A frozen fog advisory is in effect for Southern Oregon this morning, Tuesday, January 13, with conditions affecting Grants Pass and nearby valleys during the early hours of the day. Like many residents, we found ourselves pausing at the alert and asking a simple question: what exactly is frozen fog? It is not a term many people here grew up hearing, yet this winter it has appeared repeatedly in weather advisories across the region, making it worth a closer look.
Frozen fog forms when temperatures drop below freezing and moisture near the ground turns into fog made up of supercooled water droplets. These droplets remain suspended in the air, but unlike regular fog, they freeze as soon as they come into contact with surfaces. That means roads, bridges, sidewalks, vehicles, and even trees can quickly develop a thin layer of ice without any snowfall or freezing rain. In many cases, the pavement still looks dry, which is what makes frozen fog especially deceptive.
In Grants Pass, local geography plays a major role. Cold air settles into the Rogue Valley overnight, particularly during calm and clear winter nights. Moisture left behind from recent weather systems becomes trapped near the surface, and as temperatures dip below freezing, fog develops and lingers. When that fog freezes on contact, it creates slick conditions that often show up during the morning commute.
The advisory issued for today warns of reduced visibility in some areas, especially near rivers and low-lying roads. More concerning, however, is the risk of black ice. Black ice is nearly invisible and can form quickly on bridges, overpasses, shaded roadways, and residential streets. Drivers may not realize the danger until traction is suddenly lost. Even short trips can become hazardous under these conditions.
Frozen fog is different from winter weather most people are familiar with. It does not fall from the sky like snow or freezing rain. Instead, it forms quietly at ground level, coating surfaces as fog droplets freeze on contact. Trees and power lines may show a white, frosty buildup known as rime ice, while roads develop a slick glaze that is easy to miss. That subtlety is part of why frozen fog has caught so many people off guard this season, ourselves included.
While frozen fog does not usually cause widespread damage, it poses real safety concerns. Traffic accidents often increase during these events, and pedestrians face a higher risk of slips and falls. Even after the fog lifts, icy patches can remain in shaded areas well into the morning hours, long after the sun is up.
As temperatures rise later today, conditions are expected to gradually improve, but caution is still advised. If you were unfamiliar with frozen fog before this winter, you are not alone. With these advisories becoming more common in Southern Oregon, understanding what frozen fog is and why it matters can help residents of Grants Pass stay safer during cold, foggy mornings like this one.