How does upslope fog form?

Upslope fog forms specifically through the process of air rising and cooling adiabatically. As air moves up a slope, whether it be a hill, a mountain, or any elevated terrain, it expands due to the decrease in atmospheric pressure at higher altitudes. This expansion causes the air to cool. If the air contains enough moisture, the cooling can lead to condensation, forming fog.

This phenomenon occurs particularly in areas where moist air is forced to ascend, such as along mountain ranges. As the air cools to its dew point temperature, tiny water droplets form, resulting in the characteristic fog that often blankets the slopes.

While other factors such as radiation cooling and the mixing of warm and cold air can contribute to fog formation in different contexts, they do not directly describe the upslope fog mechanism. For instance, radiation fog typically forms on calm, clear nights when the ground loses heat and the air close to the surface cools. Similarly, precipitation evaporation refers to fog formed by moisture added to the air from falling rain evaporating before reaching the ground. These processes do not explain the unique conditions under which upslope fog develops.