photo by SWK in Urbana, IL |
Today a major storm is pounding the southeastern part of the U.S. A major concern is that the abnormally cold temperatures that will persist through Tuesday will cause formation of ice and freezing rain that will bring down the power lines. Freezing rain is a hazard in the mid-latitude regions of the globe. Although major freezing rainstorms are relatively rare, they are amongst the most costly of hydrometeorological disasters. The Ice Storm of January 5-9, 1998 in Quebec and Ontario caused 25 deaths, left a million householders without power, caused nearly $US 3 billion in damages, and another $US 3 billion in short-term lost economic output and insurance claims. Some climate change scenarios predict that with warming, the amount of freezing rain will increase. For example, study of possible changes in south-central Canada during the winter predicts an increase of 85%, 60%, and 40% in northern Ontario, eastern Ontario (including Montreal, Quebec), and southern Ontario, respectively (Cheng, C.S. et al., Natural Hazards Earth System Science, 7, 71-87, 2007).
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From Wiki, which adapted this figure
from two figures in Gay, David A and R.E. Davis,
Climate Research 3(1) 209-220, 1993 |
What is freezing rain? Under certain atmospheric conditions, raindrops become supercooled (cooled below the freezing point without turning to ice) while passing through a sub-freezing layer of air, and then freeze upon impact with cold objects on the surface. The figure to the right shows conditions under which freezing rain occurs, and contrasts these conditions with those that produce snow, sleet and rain. Freezing rain is a particular hazard to aircraft because it can add a significant amount of weight to a plane, and can resculpt the contours of the wings. If an aircraft starts to accumulate freezing rain in flight, the safe and easy maneuver is to descend into warmer parts of the atmosphere. However, the conditions that produce freezing rain near the surface are such that the plane must climb to higher elevations to find warm air, a much more difficult maneuver if the plane has started to ice up. In 1994, American Eagle Flight 4184 was in a holding pattern 65 miles southeast of Chicago's O'Hare airport. Ice formed on the upper surface of its wings, causing the autopilot to disconnect and the pilots to lose control. The plane crashed into a field, killing all 68 passengers and crew. As a result of this accident, the airplane type (an ATR) was removed from service in most northern hubs prone to icing conditions, and pilot operating procedures in icing conditions were substantially revised, including non-use of the autopilot under icing conditions.
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