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…