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 Throstur Thorsteinsson ([email protected]) 1 Glaciology Spring 2012 Snow crystals Different types of snowcrystals Snow flake Snow flake (II) Magnification 7x Magnification 20x Snowflake (III) Ice lens Throstur Thorsteinsson ([email protected]) 2 Glaciology Spring 2012 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 Throstur Thorsteinsson ([email protected]) 3 Glaciology Spring 2012 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) Throstur Thorsteinsson ([email protected]) 4 Glaciology Spring 2012 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 Throstur Thorsteinsson ([email protected]) 5 Glaciology Spring 2012 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) Byrd 140 -28 64 280 r ri (ri rs )exp(Cz) 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 Throstur Thorsteinsson ([email protected]) 6 Glaciology Spring 2012 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. Throstur Thorsteinsson ([email protected]) 7 Glaciology Spring 2012 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 Throstur Thorsteinsson ([email protected]) 8 .