Snow and perennial ice

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Temporal distribution | Spatial distribution

Temporal and spatial distribution of snow

Temporal distribution

At elevations of 2000m in the Alps, snow covers the ground for nearly seven months, from mid November to the end of May. With increasing elevation, the period of time when there is a persistent snow cover increases. At around 3000m the nival zone is reached, where the snow stays all year around.

1 - Snow cover during a year at different altitudes in the Alps

		Usually at Weissfluhjoch, snow is present from mid-October to mid-July. The time point of the start of continuous winter snow cover, its depth and the beginning of snow melt differs considerably from year to year.
2 - Development of the snow cover at Weissfluhjoch (2540 m). The maximum and minimum curves show extreme values in 67 years of manually acquired measurements. Source:http://wa.slf.ch/ (German)

Spatial distribution

The local pattern of snow on the ground is the result of precipitation and redistribution processes.

The small scale distribution is determined by:

Wind (Fig. 3 and 4)
Vegetation (Fig. 5 and 6)
Terrain configuration
Avalanches (Fig. 7)

In localities of high wind speeds, as on the wind-ward side of ridges, snow is eroded and deposited in areas of low wind speeds, as in the lee of ridges.

3a and b - Wind drift. Snow drifting from the ground occurs at wind speeds above 4-10 m/s. (64K and 48K) Photos: S. Fehler, SLF		4a and b - At the leeward-side of any obstacle, wind speed is low and snow is deposited and accumulates on the ground or, as in this case, on a roof. (56K and 52K)


In the forest, the distribution of snow on the ground is determined by how much snow is intercepted by the crowns of trees. The amount of snow intercepted depends mainly on air temperature and the surface structure of intercepting branches.

5a and b - Interception of snow on the branches of spruce trees. Around 30% of the falling snow is intercepted by evergreen trees such as spruce. (188K and 76K)		6 - Snow depth along a profile from an opening towards a spruce tree reveals the heterogeneous distribution of snow as a result of interception by tree crowns. (160K)


Does the snow profile in Fig. 6 correspond to the 30% rule? What should a branch look like so that snow gets caught efficiently and stays for a long time? Which temperatures facilitate trees in keeping their snow cover (think of the snow structure and the behavior of snow that you observed by different temperatures)?


Answers

Avalanches

Avalanches redistribute snow masses from the release zone down to lower elevations. Establishing speeds of up to 350 km/h, they break trees along their pathway. The release of avalanches depends on the varying deposits of snow in relation to wind, topography and/or forest cover.

7 - An avalanche in action (48K)

8 - A skier sets off a slab avalanche (76K).
Photo: T. Stucki, SLF

9 - Superficial, loose wet snow slide (72K)
Photo: H.-J. Etter, SLF

10 - This landscape has been formed by avalanches: The outer Dischma valley near Davos (136K).
Photo: T. Wiesinger, SLF

Find the slope inclination where avalanches may release.
Move the slider to change the angle and check your observation.

In very unstable situations avalanches may release at smaller slope angles!

The large scale distribution of snow depends mainly on:

Continentality

Continentality is a climatic effect influenced by the energy storage capacity of the oceans. The oceans store thermal energy in warmer seasons and emit a part of this energy in the colder seasons. With a longer distance from the coast

temperature differences between summer and winter increase
precipitation decreases.

Continental climate is charactrerised by:		Maritime climate is characterised by:
thinner snowpack lower densitiy of new snow (ca. 70 kg/m³) lower air temperatures (during winter) hidden depth hoar and persistent instabilities		thicker snowpack higher density of new snow higher air temperatures (during winter)

The snow cover constantly changes, not only in its duration and depth, but also in its physical and chemical characteristics. Hence it offers a constantly changing environment, dominating alpine ecosystems for more than half of the year.

The development of the snow cover and its dimensions vary quite considerably between winters. However, the local snow cover shows a relatively constant pattern in its temporal and spatial extensions.

29 August 2011 © ALPECOLe 2002-2007