Geography 327      Hydrology

Snow


snowfall


snow crystals


measurement


hydrological significance of snow


  1. the spatial distribution of snow fall, especially in terms of altitude
  2. depth of the snowpack during the melt season
  3. the rate of melt


Snow metamorphosis


gravtitational metamorphosis

Snow settles as it accumulates and thus the depth of snow on the ground is always less than the initial amount of snowfall, especially where snow falls as large flakes and then settles under the usually milder conditions near the ground. The rate of settling is directly related to snow density and depth.

equi-temperature (ET or destructive) metamorphosis

ET metamorphosis occurs in snowpacks which are close to isothermal, that is, where air temperatures around 0o and thus close to temperatures at the base of the snow pack, which are generally around 0o given the low thermal conductivity of snow (i.e., its a good insulator). Highest rates of ET metamorphosis occur as air temperatures approach 0o and it does not occur below -40o. Snow flakes are destroyed as the sharp corners and spikes sublimate. The resulting water vapour is diffused from the higher vapour pressure over the more intricate and curved surfaces towards the lower vapour pressure around the more dense, and thus colder, interior of the snow crystal. This very local gradient dominates any vapour pressure gradient in the equi-temperature snow pack. The compact snow grains occupy much less space than the original snow flakes and thus the snow pack becomes more dense and stable.

temperature gradient (constructive) metamorphism

TG metamorphosis occurs in response to a temperature, and therefore vapour pressure, gradient between the insulated ground surface and cold air. Thus maximum TG metamorphosis occurs with persistent cold dry weather. Water vapour from the base of the snow pack is diffused towards the surface, encountering colder air and sublimating to form depth hoar, a layer of large coarse "beaker" crystals. Depth hoar normally forms near the base of the snow pack, where the vapour pressure gradient is strongest and most persistent. The upper part of the snow pack is subject to diurnal variations in temperature and thus vapour pressure gradient. Depth hoar has large compressive strength but low shear strength, so it can support the weight of overlying snow but can be sheared, for example, by tunneling animals or by the shear stress in a sloping snow pack causing snow avalanches. Small mammals move around in the depth hoar or pukak (native Alaskan word used by ecologists) to avoid predators and take advantage of the warmer temperatures in the snow pack.

melt metamorphosis (firnification)

With ET metamorphosis, the density of the snow pack increases to 0.58 to 0.60 g cm-3. Further increase in density results from melt and refreezing during the melt season and throughout the summer in firn, snow that survives the melt season.


Snow melt and melt water forecasting


methods of snow survey



Prairie snow melt


energy budget of the melting snowpack


  1. incoming radiation
  2. outgoing radiation
  3. sensible heat flux
  4. latent heat flux
  5. soil heat flux
  6. internal energy (changes in storage)
[ Course Outline | Next Topic ]