Snow Hydrology Midterm1

162 km/hr

97.2 miles/hr

change melting pt =

(decrease in meltig pt / unit pressure)x(pressure change)

heat energy =
(mass)x(specific heat of ice)x(temp change)

heat energy =
(mass)x(latent heat of fusion)

heat energy=
(mass)x(specific heat of h2o)(temp change)

heat energy =
(mass)(latent heat of vaporization)

heat energy =
(mass)(latent heat of sublimation)

104.5*

lone pair orbitals (2)
--- two O electrons

covalent bonding orbitals (2)
--- one O electron
---- one H electron
(+) charge

tetrahedron

chains of molecules linked together

0*C - 4*C:
--cutting of H bonds (infolding of crystalline lattice)
FASTER RATE than
thermal expansion

@ 4*C
---= the most efficient packing of water molecules due to cutting of h bonds
---increase in density reaches maximum

> 4*C
---density decreases again because thermal expansion occurs at faster rate than
infolding of crystal lattice (cutting of h-bonds)

tratrahedral crystalline element of ice is packed hexagonally, forming Ih

some h-bonds holding molecules in crystalline shape are broken,
and ice becomes liquid with no change in temp
... the cutting of h-bonds causes crystalline shape to fold in on itself

1. added energy causes individual h2o molecules to vibrate faster
= thermal expansion

2. more h-bonds are cut, causing additional infolding of the crystalline lattice

Joules / second

cubic meter h2o

PV = nRT

P= pressure
V= volume
T = temp

***TEMP ALWAYS IN *KELVIN*****

n = #moles
R = gas constant

weighted mean density:
= (density)(layer height)
sum all values and divide by depth of snowpit

weighted mean temp
= (temperature)(layer height)
sum all and divide by depth of snowpit


SWE calculated as the
(mean density)(snow depth)(density h2o)

Snowpack behind SNOWFENCE:
-depth determined by height of snowFENCE, not amt of snowfall

-snowdrift forms and reaches mature shape earlier in the season
==> - deeper snowpack
===> higher SWE

HIGHER DENSITY
-higher density than natural bc of wind transport shears snowflakes into smaller fragments -- packed more efficiently during deposition

*HIGHER TEMP
--snow accumulates while ground is still warm and is warmed by ground
+ higher density = more efficient heat transfer from ground thru snowpack

higher snow density = higher thermal conductivity of snowpack

(more efficient heat transfer from ground thru snowpack)

*why snowpack behind snofences are warmer than natural snopacks

wind during snow events causes precip gauges to under-estimate "true snowfall" amt

Pa = (K)* (Pg)

Pa = adjusted precip

Pg = gauge measured precip

K = 1/CR
(adjustment factor for wind induced error)

CR = catch ratio of gauge

catch ratio of gauge

CR = alter shield and snow = exp (0.0055-0.133Ws)

CR = no shield and snow = exp (-0.251-0.176Ws)

CR = alter shield and rain = exp (0.0055-0.133Ws)


Ws = Wind speed at gauge orifice

tension = crown

shear = bed

compression = stauchwall

SLOPE ANGLE:
-typically the upper slopes of gullies have steeper slope angles than the surrounding terrain
--provides local loading and starting zones

gullies have low slope angled bottoms where deep avalanche debris can accumulate
& slides can be triggered from below compression zone

EXPOSURE
due to wide variety of aspects found along upper perimeter of gully
== variable heating regimes within confines of gully
----> these differential energy patterns can lead ot formatiom of weak layers and crusts different slopes

WIND
-bc of wide range of aspects, distribution patterns of wind deposited snow are highly variable w/i gully boundaries
---some boundaries / slopes may be lee deposition & others are scoured
-the natural shape of gully can easily mask wind loaded accumulation zones

RUNOUT ZONES
-small gullies can serve as place where runout zones deposit avalanche debris
-- due to confined geometry of gully, small avalanches can produce significant piling of avalanche debris into small areas

LACK OF PERCEIVED DANGER
-traveling in gully bottom gives flase sense of security due to lower slope angles

shear stress =
(snow density)(gravity)(snow height)(SIN of slope angle)

increasing density = increase shear stress

increase height = increase shear stress

snow conditions don't look good!!!

1 m = a lot fo snow
80kg/m3 = low density ("light")
+ 20m/s winds will redistribute a LOT of snow

= A STABILITY EVALUATION
determines snowpack stability for a given slope

-- should be conducted on a slope that is similar (slope angle / aspect /wind depo ...etc)
to slope that will be skied

Rectangular block of snow
(1.5m wide in downslope direction
2m wide across slope)
is isolated by cutting out sides and cutting the back out (using saw / ski / rope)
-after block is built it's loaded to produce a weak layer of failure in stages that give a rough numerical rating as and index of stability

1 - failure under weight of block alone

2 - one person on skis steps onto blocks

3 - person weights skis by making rapid knee bend

4 - person with skis jumps

5 person with skis jumps 2nd time

6 - person withOUT skis jumps

7 - no failure observed

year progresses = temp becomes higher

both incoming and outgoing longwave increased as year progressed due to higher temps

Outgoing rad > Incoming rad
bc
--snowpack warmer than air
--snow has higher emissivity than air, so emits more LW rad

shortwave more variable bc affected by cloud cover

primarily due to decrease in albedo
and increase in incoming solar rad

80%

CA - 80%

CO- 70%

due to albedo of snow and ice (high relative to other surfaces

1. ALBEDO
-new = ~0.8-0.9
-old =~0.5-0.6

ice (0.3)
water(0.05)
**blocks incoming rad from surface ((solar energy returns to space instead of being retained in atm)

2. surface temps stay depressed

3. low conductivity = acts as insulator keeping vast areas of soil unfrozen

4. climate feedbacks -- snow depresses temps causing more snow...

1. controls hydrologic cycle
(stored over winter and relased in pulse during spring melt)

2. frozen h2o = 80% freshh2o

3. major contributor to river and ground h2o

energy required (q)
= mass (m)
*(latent heat (L)

(L)
amt of heat energy released / absorbed when a substance changes phase

amt of heat energy required to raise the temp of
1 g substance
by 1 degC (or 1 degK)

gradients of temp and vapor density within snowpack

transfers of
-heat (energy)
-liquid water
-water vapor
to or from the snowpack

normally 0degC

(but some water flow can occur when temps are -3 or -4degC

1.radiation

2. conduction

3. convection

4. advection

transfer via electromagnetic waves

... the only form of energy transfer that doesn't require a medium

(thus the only way energy can be transferred thru space)

transfer by molecule to molecule contact
in response to a temp gradient
.. when substance itself doesn't mix

(takes place in both solids and liquids but solids more important)

transfer when the substance mixes

sensible heat flux (in response to temp gradient)
latent heat flux (thru change of state amound solid / liquid / gas phase)

transfer of energy by mass transfer
(eg rain on snow)

often called "HORIZONTAL CONVECTION
-- ie horizontal movement of warm air mass over cold snowpack

as temp increases, the WL that max energy is emitted DECREASES

104.5*

H = 1 P --- one e

O = 8P 8N
---2 e's in inner shell
---6 e's in outer shell

donor:
-covalently bonded H in interaction

acceptor
-accepts H of other molecule via h-bond

1. high surface tension

2. high viscosity

3. high specific heat

4. density

5. melting / boiling pts

6. solvent properties

because attracted to itself via h2o molecules h-bonds

h-bonds make water "sticky", for its size

(like honey)

amt of energy to raise temp of 1g substance by 1degC

... spec h2at h2o 5x > land

--can absorb large amt of heat with small increases in temp

-- releases a lot of heat with only a small drop in temp

**oceans / lakes moderate climates of adjacent land

max density @ 4degC
not freezing pt

--expands upon freezing

-- lakes only freeze at surface!

abnormally high

--allows h2o to exist as liquid at earth's surface

good solvent for ionic salts and polar molecules

imp in transfer of dissolved substances in hydrologic and biological systems

hydrogen (1e,1p,0n)

deuterium (1e,1p,1n)

tritium (1e,1p,1n)

oxygen-16 (8e,8p,8n)

oxygen-18 (8e,8p,10n)

-no net charge to h2o molecule

-BUT electrical field not evenly distributed
(bc O atom attracts e's more strongly than H)
---> side of h2o molecule with H= net + charge
--> side of h2o molecule with lone pair orbitals = net (-) charge

it is a POLAR MOLECULE
(molecule with partial neg and partial positive charge)
--> allows h2o to seperate polar solute molecules and solvent ability

can refer to the H-bond

... ie = ENERGY needed to break it is LOW

avg of 3.4 h-bonds per h2o molecule


.... the large number of h-bonds actin in unison have strong contributory effeft
(for liquid h2o state)

in ice each molecule is H-bonded to 4 other molecules

the orderly arrangement of h2o molecules with fixed positions for the O atoms... held together by H-bonds

1. there are two protons close to each O atom
(~9.8A)

2.the two protons in each h2o moleule are oriented so they pt toward two different O atoms

3. there is a single proton on a H-bond between two adjacent O atoms

4. under ordinary conditions any of the large # possible configurations equally possible

1. each pair of O atoms is linked by an H-bond

2. the H-bond moves back / forth between the O atoms

3. each O atom in ice is connected to 4 other O atoms with 4 H bonds

2.76 A

(angstoms)

109*

11

hexagonal form
--- only one found on Earth surface
(@normal T and P)

--> The tetrahedral structure crystallizes hexagonally

hexagonally
or
cubically

-- layer of hexagonal rings

..c-axis at rt < to plane

change in kinetic energy of participating particles

geometrically & bondwise setup with lowest energy conformation

ICE lowest energy setup =
--- 4 nearest neighbors
= open hexagonal structure
(each bond = lowered overall energy)

liquid like layers that form on solid surfaces at temperatures below the solid's melting pt

160K

large heat capacity == large amts of energy required for evap and freezing

**energy released / used in evap/freezing = major control on local air temp (particularly in mtn enviros)

freezing - released
melting - used
evaporation (vaporization)-used
condensation - released (gas to liquid)
sublimation (solid to gas) -used

graph shows the conditions at which substance exists as solid, liquid or gas

(function of T and P)

--> line seperating two phases = conditions where two phases can exist in equilibrium
--> triple pt= where all 3 curves meet
0.01 degC 0.006 atm

all phases exist in EQUILIBRIUM
(tho all phases exist simultaneously on E's surface they're not in equilibrium

0.01 degC
0.006 atm

q = mL

q = energy required

m = mass

L=latent heat

amt of heat energy released / absorbed when a substance changes phase

--latent heat is constant for a substance for a given phase change

energy required to MELT ice

solid --> liquid

energy required to evaporate / vaporize liquid

liquid ---> gas

energy required to go from
solid ----> gas

(latent heat of fusion)+(latent heat of vaporization)

q=mc(Tchange)

mass x specific heat(c) x delta T

amt of heat energy required to raise the T of one g substance by 1degC or K

energy = mcT

expressed as a number density of h2o molecules
Nv

or as a mass density
rho(v)

amt h2o vapor in air sometimes expressed as "Vapor Pressure" (e)

PV = NRT

P-pressure
V-volume
N-# moles
R-gas constant
T-temp (K)
V-vapor

substitute rho/m for P/V (in ideal gas law)

P = rho RT / m

rho = density (kg/m^3)
m = molecular weight (kg/mole)

N(v) = e/kT

N= "absolute humidity" expressed as # density
e = vapor pressure
k=
T = temp


or

rho(v) = e/R(v)T

rho(v) = absolute humidity expressed as mass density
R=gas constant

total pressure is equal to sum of partial pressures

P = Pd + Pw
(dry air prressure = Pd
Vapor pressure = Pw)


-- same with total density
rho = rho(d) + rho(w)

e(s)
max amt of h2o vapor that is thermodynamically stable

-function of air temp
*(amt h2o vapor air can hold increases exponentially with increasing air T)

the amt of h2o vapor in the atm often exceeds the SVP

("equilibrium vapor pressure is better term"

RH
ratio of actual vapor pressure
e(a)
to the saturation vapor pressure
---expressed as %

RH = e(a) / e(s) x 100%

actual amt h2o vapor
is greater than sat vapor pressure

ie RH > 100%

always near 100%

... so as h2o vapor moves from warmer to colder regions in snowpack
pore space becomes supersaturated with respect to water vapor ....
and h2o vapor deposits directly from gas phase to solid phase

DRIVE PROCESSES --- SINTERING
& FORMATION OF DEPTH HOAR

1. function of temp only

2. below 0degC SVP over water > SVP over ice

3. below 0degC plenty of condensatio nuclei for h2o but few available for ice
.... thus supersaturation w/ respect to h2o is rare
.... & supersaturation w/ respect to ice is common

SVP over liquid always greater than SVP over ice

(ie the atm is supersaturated with respect to snow crystals when NOT supersaturated with respect to h2o droplets)
h2o thus moves from by the gas phase form liquid h2o droplets to ice crystals

degree of supersaturation in atm

-degree of supersaturation in atm largely determines snow crystal type

-supersaturation in pore space of snopack determines type of snow metamorphism that occurs
(ie ET or TG snow grains)(& avalances)

1. creation of saturated conditions in atm

2. condensation of h2o vapor into liquid h2o

3. growth of small droplets by COLLISION and COALESCENCE until they become large enough to precipitate

air mass cooled by being lifted vertically

cool air holds less h2o vapor
(e(sat) is REDUCED)

warm air holds more h2o vapor

phase change where h2o vapor becomes liquid

requires creation of saturated conditions
.... and presence of condensation nuclei (small dust particles or previously formed particles of h2o or ice)

... can produce such small particles = stable in atm
(white clouds = droplets too small to precipitate)

small droplets into larger droplets
thru collision of small droplets with eachother (or with large drops)
=
drops large enough to overcome gravity and fall to ground as rain

1. h2o present in Vapor phase

2. RH = 100%

3. Tair to 0degC

4. nucleating agents

snow requires ICE NUCLEI
... hard to 'make' h2o molecules line up correctly to form a crystalline 3d shape

(rain requires dust or previous formed h2o droplets)

h2o vapor + nucleus + T<0*C + saturation
---------------->
nucleation
-------------------->
ice crystal
----------------------->(sublimation growth)
Snow cystal
-----------> (continued growth thru..)
1. Sublimatiom
2. Riming
3. Aggregation

the amt of h2o vapor in air increases above RH 100%--- ie supersaturation

,.... if RH = 112%... the amt of supersaturation is 12%

homogeneous nucleation


heterogeneous nucleation

-spontaneous process
-no nuclei necessary

-- formation of ice crystals from supercooled h2o droplets (-40*C)

1. deposition nucleation

2. immersion-freezing nucleation

3. contact nucleation

growth by deposition from vapor phase onto existing ice nucleus

Radius > 1000 A

ice nucleus imbedded w/i supercooled h2o droplet

Radius > 100A

collision contact between ice nucleous and supercooled water droplet

more efficient than freezing nucleation (can nucleate at Temp 5-10degC warmer than in freezing nucleation)

-- is "the deposition of gas or liquid onto an ice nuclei"

.5-8 micrometers diameter
--can be advected 1000s miles

--dust storms, volcanic eruptions good sources

ln N = A (T1 - T)

T1 = temp for concentration of 1 active ice nucleous per liter
(norm ~ -20degC)

silicate minerals
---clays (kaolinite & illite)
----(greenland -- 85%ice cristals had clay particles as nuclei with >1/2 kaolinite


sea salt = poor nucleating agent ... cloud drops must be cooled to at least -35degC before sea salt aerosols act as nuclei

-decomposing leaf matter = good

-microbial decomp increases effectiveness of ice nuclei

Composition of organic ice nuclei:
--insoluble in h2o
--stable in all common organic solvents
--temps above 60degC deactivates ice nuclei

-growth from vapor phase

-aggregation

-riming

snowflakes formed by collision / adhesion (or sticking) or ice and snow crystals

growth rate:
-fast compared to vapor diffusion

max size:
-@ -1 to -4degC -- substantial pseudo liquid film on ice surface .. = ice neck connection between crystals ... crystals freeze togehter (mass sufficient to fall to ground)

-- adhesion of super-cooled h2o droplet to an ice particle or snow crystal

size of supercooled droplet =
--- 2-50 micrometers diameter

adhesion efficiency
--- very high (~1)
(ie every droplet adheres to every particle it contacts)

growth rate
-rel. fast
(10-20min for crystal to grow from 250micrometer radius to a 1-2mm radius)

formula to calc growth rate ;
dm/dt = pie r^2 a b w W

dm/dt = pie r^2 a b w W

r = radius of crystal
a=adhesion efficiency (usually 1)
b = collosion rate between droplet and ice crystal

w= crystal fall velocity relative to h2o droplet
W= liquid h2o content of cloud

Rimed
(initial form of crystal is apparent)

Graupel
(original crystal shape is obliterated usually round ball (also called "soft hail", "snow pellet")

Hail
(more advanced riming)

Sleet
(partially melted and refrozen snow crystals /or/ hard and transparent ice particles of frozen drops)

1. h2o droplets have a hand time of ~ 15sec

2. air temp rel. warm (close to 0degC)

3. natural air humidity low (<100%RH)

1. inject air / h2o mixture into atm

2. produces liquid h2o drops 100-700microns diameter

3. cooling of air mass by adiabatic expansion
(temp of h2o/air shot into atm is lower than surround atm by several degC)

4. add nucleating agents that are efficient at warm temps (near 0degC)

5. makes good ski base
--produces rounded ski grains (pack efficiently
---high density snow (400-450 kg/m3

isolated from corn plant
--cultivated and freeze dried preserving cellular structure

--- protein in cell wall = nucleating agent

*very efficient
1

1. temp

2. % supersaturation

3. ice nuclei type

1. BASAL PLANE
-three a-axis' (each 120*apart)
--hexagonal symmetry
--produces plate-like & star like structures

2. OPTIC axis
--c axis (90*to basal plane)
--produces Needle or COlumelike structures

3. high supersaturation
--growth occurs where excess vapor density highest
-- growth occurs at edges and corners
-complicated crystals (dendrites)

4. low super saturation
-produces solid structures

5 atm changes after deposition
-often pass thru diff T and vapor regimes in atm -- crystal type can change after initial formation
---ex Capped Columns

solid column formed in cold air w low supersaturation

enters warmer air -- plate can grow on end making capped column

-- ice is poor solvent (unlike liquid h2o)

--impurities dissolved in h2o are rejected or precipitated when h2o freezes in atm

--incorporation of impurities involves "VACANCY SUBSTITUTION"
--> replacement of h2o molecule with one of similar ionic radius and charge (NH3 for H2o, or F for O)

-- snow and ice crystals have high surface:volume ratio
+ lower settling rate
rel. to rain
//// so snow / ice crystals better at scavenging impurities out of the atm & rain
(thus have many impurities located on outside -- not inside of crystalline lattice structure)

deep (15-25m annual snowfall)

high density (120kg/m3 new snow density)

moderate air temps (-1.3 *C)

low temp / vapor pressure gradient (<10*C/m)

snow melts as land / lies on ground

snow settles as lies on ground

snow easily blown by wind & redistributed

1. snow stakes (perm or seasonal)

2. snow probes ("rulers stuck in snow")

3. remote sensing: not functional
--active microwave has potential-- but not functional for measuring snow depth

1. Gravimetric

2. snow courses

3. snow pillows

4. radioisotope gauges

-melt and weigh snow of known volume

"federal" / "mt rose" snow smaple=
=log tube of known volume retreives snow core (want from top to bottom of pack)

manual surveys - req 2 ppl
measure snow depth & h2o content @ designated course

= perm site that represents snow conditions at a given elevation in given area

*norm 1000ft long in area protected by wind

snow pillows = envelopes of stainless steel or synthetic rubber that contain antifreeze solution ---
as snow accumulates on pillows
devices convert snow weight into SWE

SNOTEL

nuclear gauges== radioactive source in ground under snowpack
... measure radioactivity without snow and above snowpack -- attenates radioactivity as a function of mass of h2o
(swe)

gamma ard SWE measurement
-based on fact that natural terrestrial gamma rad is emitted from K, U, and thorium radioisotopes in upper 8" soil

--- rad sensed from low flying aircraft
----water mass in snow cover blocks terrestrial rad signal

change constantly
as function of
energy flux,
wind,
moisture,
h2o vapor,
pressure

porous medium
(ice + air)
---(+liquid h2o)

-generally composed of layers of different snow types

-ice is in forms of crystals and grains that are usually bonded together
((forming texture w/ some degree of strength)

mixture of water in three phases
ice
liquid h2o
h2o vapor

-- but AIR is imp component (95%+ of champagne powder)

---+ solutes / particles

Once ice particles are deposited on ground

-- (bc of changes that occur within snow pack)

“snow grains”

due to changes that occur w/I snowpack

Snow grains become bonded with their neighbors
-- act as ice skeleton providing structural strength to snopack

-best way to measure snowpack characteristics

- (dig a hole from snow surface to ground)
WORKING WALL
- side of hole facing sun (ie self shaded from sun)
- smoothed with square shovel

1. water equivalent
2. Temp
3. Albedo
4. Liquid water equivalent

measuring tape from surface to ground

z= 0 at ground (bc ground-snow interface doesn't change)

mtns 10cm - 100m over
100m distance

(the amt of ice relative to air)

mass / unit volume

measured by weighing a known volume of snow

ranges from 30kg/m3(dry champagne powder) to 600kg/m3 (snowmelt runoff)

the amt of liquid h2o stored AS snow

density * depth / density of h2o (given)

ratio of density dry snow : density ice
-- give fraction of volume composed of ice

1 - this fraction = fraction snow sample composed of air (ie porosity)

Hardness =
-measure of strength of snowpack in compression

increases with increasing density (density generally increases with depth due to compression-- so hardness generally increases with depth)
--

amt of bonding
strength of bonding

rate / location of meltwater flow

gradients of water vapor w/i snow pack

1. initial type ice particle & initial conditions of deposition

2. temp of snowpack

3. magnitude of temp gradient

4. pore space size

surface 0*C day

surface -10*C night


mid = ~ -5*C

bottom = ~0*C

1. temps w/i pack norm not uniform
(drive temp gradients of h2o vapor movement

2. when pack isothermal @ 0*C meltwater is released from bottom -->to hydro system

1. stored heat in ground from summer warming

2. geothermal heat from interior

GROUND norm near 0*C (stored summmer heat and geothermal heat from earth interior)

SURFACE responds to surrounding conditions and daytime heating / nighttime cooling


----> surface generally cooler than bottom =
VERTICAL TEMP GRADIENT

snow -
--heat flow upwards


soil
---heat flow upwards

considerable mass exchange across pore space via sublimation
vapor diffusion
re-deposition

**every snow grain sublimates, diffuses in gas phase to somewhere else in pack, and is redeposited (beleives some snow researchers)

always close to 100% bc pore spaces poorly ventilated

ie at or very near sat. vapor pressure


..... so the pressure of water vapor within pack is controlled BY TEMP of pack

controlled by TEMP
bc RH always close to 100%(ie alwyas at or near SVP)

warmer areas - higher vapor pressure
colder areas - lower vapor pressure

h2o vapor diffuses form higher pressure (warm) to lower pressure (cold) areas

-grain size / type (thru influence on water vapor gradients -- movement)

-snowpack statigraphy

-temp

+ has influence on depth / density

energy exchange w/ atm

absorbs & transmits less rad

reduces air movement

conducts less heat


**** energy exchange at surface driven mainly by radiation and turbulent fluxes*****

1. conduction thru ice skeleton

2. vapor diffusion thru pore spaces

radiation

turbulent fluxes
(sensible and latent energy exchanges)
---sensible and latent may be large in magnitude, but tend to cancel eachother out

all objects at all time emit and absorb electromagnetic energy from surrounding

-transfers occur continuously

-amt rad emitted by object is function of temp of object

0.4 - 5.0 um

shortwave emission from snowpack - negligible

3 - 100 um

ratio of reflected to incoming rad

albedo snow as high as 95% (new dry snow)

can penetrate snowpack... with rad intensity decreasing exponentially w depth

-- dry snow with density 100kg/m3 -- <10% of solar rad that penetrates snowpack remains after 10cm distance

... solar rad during day source of energy & can raise T of upper layers

incoming -- absorbed with little penetration
lost continuously at surace (cooling surface)

location of maximum energy exchange of system
... during day several cm below surface due to penetration via incomimg short
wave
(long wave loss from surface can cancel out incoming shortwave)

night- no solar rad input
--still longwave output ...
= NET ENERGY BALANCE usually NEGATIVE
(surface temp of snow less than air temp)

difficult bc costantly changing via
temp, wind, moisture, vapor pressure
variations

"Grain"
-visible ice particles in snowpack

"grain boundary"
designate where 2+ grains sintered together

GRAIN SHAPE

-- tells most about snow pack evolution (history)

--info about snow stability and avalanche potential

1. appearance
-solid / hollow
-broken
-abraded
-partially melted
-rounded / angular

2. Surface
-rounded facets / stepped or striated / rimed

3. interconnections
-bonded / unbonded / bond size
-clustered
-# bonds per grain

avg size of characteristic grains w/i snow mass

(size in mm)

-Very Fine = <0.2
-Fine = 0.2 - 0.5
-Medium = 0.5 - 1.0
-Coarse = 1.0-2.0
-Very COarse = 2.0-5.0
-Extreme >5.0

ET <0.5 mm diameter

TG > 3.0 mm diameter

rounded snow grains
tend to be 0.1-0.2 mm diameter
"ET"

-tend to bond together in simple chains, providing structural stability

-small T gradients
--> small vapor pressure gradients
--> slow grain growth within pack

depth hoar forms
"TG" metamorphism

-large temp gradient
--->large vapor pressure gradient
= produces angular / faceted grains

3-8 mm

(some can be greater than 15 mm ... results in poor sintering / low cohesion / low structural strength
"SUGAR SNOW"

1. surface hoar

2. surface crusts

3. Firnspiegel

4. near surface faceted crystals

5. Sastrgi

large feather shaped crystals

growth during clear cold nights
(( heat lost by LW rad at surface lowers surface below air temp))
-- frozen dew formed by vapor deposition on radiation cooled surface

--complicated dendritic shape
1 - 10 mm
low density

*when buried under snowfall = weak layer in pack
... efficient in making shear instability
== fatal avalanches

hard well bonded layers
hardness varies

meltwater on spring snow refreezes during cold night

CAUSES
1. surface melting / refreezing
2. rain on snow event
3. wind deposition
4. wind erosion of snow surface

buried layer can act as sliding layer for avalanche bc stronger than surrounding snow

thin ice sheet formed on snow surface by rapid refreezing of solar heated snow
**usually high elevations during spring

-- air space between thin ice layer and snow beneath "GLACIER FIRE"

(air gap forms due to liquid h2o produced that infiltrates into underlying snow)

1+ side faceted
(similar to depth hoar)

when buried = forms weak layer (avalanches may run on)

FORMATION THRU
1. radiation recrystallization during bright sunny days
2. diurnal recrystallization from diurnal T gradients
3. melt-layer recrystallization

0.5-1.5 mm diameter

formation CONDITIONS
1. sunny days
2. South facing slopes
3. clear days
4. subfreezing air T so there is no surface melt
5. clear cold nights
6. low density new snow at surface (facilitates vapor transport)

BASICALLY
solar rad penetrates snow surface and warms snowpack below
--surface is cooled thru loss LW rad
--->Results in LARGE Tgrad
--->resulting large vapor pressure grad produces faceted crystalls
Conditions for radiatiion recrystallizatoin
--low atm humidity

ridges of snow formed when wind erodes and drifts snow --

generally sequence of long sharp wave-like ridges of hard snow

--perpendicular to wind
----w/ gentle slope to windward and
STEEP slope to leeward

crystalline changes that take place due to effects of
T emp
&
P ressure

Processes
1.sintering
2. structural strength of snowpack
3. permeability of pack
4. reflectivity of surface
5. thermal conductivity of pack
6. snow density

which AFFECT:
1. avalanche stability & release
2. melt-water runoff
3. radinet energy penetration into pack
4. release of solutes from pack

-crystalline solid close to its melting temp

-thermodynamically unstable
(large surface area: volume ratio
-results in high surface free energy
-thermodynamic equilibrium occurs when surface area:volume ratio minimized)

surface area to volume ratio minimized
(ie sphere or rounded grain)

-less supersaturation in pack than atm
-large suface to volume ratio
(thus high surface free energy -- thermodynamically unstable)
(crystals with higher s:v ratio (ie DENDRITES) = most unstable = change most quickly

-- radius of curvature
(effects can be large bc many new crystals - branch form).
,.....AS branches disappear = general decrease in avg particle size
........AFTER branches disappear size increases

-larger particles grow at expense of smaller particles b/c
----sublimation more efficient from smaller bc higher radius of curvature(thus higher vapor pressure)
--- water vapor moves efficiently away from smaller particles to larger particles (with lower pressure)
--condensation or redeposition more efficient over larger particles bc of lowervapor pressure
**thus with mixed particle sizes, avg size INCCREASEs over time - after initial decrease in particles iwth high surface:volume ratios

"Equitemperature metamorphism"

-rounded grains that bond together into simple chains
aka destructive metamorphism

-modest vapor gradients
-rapid destruction of original snow crystal shape and bonding between grians

.. all snow eventually ends up as this

-T GRADIENT <10*C/m
-Vapor diffusion direction governed by radius of curvature (loss in convexities, gain in concavities of ind. grains)

-lower S:V ratio of grains
-lower surface free energy
-snow grains fill in pore space -- increase density

-rounding of grains
-bonding or sintering of grains

-strength snopack increases

-rate ET metamorphism inreases with increasing temp

SVP greater over points (lowest at hollows)

--- more vapor pressure can be supported over
Convex > flat > concave

*for given T, SVP greatest over pt
*vapor moves from points to Hollows
*mass moves from pts --> hollows
-sintering / bonding occurs via this process
(two round shapes touch = hollow or concavity is formed)

"angular grains with poor sintering"

(constructive metamorphism... sugar snow)

T Gradient >10*C
snow density < 350kg/m3
--radius of curvature not important
SIMULTANEOUS HEAT AND MASS TRANSFER

grows by movement h2o vapor from high density areas to low density areas
when vapor density gradient is GREATER across pore spaces than the gradient between pts and hollows (that drives ET meta..)

generally occurs from Warm areas --> cold areas

when vapor goes from warm to cool, the large amt of supersaturation results in immediate deposition whenever supersaturation contacts a nucleating surface
--Deposition occurs before water vapor can move from convexities to concavities

-deposition generally occurs as FACETS
---angle of deposition ~120* = stepped then scrolled and finally cup-shaped appearence

CUP GROWS TOWARD THE VAPOR SOURCE (ie toward the warm part of the snowpack

relocation of mass caused by T gradients in snow
on scale of INDIVIDUAL GRAINS

-density rel. constant
-necks between snow grains = same size
-crystal size usually > 3mm
-structural strength decreases
-TG grains grow toward vapor source
-grains usually vertically oriented
(resists compaction by gravity-- further resisting increase in density

-- almost always forms WEAK LAYER In snowpack
-norm forms at bottom

1. DRY - no liquid h2o
T<0

2. WET - liquid h2o present
-T =0

location with 1+ avalanche paths

fixed locality within which avalanches move

STARTING ZONE
- where unstable snow fails and begins to move
-avalanche gains mass w/i starting zone and accelerates thru starting zone
-slope angles often 30-45 degrees

TRACK
"zone of transition"
-slope below starting zone that connects starting zone to runout zone
-av mass generally remains constant and no longer accelerating (velocity constant)
-- highst av speeds norm occur here
-slope 20-30*

RUNOUT ZONE
"zone of deposition
-deceleration rapid
-debris deposited
-av stops
slope < 20*

1. point release

2. slab avalanche

"sloughs"
composed of LOOSE snow

-AV starts at pt

spreads downhill laterally forming inverted V shape

incolves ONLY SURFACE snow

= catastrophic failure of cohesive snow

1. POWDER AV

2. DRY SLABS

3. WET SLABS

most / all material in AV suspended above ground by turbulent eddies

cohesive material has no / little liquid h2o and a "CORE" of higher density snow (that's in contact w/ ground)

1. HARD SLAB
moves as a cohesive unit downslope

2. SOFT SLAB
breaks immediately into small blocks

liquid h2o present

-much higher friction at sliding surface
(often "plowing into soft sliding surface ... causing formation of grooves in sliding surface
& entraining rocks dirt .. other materials

top fracture surface of slab

snow remaining on the slope above crown surface

sides of slab

(often jagged appearence)

sow remaining below slab bottom

-- main sliding surface of slab
(norm smooth appearance bc of friction from sliding slab)

downslope wall of slab

-often wedge shaped (usually obliterated by slab

elastic

viscous

brittle


--- snow can deform viscously and elastically before brittle failure occurs

1. compression ((fracture surfaces pushed together))
2. tension ((( can easily fracture)))
3. shear

SHEAR FAILURE!

snowpack moves at different velocities under the influence of gravity

== generally bottom of snowpack doesn't move
---velocity increases with height in pack

TENSION FAILURE

entire snowpack moves under gravity influence

(decrease surface roughness = glide increases for given slope)

have CORE & DUST CLOUD


-CORE
--high density mateial (snow/air) @ AV bottom

DUST CLOUD
-air with suspended snow particles
--surrounds exterior of core
density <1% of AV

dust cloud - no core

often form by faling ice from steep snowfalls

(tau)

= density * depth * gravity * sin(slope<)

V^2 = zeta * R * (sin(slope<) - MU cos(slope<))

zeta = coefficient turbulent friction
R = hydraulic radius
MU (greek letter) - coefficient of kinetic friction @ bed surface)

V elocity (m/s)

Powder 20-70

flowing 15-60

wet 5-30

"I"

I = density * V^2

(v = AV velocity
density is snow density

-test skiing

-explosives

-hasty pit

-shovel shear test

-tap test

-burp the baby

-Rutschblok Test

= "STABILITY FACTORS"

current AV activity

loading tests

fracture propagation

= "SNOWPACK FACTORS"

past AV activity

Snowpack depth

slope use

ram penetrometer

snow temp

snowpack profile

= "METEOROLOGICAL FACTORS"

surface conditions

depth & h2o equivalent new snow

type new snow

density new snow

snowfall intensity
Maximum Precipitation Intensity (> 0.5 cm/hr)

Settlement of New Snow

Wind Speed

Wind Direction

Air Temperature

Relative Humidity

Solar Radiation (slope and aspect)

winds @ altitude > 1km

-pressure-grad forces in equilibrium with coriolis forces

-goverened by large scale weather systems (independent of surface topography)

friction between E's surface and atm

results in NO SLIP condition (Wspeed =0)

between E's surface and geostrophic winds

-wind speed increases from 0 (@ E surface) to its geostophic value @ the top of the boundary layer

-thickness of layer and distribution of wind speed depend on surface roughness

-ROUGH TERRAIN -
-- boundary layer thicker
--wind speed increases rel. slowly with height

FLAT OPEN TERRAIN
-boundary layer thinner
wind speed increases rel. fast with height

-usually measured at 10m above terrain

(flat , open terrain)

"downward transfer of momentum to the surface

(ie drag of wind on earth's surface

[N m^-2]

velocity gradients in boundary lary imply existence of SHEAR STRESSSES

MAX @ earth's surface

decreases with height above ground

-- becomes 0 in geostrophic wind

--shear stress by wind on snowcover causes movement of loose snow

in geostrophic wind

at earth's surface

minimum wind speed that snow is moved / redistributed

(function of physical conditions of snow surface)

increasing temp and humidity

particles are in small pieces -- packing to a higher density

... and increase threshhold wind speed

sintering or bonding processes among deposited snow grains

** the rate of threshold wind speed decreases with time
** the increase in threshold wind speed is slower at colder temps

Thr.wind speeds lower when new source of snow particles

--new snowfall
--snow on trees
--low snow strength layer

1. creep

2. saltation

3. turbulent diffusion

Rolling

height <1cm

Wind speed = <5 m/s

%load <10%

bouncing

height: 1-100cm

wind speed: 5-10m/s

%load ~80%

suspension

height: >100cm

windspeed: >15m/s

%load <10%

-slope

aspect

elevation

surrounding terrain (view factor)

reflected energy from other surfaces

long wave emission from cliffs

wind fetch

depth

density

temp

chemistry

rate change

1. snowfall amt

2. redistribution

3. AV's, sloughs

4. ablation

5. elevation

6. topographic effects

intensity, duration

depend on elevation / slope < / wind speed

erosion in convergent flow areas

deposition in divergent flow areas

- changes spatial distribution of snow

- 30-60% alpine snow cover transported by AV's i Caucasus Mtns

- increase / decrease melt rate

-often result in deeper snowpacks
(provide meltwater later in season

sublimation & evaporative losses from snow surfaces , interception in trees

ground melting

snow melt

differential snowmelt during mid-witer
(melt on south-facing slopes , but not north)

differential energy balance thru basin

increased orographic precip in increasing z in mid-lat mtns

below treeline = increased SWE w/ elevation

above treeline = wind redist. supercedes elevation effect (little correlation with z and swe

static parameter that affects all of above
topographic affects mediated by processes (blowing snow, AVs...)