Fog is essentially a cloud that forms at ground level, made up of billions of microscopic water droplets suspended in the air. When a blanket of fog settles over a snowpack, the general answer to whether it melts the snow is yes, often quite effectively. This rapid melting has earned fog the nickname “snow-eater.” However, the process is far more nuanced than simple warm air melting snow. The speed and extent of the thaw ultimately depend on a precise balance of temperature, humidity, and the physical state of the water droplets within the fog itself.
The Primary Melting Mechanism
The primary reason fog is so effective at melting snow is due to a thermodynamic process involving the release of latent heat. Latent heat is energy stored within water vapor, the gaseous form of water, and when this vapor shifts to a liquid (condensation), it releases a significant amount of stored energy into the surrounding environment.
This energy release warms the snowpack and causes it to melt. Fog is formed when water vapor condenses into tiny liquid droplets, and this condensation is constantly occurring, even directly onto the cold snow surface. The energy released by this phase change is far more efficient at warming the snow than the ambient air temperature alone. The transfer of heat is effective because the snow acts as a colder surface, encouraging the continuous condensation of water vapor directly onto its crystalline structure.
When Fog Accelerates Thawing
Fog accelerates thawing when it forms under conditions known as advection fog, often associated with a warm front moving in. This scenario involves warm, moist air blowing across the colder ground surface, which cools the air to its dew point and forms fog. The resulting fog consists of liquid water droplets suspended in air that is generally near or just above the freezing point of water.
High relative humidity is a necessary ingredient for maximum melting, as it ensures the air remains saturated with water vapor. This keeps the condensation process going. The liquid water droplets in the fog act as an efficient medium for transferring this heat energy directly to the snow. This combination of factors allows the fog to rapidly thin the snowpack, sometimes more quickly than rain or mild temperatures alone.
Conditions Where Fog Fails to Melt Snow
Not all fog melts snow; some atmospheric conditions can inhibit the process or lead to ice accumulation. One exception is “freezing fog,” which occurs when the air temperature is below freezing, but the fog droplets remain in a liquid state. These are referred to as supercooled water droplets.
When these supercooled droplets come into contact with a solid surface like snow, they instantly freeze upon impact. Instead of melting the snow, this process deposits a layer of ice, known as rime ice, on the snowpack surface, which can increase the snow’s thickness and density.
A second type, “ice fog,” forms in extremely cold environments, usually below -10 degrees Celsius, where the water vapor bypasses the liquid phase entirely. In this case, the fog is composed of tiny ice crystals instead of liquid droplets, and the latent heat of condensation mechanism is absent. This icy fog has little to no melting effect and may even act as an insulator, slowing any existing thaw.