Kootenai Valley Fog; a river 'mist'ery!

“In nature, everything has a job. The job of the fog is to beautify further the existing beauties!” — Mehmet Murat Ildan

Recently when I was crossing the Kootenai River Bridge in Bonners Ferry, the air above the river was thick with a mysterious fog. It was surprising because the evening before the skies were clear in the cold evening and the Kootenai River shimming in the surrounding night light. But that morning, just a few hours later, I could see little more than the fog. The fog was so thick I had difficulty seeing the sky, the river and highway, the fog was partially blocking my view.

After crossing the river bridge I decided to travel east down Cow Creek Road and go to the Katka overlook to get a better view of the morning river mist. From the Katka overlook I could see the morning fog cover the river and part of the Kootenai River Valley all the way to Bonners Ferry and beyond to the foothills of the Selkirk Mountains. After watching the fog above the Kootenai River for a while it slowly lifted in the sky and dissipated in the warming morning sun.

Many people in Boundary County witness this phenomenon of river or valley fog formation on the Kootenai River. Fog is a ground-level cloud that forms when the air temperature lowers to the dew point. Fog is especially known to those who live in the valleys.

Fog can develop anytime of the year, but river or valley fog is more prevalent in the fall, followed by winter. It is common for people, who live in valleys, to exit their homes during the night and find fog flowing freely around their property. It is also common for these same people to drive into patches of fog during early morning rides to work. Fog usually develops during a night in which the sky is clear, the air temperature is cool, and the wind is calm, growing sometimes to a couple hundred feet in height depending on the moisture present in the air. It then dissipates as the sun rises the following day. In deep valleys, fog can persist for several days.

In the fall, air temperatures are cool, but temperatures of water bodies such as rivers and streams are still relatively warm. This is due to water taking longer to cool and warm throughout the year compared to the air in the atmosphere. In general water bodies warm and cool much slower than air. The maximum and minimum temperatures of air in the atmosphere occur, on average, 6 weeks into the summer and winter respectively. For water bodies, since they reach maximum temperature close to the beginning of the fall, it is ideal that fog formation would be at its peak during the fall season. Water bodies are still warm but nights are getting cooler, conditions that can support the formation of fog.

The water in the Kootenai River, most of the time, will be relatively warmer than the air in the valley during fall. Molecules from the water body are always evaporating, putting moisture in the air. If a cool, calm, and clear night comes, and the air temperature approaches the dew point temperature, fog will begin to form.

After fog develops overnight, it usually persists until sunrise; afterward it begins to dissipate. When the sun rises, the energy from the sun begins to warm the ground and the top layer of the fog. As the sun’s energy begins to warm the ground, the air molecules closest to the ground begins to warm by conduction from the ground. This causes the air molecules at the ground surrounding the fog to become relatively warm. This warming of the air causes the air around the perimeter of the fog to rise.

As the sun continues to rise, more of the sun’s energy is able to reach deeper and further into the perimeter of the fog. Eventually the fog evaporates.

Enjoy the beauty of Boundary County ... even if it is a little foggy in the morning!