Freezing rain tends to occur in very narrow (10 to 160 km) bands Small changes in temperature, wind speed, and moisture content can dramatically alter the location, intensity and
duration of freezing rain Supercooled water Water does not necessarily freeze at 0 deg C Ice does, however, melt at 0 deg C For water to freeze,
ice nuclei (microscopic dust particles, soot, etc.) Ice nuclei are most effective at producing ice at temperatures less than -10 deg C There are very few ice
nuclei in small raindrops When the temperature falls below 0, the droplets supercool Supercooled water When supercooled droplets come into contact with a surface,
such as a road or sidewalk, they freeze onto that surface Freezing rainstorms occur when supercooled drizzle or rain fall onto objects and freeze
See the link to the following animation: http://www.youtu be.com/watch?v= fSPzMva9_CE Synoptic conditions for an Ice Storm Occurs to the north of a warm front and
to the west of a surface cyclone (about 72% of the events) North of a cold front in a cold-air dome associated with a high pressure system (about 52% of the events) Many Ice Storms are associated with each of the above! (see attached maps) Vertical temperature structure
during an Ice Storm Normally, the temperature decreases with increasing elevation During freezing rain events, however, the temperature increases with height near the surface, and then, farther above, the temperature decreases with height The layer in which the temperature increases with height is called an inversion
Atmospheric stability (see notes on tornadoes) Conditional Instability Air is stable to a certain height,
however, if a lifting mechanism can cause air to rise, to a level where condensation is reached the air is now
saturated. How does an inversion form? Warm, moist air from the south flows above the dome of cold air At each location, precipitation forms as snow At point D, all precipitation falls as snow At point A, all ice melts and the
precipitation is rain Freezing rain and ice pellets At point C, the snow partially melts in the shallow warm layer aloft. The supercooled water refreezes on the existing ice nuclei, resulting in ice pellets (sleet is a US term) At point B, the snow melts completely in the deep warm layer. Once the water falls into
the cold (less than 0 deg) near-surface layer, it is supercooled and freezes on impact with the surface (freezing rain) D C B A
What makes an ice storm so severe? Persistence Intensity
Phase (supercooled water) Location The Ice Storm of 1998 93% of freezing rain storms in North America last for less than 5 hours, yet the 1998 ice storms endured for nearly 5 days! Freezing rain occurred for more at than 80 hours at Ottawa and Montreal (about double
the annual average) A nearly stationary frontal system existed to the south of the Saint Lawrence River Valley for five days Warm, moist air flowed over a cold air dome in the Saint Lawrence River Valley Normally, weather systems at our location tend to move from west to east, so that precipitation durations may be only 6 to 18 hours:
http://www.ssec.wisc.edu/data/geo/index.php?satellite=east&channel=i r4&coverage=conus&file=jpg&imgoranim=8 Time series of temperature and wind soundings for Ottawa, Ontario for the period of January 4 through January 9, 2000; Shaded red region indicates temperatures greater Than 0 deg C; vertical axis extends from the surface to 3 km above the ground
Origins of the warm and moist air that is essential for producing the heavy precipitation: The following animation illustrates that some of the air responsible for the precipitation over Montreal originated in the eastern Atlantic ocean with a
history of passage off the east coast of Africa The total amount of time involved in the animation is 10 days Intensity of the Ice Storm
85 mm in Ottawa 73 mm in Kingston 108 mm in Cornwall 100 mm in Montreal All during a 5-day period (double the largest previous ice storms)
Reports of thunder during the Ice Storm! Location The Ice Storm produced more than 1 billion dollars in damage Had the storm shifted its impact by only 100 km, the damage would have been only minor
Ice Storm of 1998 accumulated precipitation (mm) for the six-day Period ending at 7 AM, January 10, 1998; The 0 deg C temperature Lines (isotherms) at 1.5 km are shown in solid. The 0 deg isotherms At the surface are shown in dashed. Impacts of the 1998 Ice Storm More than 25 fatalities, mostly from hypothermia
900,000 without power in Quebec, 100,000 without power in Ontario 100,000 took refuge in shelters Impacts of the Ice Storm Millions of residents
forced into mobile living, visiting relatives to take a hot shower, or to share a meal 14,000 troops deployed to assist the clean up and evacuation, and security
Impacts of the Ice Storm Millions of trees were downed 120,000 km of power and telephone lines 130 major transmission towers, each worth $100,000
30,000 wooden utility poles at $3,000 each Impacts of the Ice Storm Infrastructure of the electrical grid had to be rebuilt, not just repaired Massive livestock losses
Agriculture devastated with many maple syrup producers losing their livelihood Climatology of Ice Storms (median number of hours per year that freezing rain is reported in North America Preferred areas include
the Saint Lawrence River Valley, Newfoundland, the eastern Great Lakes, the eastern valleys of the Appalachians, the Mississippi River, and inland sections east of the Cascade Mountains
(from Cortinas et al. 2004) Even Locations such as Oklahoma are vulnerable to ice storms Adair County, Oklahoma; January 2009
The topography (terrain) of North America (m) Montreal (YUL) has a particular vulnerability to ice storms because of its location in the Saint Lawrence Valley The wind directions are nearly bimodal. Basically, there are two wind
directions (southwesterly and northeasterly) Mitigation: Listen to the media for freezing rain warnings (as much as a day in advance)
Safety measures (if power and heat are lost): Close off unneeded rooms
Stuff towels in cracks under doors Cover windows at night Eat to maintain energy for heat; drink fluids to avoid dehydration Wear layers of loose-fitting, lightweight, warm clothing; remove layers to avoid overheating, perspiration and subsequent chills If using an alternate heat source, use fire safeguards, and properly ventilate
Readings on ice storms: http://www.msc.ec.gc.ca/media/icestorm98/index_e.cfm http://lwf.ncdc.noaa.gov/oa//reports/janstorm/janstorm.html
Rauber, Walsh, and Charlevoix, chapter 11. Cortinas, J., et al. 2004: An analysis of freezing rain, freezing drizzle, and ice pellets across the United States and Canada: 1976-1990. Weather and Forecasting., 19, 377-390. (available at http://ams.allenpress.com/amsonline/? request=index-html) DeGaetano, A. T., 2000: Climatic perspective and impacts of the 1998 northern New York and New England ice storm. Bull. Amer. Meteor. Soc., 81, 237-254. (available at
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