...THE WHITE DRAGON'S WAKE...
INTRODUCTION
TYPES
OF AVALANCHE
THE
MECHANISM OF AVALANCHE RELEASE
FORECASTING
SNOW AVALANCHES
AVALANCHE
CONTROL TECHNIQUES
AVALANCHE
HAZARD RATINGS
AVALANCHE
TRAINING FOR DOGS
INFO
ON THE PAGE OWNERS
- Other methods of forecasting...
Examining
the snow cover
Snow
cover structure is investigated directly by digging pits and examining
the exposed stratigraphy. Snow temperature, density, strength properties,
and crystal type are all important or determining stability. Time variations
in these properties are examined by a succession of pits in a given study
area, the result being plotted in a time profile. Indirect evidence on
snow structure can be gathered by instruments probing from the surface.
The most useful of these is the ran penetrometer which measures snow strength
variations with depth by means of a pointed rod driven by a falling weight.
Periodic observations at representative study plots (usually on level ground)
are compared with snow profiles from actual avalanche fracture lines to
determine and anticipate stability trends.The basic structure leading to
slab avalanche formation is a cohesive snow layer resting on a weak substratum
which offers poor support or attachment. Actual combinations of slab layer
and substratum strength vary widely. A heavy,, hard slab of great thickness
may exert enough shear stress at its base to rupture a relatively strong
supporting layer which would provide adequate anchorage for a lesser overburden.
On the other hand, even shallow layers of soft, weak snow may break free
as a slab avalanche if the substratum is sufficiently fragile.
A common source of weakness is depth hoar formed in the early winter snow
cover. This provides very poor support for subsequent snowfalls
which often slide off fully developed depth hoar regardless of their individual
character. Thin layers of depth hoar, surface hoar, or
graupel can also provide a fragile bond (good lubricating layer) when sandwiched
between stronger layers. The general process of constructive metamorphism
always weakens snow layer strength and bonds; it may precipitate
an avalanche long before recognizable depth hoar crystals actually appear.
Another frequent cause of slab avalanching is an ice layer
or crust which provides a smooth sliding surface. Crusts formed by refreezing
following a rain storm offer especially poor anchorage to subsequently
deposited snow layers. The bond between slab layer and
a crust can be poor at low temperatures., while it rapidly gains strength
if the interface is near the freezing point. Other patterns
of snow stratigraphy also lead to slab avalanche formation in dry snow
though these are the most important.
