A fascinating and rare weather phenomenon is occurring across Boise and the Treasure Valley as rime ice forms within the deep inversion layer that has trapped dense fog for several days, while some areas are also experiencing a dusting of snow or ice crystals on the ground.
Rime ice is creating white, feathery coatings on trees, power lines, and other surfaces as supercooled water droplets in the persistent fog freeze instantly upon contact with objects. Meanwhile, the same atmospheric conditions are producing light accumulations of ice crystals and snow-like precipitation at ground level in some locations.
What is rime ice?
Rime ice forms when supercooled water droplets in fog come into contact with surfaces at or below freezing temperatures. Unlike hoarfrost, which forms from water vapor, rime ice develops directly from liquid water droplets that are cooled below 32 degrees but remain liquid until they strike an object.
The droplets freeze instantly upon impact, creating white, rough, and feathery ice deposits that build up on the windward side of objects. The formation typically occurs in temperatures between 15 and 32 degrees Fahrenheit with light winds and dense fog conditions.
Ground-level ice crystal formation
The same deep inversion creating rime ice is also producing light dustings of snow or ice crystals on the ground in some areas. This occurs when supercooled water droplets in the dense fog aggregate and freeze into small ice crystals that fall to the surface.
Unlike traditional snowfall that forms high in the atmosphere, these ice crystals develop within the fog layer itself, close to the ground. The crystals are typically very small and light, creating a thin, powdery coating that resembles a light dusting of snow.
This ground-level precipitation is most common during the coldest parts of the day when the fog is densest and temperatures are lowest. The phenomenon can create slippery conditions on sidewalks, roads, and other surfaces, even though accumulations are minimal.
Perfect conditions in the Treasure Valley
The current weather pattern has created ideal conditions for both rime ice formation and ground-level ice crystal development. The persistent upper-level ridge has established a strong temperature inversion, trapping cold, moist air near the surface while warmer air sits above it.
Within this inversion layer, dense fog contains abundant supercooled water droplets that remain liquid despite sub-freezing temperatures. When these droplets encounter trees, fences, power lines, or other objects, they freeze immediately as rime ice. When they aggregate and freeze within the fog itself, they fall as light ice crystals.
The calm wind conditions associated with the high-pressure system allow both phenomena to occur simultaneously, creating a unique winter landscape with white-coated vegetation and light ground accumulations.
Impacts and safety considerations
While rime ice and ground-level ice crystals create beautiful winter scenery, they can pose some hazards. The ice buildup can add weight to tree branches and power lines, potentially causing damage if accumulations become heavy enough. That will not occur with our inversion.
The light dusting of ice crystals on the ground can create unexpectedly slippery conditions, particularly on sidewalks, parking lots, and roadways. Even minimal accumulations can reduce traction for both pedestrians and vehicles.
Drivers should exercise extra caution as rime ice can form on vehicles, reducing visibility through windshields and mirrors, while the ground-level ice crystals can create slick driving surfaces.
Both phenomena typically occur during the coldest parts of the day, often in early morning hours when temperatures are lowest and fog is densest.
How long will it last?
Both rime ice formation and ground-level ice crystal development will likely continue as long as the current inversion pattern persists. The stagnant weather pattern will remain in place through the weekend, with potential relief arriving Thursday when an upper-level shortwave trough moves through the region.
Once the inversion breaks down and fog clears, rime ice deposits will gradually melt or sublimate back into the atmosphere, and the light ground accumulations will also disappear.
Understanding the atmospheric setup
The current conditions in the Treasure Valley represent a textbook example of how a deep temperature inversion can create multiple winter weather phenomena simultaneously. The trapped cold air provides the necessary temperatures for supercooling, while the abundant moisture in the fog supplies the water droplets needed for both rime ice formation and ice crystal precipitation.
This combination of rime ice on elevated surfaces and light ice crystal accumulations on the ground creates a unique winter weather event that showcases the complex interactions possible within a strong inversion layer.
