Ice Storms

• "Ice Storm": ice accumulation of at least 0.25" (0.64 cm) or structural damage due to freezing precipitation; roughly one in four winter storms meets this criteria

• Ice storms impact public safety and the power, insurance and transportation industries; severe ice storms can lead to extensive power outages, halt air and ground transportation, and cause considerable property damage

• Strong winds following an ice storm can dramatically increase the amount of damage to trees and structures (including power lines) burdened with ice

• Supercooled water: water can exist in liquid state below 0 C, down to as cold as -40 C; it is nearly impossible for pure water to freeze at temps above -40 C

• Condensation nuclei: particles such as wind-blown clay eroded from soil, organic particles from rotting leaves and car exhaust, and microscopic bacteria, that water vapor can condense upon.

• Ice nuclei: those particles that have a lattice structure similar to that of ice; most effective when temps are colder than -15 C (5 F), are marginally effective between -15 C (5 F) and -5 C (23 F), and hardly effective at all when the temp is between -5 C (23 F) and 0 C (32 F).

• Ice nuclei are sufficiently rare in the atmosphere, such that there may not be any nuclei that can activate ice formation between temps of -10 C (14 F) and 0 C (32 F); the droplets remain supercooled

• Melting Process: leads to freezing precip when snow falls from high in the clouds into an atmospheric layer where the temp exceeds 0 C; the snowflakes melt into rain and then pass through a sub-freezing layer (< 0 C) near the ground; the droplets supercool and freeze on contact with surface objects ... freezing rain ... highest frequency of occurrence is over southeastern Canada and the Maritimes, and the Northeast U.S.

• Supercooled Warm Rain Process: leads to freezing precip when tiny cloud droplets grow to precip size by "collision & coalescence" and fall out of the cloud ... freezing drizzle ... cloud top temps no colder than -10 C to -15 C

• Freezing drizzle is especially hazardous to aircraft; ice accumulation will make the aircraft heavier and change the "shape" of the wings, degrading flight performance and potentially causing a crash; is quite common over central Canada, down into the central U.S.

• Temperature profile of the atmosphere will determine winter precip type ... snow, rain, ice pellets (sleet), freezing rain or freezing drizzle.

Ice Storms (continued)

• Weather Patterns Associated with Freezing Precip:

  1) Arctic Front: warm air overrides cold air dome, creating a narrow (less than 100 miles wide), shallow layer that produces freezing drizzle; deeper clouds (esp. in southern U.S.) can produce freezing rain (Figure 11.3A); about 33%

  of all freezing precip events;

2) Warm Front: warm air overrides warm frontal boundary; freezing precip fall just north of the 0 C isotherm in a narrow band approximately parallel to the front; can also extend northwest around the low pressure center (Figure 11.3B); this pattern occurs in about 33% of all freezing precip events and about 50% of these have a strong high pressure to the north of the cyclone (Figure 11.3C), which can induce strong surface winds, raising the potential for destruction if combined with significant ice accumulation;

3) Western Quadrant of Arctic High Pressure: southerly flow occurs on the western side of high pressure; this transport of warm, moist air over the western part of the arctic air mass can produce freezing precip in the stronger southerly flow, in a more circular area (instead of a narrow band), extending no more than 120 miles north of the 0 C isotherm (Figure 11.3D); about 10% of all freezing precip events;

4) Cold Air Damming: sub-freezing air is "trapped" at lower levels east of a mountain range (e.g., Appalachians, Rockies); with a strong Arctic high over southeastern Canada or the Northeastern U.S., cold air damming can extend down the east coast to Georgia (Figure 11.4A); with a low pressure passing off the southeast U.S. Coast, cold air is trapped below warmer, Atlantic air brought in by the storm's east to northeast wind flow (Figure 11.4B); about 15% of all freezing precip events;

5) Cold Air Trapping: with low pressure passing to the west of the Appalachians, cold air trapping will occur on the eastern slopes, leading to odd looking warm fronts on surface maps; precip falling into and evaporating in the dammed up arctic air only serves to cool and strengthen the damming (Figure 11.4C); also is common in the Pacific Northwest, esp. in the Columbia River Basin; about 10% of all freezing precip events

• Summary: in all the weather patterns above, the zone of freezing precip is narrow; it continually moves with the evolving weather system; in most cases, there is a transition from freezing precip to sleet or snow, or from freezing precip to just rain, over the course of a few hours

• Worst Ice Storm in North American History - January 4-9, 1998