4.1.10 Day 2, 6AM: Surface hoar

Some of the water vapor particles moving up through the snowpack escape into the atmosphere right above the pack and freeze upon contact with the colder air temperatures. This results in the formation of surface hoar at the top of the pack. Surface hoar is large, rounded, feathery crystals (snow grains) with flat edges that grow rapidly.

To summarize, surface hoar forms when:

  • Atmospheric conditions are calm and clear and the snow at the top of the pack has cooled rapidly overnight due to longwave radiative cooling
  • A strong temperature gradient in the snowpack is accompanied by a vapor pressure gradient that drives water vapor out of the snow and into the atmosphere very close to the snow surface
  • The water vapor freezes, forming hoar crystals

Early morning scenario during frost and surface hoar formation.