Many times as an amateur photographer on a cold fall or winter evening I have seen skies that are clear and the Kootenai River shimmering in the surrounding night light and anticipate what is to come. The next morning I come out of the house to see a clear sky. As I left the house the air was thick with anticipation but when I got outside to see a clear picturesque morning I saw nothing but fog. The sky was a veil of haze, I couldn’t see the river, couldn’t see the sky, the fog has blocked my view. It was like a river of clouds.
Numerous people in Boundary County witness the phenomenon of 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 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 homes. 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 cools, 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, six 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.
During fall the water in the Kootenai River; most of the time, will be relatively warmer than the air in the valley. 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. As the sun’s energy begins to warm the earth, the air molecules closest to the ground begin 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.
Energy from the sun is able to warm the top layers of the fog which causes the air temperature in those layers to increase. The air temperature increases away from the dew point temperature, and causes the fog to start evaporating.
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 veiled in haze.