Transcript avalanche terrain - CWU Department of Geological Sciences

American Institute for Avalanche Research & Education
Level II Avalanche Course
Goals
1) Snowpack development and
metamorphism.
2) Standardized observation
guidelines & recording formats
for factors that indicate snow
stability.
American Institute for Avalanche Research & Education
Level II Avalanche Course
Video
American Institute for Avalanche Research & Education
Level II Avalanche Course
Objectives
 Formation of new snow and surface hoar
 Metamorphism of snow on the ground due to direct and
indirect weather.
 Observation guidelines and recording formats for
snowpack factors.
American Institute for Avalanche Research & Education
Level II Avalanche Course
Enable judgment-based decision making.
A complete understanding of the ‘science and technology” builds
the foundation for practical decision making in the field later.
This course is designed to bridge the gap between the basics
taught in the AIARE Level 1 Avalanche Course and advanced
decision making which is the focus of the AIARE Level 3
Avalanche Course.
You will understand how the mountain snowpack forms and
evolves, how avalanches are triggered, and how to observe and
record snow stability data. A stability analysis and forecasting
framework will be introduced.
American Institute for Avalanche Research & Education
Level II Avalanche Course
Major points:
• Avalanche Types and Characteristics
• Avalanche Terrain
• Creation of the Mountain Snowpack
• Metamorphism of the Mountain Snowpack
• Avalanche Danger Scale
• Avalanche Danger Factors
• Travel Techniques
• Planning and Preparation
• Decision Making
Snow stability
Snow stability is defined
as:
The likelihood that an
avalanche will start.
When we discuss snow
stability we are estimating how
easy it will be to trigger a slide
and, in general terms, where
avalanches might occur.
Snow stability does not take
into account how large an
avalanche might be, its
destructive potential, or
characteristics.
Snow stability
Snow stability is rated on a five
step scale:
Very poor
Poor
Fair
Good
Very good stability.
Very poor = least stable condition
Very good = most stable.
Even very good snow stability
(e.g., the best stability that can be
expected) avalanches are still
possible
Avalanche Hazard
Avalanche hazard is
defined as:
The likely consequences if
an avalanche does start.
Avalanche hazard takes
into account snow stability
and adds a variety of
factors such as
characteristics, terrain,
destructive potential, and
what kind of effect an
avalanche might have
Avalanche Hazard
Level 2 course curriculum introduces
snow stability analysis and forecasting.
To assess avalanche
hazard, one has to take
into account snow
stability analysis and
forecast as well as a
variety of other factors
that influence the
destructive potential an
avalanche might have
should one occur and the
likelihood that people or
facilities will become
involved in a avalanche.
Discussion
Why separate stability and hazard?
Reasonable judgment based decisions are based on analysis
and forecasting of avalanche hazard. Stability is a critical part
of the process that leads to a hazard analysis and forecast.
Stability is a discrete factor in the hazard analysis process.
It is essential that we deal with stability as a separate and
distinct subject before we delve into hazard.
Mountain Snowpack
Learning Outcomes
The basics of the
formation of new snow.
Riming and graupel
formation.
Surface hoar formation.
Creation of a layered
mountain snowpack.
Recognize basic new
snow crystals.
Recognize surface hoar.
Mountain Snowpack
Snow crystals form in the
atmosphere.
These crystals are created
when water vapor
condenses (deposits) as ice
on a crystalline nucleus.
The process of vapor becoming
a solid is called condensation.
Often refer to this process as
“deposition.”
Depending on the
temperature and humidity in
the regions where snow is
forming, new snow crystals
take a variety of shapes and
sizes.
Individual particles of
atmospheric snow are
generically called crystals.
Mountain Snowpack
The word snowflake refers
to a larger structure which
is formed when individual
crystals join together into a
“raft” as they descend to
the ground.
In reality atmospheric (new)
snow comes in a variety of
shapes and sizes.
The classic “stellar” shape is
what most people visualize
when we talk about a new
snow crystal.
We recognize a number of
sub-classes which reflect
the main types of new
snow.
Mountain Snowpack
When a new snow crystal gains enough weight to
overcome gravity and any updrafts that might exist in the
airmass, the crystal falls from the atmosphere and
eventually lands on the ground.
This creates what we refer to as the snowpack which is
really just the total accumulation of new snow that has
fallen to the ground to date in a given winter.
Sub-classes, shape, and atmospheric conditions of snow
Sub-class
Shape
Growth Environment
Columns
Short prismatic crystal, solid
or hollow
High supersaturation at -3o
to -8o C, below –22 o C
Needles
Needle-shaped,
approximately cylindrical
High supersaturation at -3
to –5o C
Plate-like, mostly hexagonal
High supersaturation at 0o
to -3o C, -8 to -25o C
Sixfold, star-like, planar or
spatial
High supersaturation at 12o to -16o C
Irregular
Crystals
Clusters of very small
crystals
Varying environments
Graupel
Heavily rimed particles
Plates
Stellars
(dendrites)
Super-cooled water in
airmass causes riming
Mountain Snowpack
Sub-classes
Columns
Needles
Plates
Stellars (dendrites)
Irregular Crystals
Graupel
Each of the sub-classes in
turn has numerous
variations.
More than one sub-classes
and/or variation can form in
a single storm as the
temperature and humidity
regimes change.
It is not uncommon to see
several different types of
new snow during a single
storm, that change back
and forth over short periods
of time (minutes to hours).
Mountain Snowpack
Sub-classes
Columns
Needles
Plates
Stellars (dendrites)
Irregular Crystals
Graupel
When a new snow crystal
gains enough weight to
overcome gravity and any
updrafts that might exist in
the airmass, the crystal falls
from the atmosphere and
eventually lands on the
ground.
This creates what we refer
to as the snowpack which is
really just the total
accumulation of new snow
that has fallen to the ground
to date in a given winter.
New snow, stellar
Mountain Snowpack
Riming
Under some conditions, tiny
water droplets form in the
atmosphere and remain in a
liquid state at temperatures
below 0o C.
These water droplets are
described as super-cooled.
This process is called
riming. The tiny ice
spheres are referred to as
rime.
When a super-cooled water
droplet comes into contact
with any surface or object, it
immediately adheres to the
surface or object and
freezes, forming a small
spherical piece of ice.
New snow, plate (rimed)
New snow, needle (rimed)
New snow, capped columns
Mountain Snowpack
Riming
The most visible form of rime is
when super-cooled water is
driven against a surface by
wind.
Under these conditions, rime
accretes on the windward side
of the surface and creates a
kind of icy stalactite formation
which grows larger as additional
rime is added.
Rime can also deposit on the surface of
the snowpack where it often takes the
form of a white, crunchy crust.
These rime formations are often
seen on rocks, trees,
communication towers, etc. in
wind exposed areas, especially
in maritime climates.
Mountain Snowpack
Graupe
l
Under heavy riming, a new snow
crystal can accumulate so much
rime that its original form
becomes completely obscured,
eventually forming a roughly
spherical (seldom a perfect ball)
pellet.
Sometimes referred to as “pellet
snow” this is what we call graupel.
Atmospheric snow simply called
new snow and use one symbol for
all types of new snow when
making field notes.
We will identify sub-classes when
we can clearly identify the grain
type and its significance in terms
of stability
The notable exception is graupel.
Mountain Snowpack
Symbol used to identify
new snow in field notes
+ New snow
+r Rimed new snow
Graupel
These are the most
common sub-classes.
Additional specialized
symbols for new snow subclasses are in the American
Avalanche Association’s
Observation Guidelines
(SWAG) book.
The SWAG book is onreserve.
A lower case “r” is added when riming is
prevalent but the grain is still recognizable
as new snow.
Mountain Snowpack
Surface Hoar
V Surface hoar
Another addition to the
snowpack that is
technically not a new
snow crystal but which
can form a significant
layer is called surface
hoar.
To understand surface
hoar formation, we need
to understand the
concepts of relative
humidity and saturation,
dew point, and the
formation of dew.
Mountain Snowpack
Relative Humidity
Definition:
The actual amount of water
vapor that at airmass at a given
temperature does hold to the
amount it could hold if it were
saturated at that temperature.
One of the components of
the atmospheric mix is water
vapor.
The amount of vapor in the
mix varies from time to time
and from place to place.
When the atmosphere
contains little water vapor we
say it has low humidity.
Conversely, when there is a
lot of water vapor present,
we say the air has a high
humidity.
Mountain Snowpack
Relative humidity
When there is so much water
vapor in the air that
condensation occurs and
clouds, mist, or fog form we
say the airmass is saturated.
How much water it takes for
saturation to occur depends on
the temperature of the air.
Warm air can hold more water
vapor than cold air so it takes
more vapor to saturate a warm
airmass and less vapor to
saturate a cold airmass.
Mountain Snowpack
Relative Humidity
When an airmass is saturated,
we say it has reached 100%
relative humidity (RH).
Misconception: The air doesn't
"hold" water vapor in the sense
of having some attractive force
or capturing influence.
This tells us that the air at this
place and time is holding all the
vapor it possibly can.
Water molecules are actually
lighter and higher speed than
the nitrogen and oxygen
molecules that make up the
bulk of the air, and they
certainly don't stick to them and
are not in any sense held by
them.
When a given airmass is less
than fully saturated with water
vapor, its RH is less than 100%.
For example, if an airmass
contains only half the water vapor
required to bring it to saturation,
we would say it has 50% relative
humidity.
Mountain Snowpack
Relative Humidity
Relative humidity is a ratio
that describes the actual
amount of water vapor that
at airmass at a given
temperature currently holds
compared to the amount it
could hold if it were
saturated at that
temperature.
A warm airmass, say one at
30oC, that is at 50% relative
humidity will have more
water vapor in it than a cool
airmass, say one at -10o C
that is at 50%.
In both cases, when the air
becomes saturated it will be
at 100% RH even though the
total amount of water vapor
will be different.
That’s why we call it relative
humidity.
Water is a a certain amount of water vapor can be resident in the air as a
constituent of the air. of dry air can be expressed as volume percentage
Mountain Snowpack
Say we take a cubic metre
of air that is -10oC and we
know it to be saturated.
Relative Humidity
Actual amount of vapor 200 g
Could hold 200 g at saturation
200 g
X 100 = 100%
200 g
=
If we extract all the water
vapor from it, weigh the
water vapor, and we find
that there was 200 grams
of water vapor.
We now know that it takes
200 grams of vapor to
bring air at -10 to 100%
RH.
Mountain Snowpack
Relative Humidity
Actual amount of vapor 100 g
Could hold 200 g at saturation
100 g
X 100 = 50%
200 g
=
If, at another time or
place, we take a different
cubic metre of air that is
at -10oC, extract all the
water vapor from it and
find only 100 grams of
vapor.
We can deduce that the
RH in this second
example is 50%, because
it actually contains only
50% of the vapor it could
contain at saturation
Mountain Snowpack
Relative Humidity
Relative humidity changes when:




Water vapor is removed from
an airmass (RH decreases)
Water vapor is added to an
airmass (RH increases)
The airmass is warmed (RH
decreases)
The airmass is cooled (RH
increases)
In our examples, we
assume that only one
variable is changed at a
time.
Thus, the air temperature
remains constant or the
amount of vapor remains
constant when changing
temperature.
There are other factors that
affect RH (e.g.,
atmospheric pressure) but
are significant in in the
formation of surface hoar.
Mountain Snowpack
Relative Humidity
Relative humidity changes when:




Water vapor is removed from
an airmass (RH decreases)
Water vapor is added to an
airmass (RH increases)
The airmass is warmed (RH
decreases)
The airmass is cooled (RH
increases)
In our examples, we
assume that only one
variable is changed at a
time.
Thus, the air temperature
remains constant or the
amount of vapor remains
constant when changing
temperature.
There are other factors that
affect RH (e.g.,
atmospheric pressure) but
are significant in in the
formation of surface hoar.
Mountain Snowpack
Dewpoint
The temperature that a
given airmass must be
cooled to attain saturation
(100% RH).
If the current temperature of
an airmass is –10ºC, and if
cooling it to -14ºC would
bring it to 100% RH.
Then the dewpoint of that
airmass is -14 ºC. At a
temperature of -14 ºC the
airmass would become fully
saturated with water vapor.
If we cool an airmass the
concentration of water
vapor will rise.
If we cool it enough, it will
eventually become
saturated (even though no
water vapor has been
added).
Mountain Snowpack
Formation of Dewpoint
Sometimes, only a very small
portion of the airmass gets
cooled to its dewpoint.
In summer, this occurs where the
air is in contact with a cool
surface (e.g., front lawn or car).
When this happens, we may not
see fog or mist but the thin layer
of air in contact with the lawn or
car will drop moisture onto the
cool surface just like the fogbank
makes your skin feel damp.
When an airmass is fully saturated,
it contains so much water vapor that
anything that it touches will become
damp or wet.
If we cool an airmass just a bit
beyond its dewpoint, condensation
occurs and clouds form.
If this occurs near or at the ground
we would call the clouds mist or fog.
Further cooling (and the presence
of a proper nucleus) will lead to
precipitation (rain if above freezing
and snow if below freezing).
Mountain Snowpack
Formation of Dewpoint
Place a glass in a refrigerator.
When it has cooled, bring the glass
into a warm room.
Dew will form on the surface of the
glass where it is in contact with the
air.
What’s happened is the glass has
cooled a very thin layer of air at the
air/glass interface to the dew point
and water vapor in the air has
condensed onto the cool glass.
The droplets of dew you
see on your lawn on a
summer morning comes
from the air that was in
contact with the lawn
during the night.
How much dew you get
(how many droplets of
water there are on the
grass and how large the
droplets are) depends
primarily on how much
water vapor was in the air
and how cool the lawn got.
Mountain Snowpack
Formation of Surface Hoar
What is surface hoar?
Put a glass into a freezer,
and you let the glass get
very cold, ice will form on
the glass instead of water
when you bring it into the
warm room.
In this case, the water
vapor becomes ice without
going through a liquid
phase.
Surface hoar is the winter
equivalent of dew.
Mountain Snowpack
Formation of Surface Hoar
Under certain conditions, the
surface of the snow cools a
thin layer of air at the
snow/air interface to the dew
point.
This causes water vapor to
deposit as ice on the
snowpack in the same way
that ice formed on the
freezing-cold glass in the
example above.
Surface hoar is not limited to
forming on snow; it is often seen on
trees, bushes, rocks, etc. and is
sometimes referred to as “hoar
frost” in non-technical circles.
The surface hoar you see on
the snowpack in winter
comes from the air that was
in contact with the snowpack.
Surface hoar layer on surface
Mountain Snowpack
Formation of Surface Hoar
V Surface hoar
Surface hoar crystals have a
characteristic “icy” look and
often glitter as they refract
sunlight.
In its classic form, surface
hoar has a feathery vee
shape but it can also form
as needle, plate, and hollow
six sided varieties.
Generally, striations are
visible on the crystals; these
are caused by successive
drops of moisture from the
air onto the surface.
Mountain Snowpack
Conditions that promote surface hoar growth
Clear skies: promote cooling of the spx through radiation loss that produces
a cold surface for surface hoar growth.
Calm winds: too much wind prevents the air to reach the dewpoint. A very
light exchange of air at the surface promotes growing large surface hoar
quickly as the exchange replenishes vapor supply.
Sheltered terrain: reduces wind effects.
Cooling air temperatures: increases relative humidity.
Calm winds: allows humidity to concentrate undisturbed near the surface of
the snow.
High relative humidity: more moisture available for surface hoar growth.
Proximity of water vapor sources: open water, moist ground, and warm
vegetation. help increase the relative humidity of the airmass.
Snow climates
Maritime
Continental
Intermountain
There are three main
snow climates, each of
which has particular
weather, snowpack, and
avalanche
characteristics.
Weather
Precipitation
Wind Transport
Maritime
Low Rate
Mod-High Rate
Large Accum.
Small Accum.
Med–Large Acc.
High Riming
Little riming or
surface hoar
Much surface
hoar formation.
Much pre storm
Little pre
Little-some pre
Much in-storm
Some-much in
Much post
Some-much in
Warm
Cold
Snowpack
Depth/Distribution
Layering
Deep, uniform
Uniform
Rounded
Temperatures
Intermountain
High Rate
Little post-storm
Temperatures
Continental
Warm
Shallow, variable
Strong over weak
Faceted
Cold
Some post
Cool
Shallow - mod,
var. early winter.
Deep - uniform
later
Variable, faceted
early, more
uniform, rounded
later.
Cool
Maritime
Avalanches
“Direct action”
“Delayed action.”
Many in-storm
events, associated
with significant
storms.
Some in storm events,
often associated with
minor storm.
Some post storm
events, usually
ending within 24 –
36 hours
Avalanche
Danger
Continental
Quick to
rise
Quick to fall
Intermountain
Direct and delayed
action
Many post storm, days
or even weeks later,
often associated with
little or no significant
weather.
Slow to rise
Often very
slow to fall
Quick to
rise
Often slow
to fall early
season;
quicker to
fall late
season.
Snow climates
Maritime
Continental
Intermountain
Since the different subclasses new snow often fall
during a storm and since each
of these may have
significantly different
characteristics, it is not
unusual to see different layers
form in the snow that falls
during a storm.
Even if the storm snow is
homogeneous, in most cases
it differs from the surface of
the snowpack it falls onto.
This forms the first of what
may be many layers in the
mountain snowpack, the
interface between the storm
snow and the old snowpack
surface being the boundary.
Snow climates
Maritime
Continental
Intermountain
Since there are layers in
the snowpack, and if they
are different from one
another, the layers may
not bond to each other. It
is this layering that is the
basis for the formation
and release of
avalanches.
Riming may occur and the
snow climate has an influence
on the type of snow that
forms, weather conditions
under which it is deposited,
and the likelihood that surface
hoar will form.
Successive storm snow
deposits, the weather
conditions present during and
between storms, riming,
surface hoar deposits, and
the snow climate combine to
create a succession of layers
in the snowpack as it
develops over the winter.
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