UPDATE: The Dense Fog Advisory has been canceled for Winnebago, Boone, McHenry, Ogle, Lee and DeKalb counties.
Even though there has been some improvement in visibility across the area, the Dense Fog Advisory has been extended through early Tuesday morning. Temperatures struggled to to make it out of the 20s during the afternoon, leaving much of the area encased in freezing fog throughout the afternoon.
The freezing fog coated trees, branches, power poles, homes and fences with something known as ‘rime ice’. Rime ice is different than hoar frost – what we are used to seeing most of the time. Rime ice forms as supercooled water droplets freeze on contact to surfaces. Supercooled water droplets are droplets of water where the temperature is below freezing, but the water has not frozen. That’s because those water droplets are undisturbed. But once those water droplets come into contact with something, now becoming disturbed, they freeze giving the appearance of the ice spikes that many witnessed today.
Hoar frost forms in a way similar to how dew forms, on clear nights with calm winds and plenty of surface moisture. Hoar frost forms when water vapor deposits on surfaces and freezes on contact. This gas phase (water vapor) turns directly to ice without going through the liquid phase.
There has been some improvement in visibility Monday evening, but with a clearing sky behind a narrow band of snow it’s possible that areas of fog will develop once again. Winds will turn more northwesterly later this evening which should help bring in a little more dry air. But the dry air over moisture down near the surface may not allow skies to clear as quickly, allowing low clouds to form once again. It might not be quite as foggy Tuesday morning as Monday morning, but anticipate locally dense freezing fog for the morning commute.
Here are a few photos of the wintry wonderland from earlier today.