Fog is a common meteorological phenomenon defined as a cloud that forms at or near the Earth’s surface, consisting of tiny water droplets suspended in the air. For fog to develop, the air must be near its saturation point and cool enough for the water vapor to condense into visible droplets. This cooling process brings the air temperature close to the dew point and distinguishes the various classifications of fog. The reduced visibility caused by these droplets presents a hazard to transportation and daily life.
What Distinguishes Upslope Fog
Upslope fog is unique because its formation is directly tied to the forced movement of air over terrain. Radiation fog, for example, forms when the ground cools the air immediately above it during clear, calm nights. Advection fog, by contrast, develops when warm, moist air moves horizontally over a much cooler surface.
Upslope fog, sometimes called “Cheyenne fog,” is defined by moist air being forced upward along a gradually rising slope. The terrain, whether a hill, a mountain, or a large, elevated plain, acts as the physical barrier directing the air mass to ascend. This type of fog is essentially a stratus cloud layer that has formed at ground level.
The Process of Adiabatic Cooling and Formation
The formation of upslope fog is governed by adiabatic cooling. As a parcel of air is pushed up the sloping terrain by the wind, the surrounding atmospheric pressure gradually decreases. This decrease in pressure allows the air parcel to expand outward. The expansion of the air parcel requires energy, causing the temperature of the air to drop. This cooling occurs without any exchange of heat with the surrounding environment, hence the term “adiabatic”.
As the air continues to rise and cool, its temperature eventually drops to the dew point. Once the air reaches its dew point, the excess water vapor condenses around microscopic particles in the air, forming the visible water droplets that comprise the fog. For this process to be sustained, a continuous flow of moist air must be directed up the slope, requiring a favorable wind direction blowing perpendicular to the terrain. Upslope fog can form even with relatively high wind speeds, because the increased lift and cooling effect counteract the mixing that might dissipate other fog types.
Regional Prevalence and Effects on Visibility
Upslope fog is a recurring weather event in specific geographic locations that feature extensive, gently rising terrain. A prime example is the eastern slope of the Rocky Mountains, particularly across the Great Plains region. Moist air often flows westward from the Gulf of Mexico, encountering the gradually rising topography of the plains.
Unlike radiation fog, which typically burns off shortly after sunrise, upslope fog can be persistent and deep, often lasting for days. This longevity is due to the sustained force of the wind continuously lifting the air and maintaining the adiabatic cooling process. The resulting low visibility, often less than a quarter mile, poses a hazard to ground transportation and aviation. The fog’s depth can be substantial, sometimes building downward into valleys and severely restricting viewing conditions.