Glaciology Spring 2012
Snow and ice Transformation of Snow to Ice
Introduction to Glaciology Þröstur Þorsteinsson
http://nsidc.org/cryosphere/glance/
Types of snow Snow crystal types (II)
Include: Graupel - Loose collections of frozen
Snow crystals -- Individual, single ice crystals, water droplets, sometimes called "soft often with six-fold symmetrical shapes. hail." These grow directly from condensing water vapor in the air, usually around a nucleus of dust or some other foreign Rime - Supercooled tiny water droplets material. Typical sizes range from microscopic to at most a (typically in a fog), that quickly freeze onto few millimeters in diameter. whatever they hit. Snowflakes -- Collections of snow crystals, For example, one often sees small droplets of rime on large loosely bound together into a puff-ball. snow crystals. These can grow to large sizes, up to about 10 cm across in Hail - Large, solid chunks of ice some cases, when the snow is especially wet and sticky.
Snow Growth habits crystals
Columns and plates
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Snow crystals Different types of snowcrystals
Snow flake Snow flake (II)
Magnification 7x Magnification 20x
Snowflake (III) Ice lens
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Rime Hail
Hailstorm in Georgia 10-25 http://youtu.be/daRMLyi8oO8
July 23, 2010 a hailstone 8 inches in diameter and 1.93 pounds fell in Vivian, South Dakota
Ice crystal Ice Ih
Ice Ih crystal lattice
O-atom
H-atom
Growth habits Quick overview
Transformation of snow to ice
Snow falls on surface Snowflakes
Packing and/or settling Thermodynamic processes Deformation Further breaking of snowflakes Minimizing free energy Happens under load
Sintering
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Transformation of snow to ice I. Packing
1. Snow falls to the surface Further breaking of snowflake -3 In calm conditions r ~ 100 kg m (wind-blown surface layer, …) -3 Windy, r ~ 300 kg m Settling; filling gaps 2. After falling to the surface I. Packing and/or settling II. Thermodynamic processes III. Deformation under load
II. Thermodynamic processes Diffusion
Minimizing free energy 1) Molecular Reducing surface area diffusion 2 air reduces free energy a) Volume diff. b) Surface diff. Makes crystals round Snowflake 2) Vapor diffusion 1 higher pressure b a at concave (odds) than convex parts
Thermodynamic - Diffusion Evolution of a stellar snow
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 The destructive metamorphism of a stellar snow crystal. The numerals give the age of the snow crystal in days. (After LACHAPELLE, 1969, 1991)
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Evolution - images III. Deformation under load
40 h 35x Fine-grained old snow, 6 weeks old. Sintering (Magnification appr. 35x) (After LACHAPELLE, 1969, 1991) The spheres are “glued” together where they touch +36h 35x
Sintering Glacier ice
When air is trapped inside bubbles, we call it glacier ice, r = 830 kg m-3
Under more load and deformation ice -3 slowly reaches ri = 917 kg m The bubbles evolve from triangular shape, at triple junctions, to round “O”, bonds but flow can make them elliptical air bubbles
Glacier ice Air
In thin section, between crossed polaroids
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Density Depth of firn-ice transition
An empirical relation for change in Accum T D Age density with depth is (kg m-2 a-1) (10 m) (m) (a)
r r (r r )exp(Cz) Byrd 140 -28 64 280 i i s Siple 500 -24 70 95 Where i refers to ice density, s to surface, C is a constant, and z is depth Iceland 30 5 - 10 Greenland 100 100
Density profile Density Byrd (Greenland) profile:
Siple Dome
C = 0.0275 m-1
Depth hoar Hoar crystal
• Big Very cold surface • Hollow Depth hoar • Low density Warm ice • Weak
heat moisture Empty space Temperature in some cases
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Depth hoar Surface hoar
LaChapelle (fig 52)
Typical Densities Grain growth
Typical densities (kg m-3) Equation for grain growth, also New snow (calm, dry conditions) 50 - 70 empirical, Damp new snow 100 – 200 2 2 D D0 k t Settled snow 200 - 300 Depth hoar 100 - 300 k is growth rate, ~exp[-Q/(R T)] Wind packed snow 350 - 400 t is time, Firn 400 - 830 D grain diameter, 0 refers to initial size, Very wet snow and firn 700 – 800 T temperature in Kelvin, R the gas constant. Glacier ice 830 – 917
Grain growth plot References
The Avalanche Handbook David McClung, Peter Schaerer
Amazon.co.uk Our Price: £9.39 Byrd station, Greenland k = 120 10-4 mm2 a-1.
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Refs Refs on web
Field Guide To Snow Crystals http://www.snowcrystals.com 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
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