3.5 Vapor diffusion

Now we'll discuss the important process of microphysical vapor diffusion, which is critical for snowpack evolution.

This simple schematic shows how ice molecules move within a snowpack when the ground is warmer than the snowpack. The process involves millions of ice and water vapor molecules.

Snow crystals (snow grains) are interspersed with microscopic air pockets. These air pockets have a given temperature and vapor pressure (the part of air pressure that's due to water vapor). The air at the bottom of the pockets is warmer than that above since it's closer to the (warmer) ground. This results in temperature and vapor pressure gradients.

As the animation below shows, the water molecules move from high to low vapor pressure by sublimating off the snow grain at the bottom of the pocket and moving upward and attaching onto the snow grains at the top. This involves changing from vapor back to ice, a process called vapor deposition. The result is that the upper snow grains grow at the expense of those below, resulting in a net transport of ice mass upwards.

If the temperature gradient is large, the upper crystals will grow quickly. The vapor molecules attach to the bottom of the crystals in flat layers rather than simply enlarging the size of the grains. These flat edges are called crystal facets

In sum, two phase changes occur with warm temperatures below and cool temperatures above in snowpack:

  • Sublimation of the lower crystals (a phase change from ice to vapor)
  • Vapor depositional growth of the upper crystals (a phase change from vapor to ice)

The process is reversed when the ground is colder than the snowpack (an unusual situation).