Avalanches III

Dry Flowing Avalanche

Avalanche Impact Forces

Air Blast: air pressure wave that runs beyond the visible avalanche front.

Air Blast

Dry snow avalanche: Avalanche with a high-density core of snow and air.

High density core at bottom is flowing avalanche.

Powder avalanche is where the dense core at bottom is absent. True powder avi, all material is suspended by turbulent eddies.

Dry Flowing

Wet Flowing

Powder Avalanche

Patterns of new snow instability

Sastrugi at South Pole Station, Antarctica. (Courtesy of Mr. Fred Walton, NOAA Corps Collection.

Sastrugi, or wind sculpted snow, are ridges formed when wind erodes and drifts the snow.

Sastrugi are sharp irregular grooves or ridges formed on a snow surface by wind erosion and deposition, and found in polar and temperate snow regions. The ridges are parallel to the prevailing winds.

Sastrugi

Sastrugi

Bill McAfee for the National Science Foundation

The Dome at Amundsen-Scott South Pole Station is framed by ridges of snow called sastrugi

Penitentes, or nieves penitentes, are a snow formation found at high altitudes. They take the form of tall thin blades of hardened snow or ice closely spaced with the blades oriented towards the general direction of the sun.

Differential ablation that leads to the formation of penitentes is that dew point is always below freezing.

Thus, snow will sublimate, because sublimation requires a higher energy input than melting.

Once the process of differential ablation starts, the surface geometry of the evolving penitente produces a positive feedback mechanism, and radiation is trapped by multiple reflections between the walls.

Penitentes, or nieves penitentes

Penitentes were first described in the literature by Darwin in 1839. On March 22, 1835, he had to squeeze his way through snowfields covered in penitentes near the Piuquenes Pass, on the way from Santiago de Chile to Mendoza, and reported the local belief (continuing to the present day) that they were formed by the strong winds of the Andes.

The hollows become almost a black body for radiation, while decreased wind leads to air saturation, increasing dew point temperature and the onset of melting.

In this way peaks, where mass loss is only due to sublimation, will remain, as well as the steep walls, which intercept only a minimum of solar radiation.

In the troughs ablation is enhanced, leading to a downward growth of penitentes. A mathematical model of the process has been developed by Betterton (2001), although the physical processes at the initial stage of penitente growth, from granular snow to micropenitentes, still remain unclear.

The effect of penitentes on the energy balance of the snow surface, and therefore their effect on snow melt and water resources have been described by Corripio (2003) and Corripio and Purves (2005).

Penitentes, or nieves penitentes

Photograph of penitentes taken in the summit crater of Mount Rainier. Formations approximately 50cm high, tilted southwards towards the sun. Photograph taken by Mark Sanderson, September 2006

Penitentes

Penitentes

Penitentes ice formations at the southern end of the Chajnantor plain in Chile.