4.3.1 March and beyond
We've skipped forward to March. An upper ridge (a high-pressure system associated with clear, dry weather) has developed in the atmosphere. Daytime temperatures are in the +3°C to +8°C range (37° to 46°F). Since that's above freezing, significant melting is occurring on the top of the snowpack.
The warmer the atmospheric temperature, the greater the rate of melting. In general, melting adds liquid to the top of the snowpack, which increases the density of the upper layer.
The vast majority of liquid from melting or rain moves down through vertical channels in the snowpack unless it encounters an ice layer, in which case the water will pool above it, freezing if temperatures fall. If atmospheric temperatures stay warm, the water will keep moving horizontally until it finds another vertical channel.
If the water encounters a capillary barrier, it will move parallel to the snow layers rather than through them. A capillary barrier forms when a layer with small pore spaces rests above one with large pore spaces. The gradient in pore sizes creates a barrier that the water runs along.
At night, when the air temperature falls back below freezing, the top layer freezes into a crust whose density is near that of water (many times higher than snowpack). The latent heat release from the crusting process enhances temperature gradients within the snowpack. This can lead to edge growth (faceting) on the snow grains and create layers of reduced stability.
The cycling of melting and freezing can occur diurnally or with the passage of storm systems that are common during the spring season.
In the spring (and early summer at higher elevations), snowpack coverage typically becomes spotty. Melting accelerates at the bottom fringes of the snowpack due to conduction from the adjacent warmer, bare ground.