A rain-on-snow event occurs when liquid precipitation falls onto an existing snowpack. This interaction is a complex physical process that drastically alters the snow structure, triggering immediate changes in its thermal and structural properties. Understanding this process is important for forecasting weather-related hazards, as the introduction of liquid water fundamentally changes the snow’s stability and its capacity to hold water.
Immediate Physical Transformation
The initial contact between relatively warm rain and the cold snow surface immediately begins a significant energy transfer. Rain is typically warmer than the snowpack, and this sensible heat input causes rapid melting at the surface layer. The most substantial energy source, however, comes from the rain’s latent heat if the snowpack is below freezing. As the water freezes upon contact with cold snow crystals, it releases a large amount of latent heat, approximately 333 kilojoules per kilogram of water.
This heat exchange rapidly warms the snowpack toward the melting point of 0 degrees Celsius. The introduction of liquid water causes a fast change in the snow’s texture and density. The snow becomes heavy, transitioning into “wet snow” or slush due to the absorbed water. This wetting and densification transforms the top layer into a saturated medium that is less reflective, which accelerates melt by increasing the absorption of solar energy.
Internal Snowpack Dynamics
Once the surface layer is saturated, the liquid water begins moving downward into the deeper snowpack layers. Water percolation is primarily driven by gravity through a network of pores and channels within the snow structure. This flow is often not uniform, favoring discrete “fingers” or channels of preferential flow, especially in cold and stratified snowpacks.
The snowpack retains liquid water until it reaches full saturation, a condition known as “ripening.” The capacity to hold water is determined by the snowpack’s depth, age, and internal structure; colder, drier snowpacks have a greater initial capacity. If the snow is still below 0 degrees Celsius, the descending liquid will cool and may refreeze, releasing latent heat and warming the snowpack from the inside. This process eventually leads to an isothermal snowpack, where the entire column is at 0 degrees Celsius.
The Hazard of Refreezing
When air temperatures drop after the event, or if liquid water encounters a sufficiently cold layer, the water refreezes, creating hazardous ice formations. The surface water can solidify into a hard, slick ice crust, which is hazardous for pedestrians and drivers.
Within the snowpack, refreezing water forms ice lenses or melt-freeze layers that persist throughout the winter. These ice layers impede the vertical drainage of subsequent meltwater, forcing it to flow laterally or remain trapped. If the water penetrates to the bottom and refreezes, it creates a layer of basal ice, which increases the slipperiness of the ground beneath the snow. These internal formations reduce the snowpack’s ability to drain, potentially leading to a faster runoff response in future melt events.
Hydrological Consequences
Rain-on-snow events significantly increase the total amount of water reaching the ground surface. This excess water input, known as terrestrial water input (TWI), is the sum of the liquid rainfall and the snowmelt generated by the rain’s heat. Stream surges resulting from these events are often 3 to 20 percent larger than those caused by rain alone.
The rapid introduction of water leads to a quick and substantial increase in streamflow. This risk is pronounced when the ground beneath the snow is frozen or already saturated, preventing water infiltration. In these conditions, the combined rain and meltwater quickly becomes surface runoff, dramatically increasing the risk of flash flooding. The combined effect of rainfall and accelerated snowmelt is a major factor in damaging winter floods experienced in snow-dominated regions.