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Glaciology Spring 2012
Throstur Thorsteinsson ([email protected]) 1
Transformation of Snow to Ice
Introduction to
Glaciology Þröstur Þorsteinsson
Snow and ice
http://nsidc.org/cryosphere/glance/
Types of snow
Include:
Snow crystals -- Individual, single ice crystals,
often with six-fold symmetrical shapes. These grow directly from condensing water vapor in the air,
usually around a nucleus of dust or some other foreign
material. Typical sizes range from microscopic to at most a
few millimeters in diameter.
Snowflakes -- Collections of snow crystals,
loosely bound together into a puff-ball. These can grow to large sizes, up to about 10 cm across in
some cases, when the snow is especially wet and sticky.
Snow crystal types (II)
Graupel - Loose collections of frozen
water droplets, sometimes called "soft
hail."
Rime - Supercooled tiny water droplets
(typically in a fog), that quickly freeze onto
whatever they hit. For example, one often sees small droplets of rime on large
snow crystals.
Hail - Large, solid chunks of ice
Growth habits Snow crystals
Columns and plates
Glaciology Spring 2012
Throstur Thorsteinsson ([email protected]) 2
Snow crystals Different types of snowcrystals
Snow flake
Magnification 7x
Snow flake (II)
Magnification 20x
Snowflake (III) Ice lens
Glaciology Spring 2012
Throstur Thorsteinsson ([email protected]) 3
Rime
Hail
July 23, 2010 a hailstone 8 inches in diameter and 1.93 pounds fell in Vivian, South Dakota
Hailstorm in Georgia 10-25 http://youtu.be/daRMLyi8oO8
Ice crystal Ice Ih crystal lattice
O-atom
H-atom
Ice Ih
Growth habits Quick overview
Transformation of snow to ice
Packing and/or settling
Further breaking of snowflakes
Thermodynamic processes
Minimizing free energy
Sintering
Deformation
Happens under load
Snow falls on surface
Snowflakes
Glaciology Spring 2012
Throstur Thorsteinsson ([email protected]) 4
Transformation of snow to ice
1. Snow falls to the surface
In calm conditions r ~ 100 kg m-3
Windy, r ~ 300 kg m-3
2. After falling to the surface
I. Packing and/or settling
II. Thermodynamic processes
III. Deformation under load
I. Packing
Further breaking of snowflake (wind-blown surface layer, …)
Settling; filling gaps
II. Thermodynamic processes
Minimizing free energy
Reducing surface area reduces free energy
Makes crystals round
Diffusion
1) Molecular diffusion a) Volume diff. b) Surface diff.
2) Vapor diffusion higher pressure at concave (odds) than convex parts
Snowflake
1
2
b a
air
Thermodynamic - Diffusion
The higher the curvature the less stable
Larger crystals grow, smaller disappear
Finally we have “mostly” spheres of nearly equal size, and r ~ 550 kg m-3
The speed of these processes is highly dependent on temperature
Evolution of a stellar snow
The destructive metamorphism of a stellar snow crystal. The
numerals give the age of the snow crystal in days. (After LACHAPELLE, 1969, 1991)
Glaciology Spring 2012
Throstur Thorsteinsson ([email protected]) 5
Evolution - images 40 h 35x
+36h 35x
Fine-grained old snow, 6 weeks old.
(Magnification appr. 35x) (After LACHAPELLE, 1969, 1991)
III. Deformation under load
Sintering
The spheres are “glued” together where they touch
Sintering
bonds
air bubbles
Glacier ice
When air is trapped inside bubbles, we call it glacier ice, r = 830 kg m-3
Under more load and deformation ice slowly reaches ri = 917 kg m-3
The bubbles evolve from triangular shape, at triple junctions, to round “O”, but flow can make them elliptical
Glacier ice
In thin section, between crossed polaroids
Air
Glaciology Spring 2012
Throstur Thorsteinsson ([email protected]) 6
Density
An empirical relation for change in density with depth is
Where i refers to ice density, s to surface, C is a constant, and z is depth
)exp()( Czsii rrrr
Depth of firn-ice transition
Accum T D Age
(kg m-2 a-1) (10 m) (m) (a)
Byrd 140 -28 64 280
Siple 500 -24 70 95
Iceland 30 5 - 10
Greenland 100 100
Density profile Byrd (Greenland)
C = 0.0275 m-1
Density profile: Siple Dome
Depth hoar
heat moisture
Depth hoar
Very cold surface
Warm ice
Empty space
in some cases Temperature
Hoar crystal
• Big • Hollow • Low density • Weak
Glaciology Spring 2012
Throstur Thorsteinsson ([email protected]) 7
Depth hoar
LaChapelle (fig 52)
Surface hoar
Typical Densities
Typical densities (kg m-3)
New snow (calm, dry conditions)
50 - 70
Damp new snow
100 – 200
Settled snow
200 - 300
Depth hoar
100 - 300
Wind packed snow
350 - 400
Firn
400 - 830
Very wet snow and firn
700 – 800
Glacier ice
830 – 917
Grain growth
Equation for grain growth, also empirical,
k is growth rate, ~exp[-Q/(R T)]
t is time,
D grain diameter, 0 refers to initial size,
T temperature in Kelvin,
R the gas constant.
tkDD 2
0
2
Grain growth plot
Byrd station, Greenland k = 120 10-4 mm2 a-1.
References
The Avalanche Handbook David McClung, Peter Schaerer
Amazon.co.uk Our Price: £9.39
Glaciology Spring 2012
Throstur Thorsteinsson ([email protected]) 8
Refs
Field Guide To Snow Crystals
LaChapelle, Edward R.
Price: $18.95, ISBN: 0-946417-13-X
Publisher: University Of Washington Press
Snow Crystals W. A. Bentley, W. J. Humphreys
Amazon.com $13.57
Refs on web
http://www.snowcrystals.com
