The Formation of Snow Flakes and the Changing Snow Pack

1. The Formation of Snow Flakes and the Changing Snow Pack Winter Ecology Power Point Biology-109 John Corbett - Keystone College, La Plume, Penna.

2. What is Snow? • Evaporation from water sources, transpirationfrom plants and respiration from animals puts water vapor into the atmosphere • Collections of water vapor condenses onto dust particles in the air forming clouds • Snow forming clouds are composed of supercooled water droplets, just below the freezing point • Clouds continue to become colder and water droplets begin to freeze

3. Physical Properties of Ice • Snow crystals are a collection of water molecules in ice lattice form • Each Oxygen atom inside the ice lattice is surrounded by 4 other Oxygen atoms in tetrahedral arrangement Blue dots – Hydrogen atoms Blue/Red dots – Oxygen atoms

4. Chemical Properties of Ice • Specific chemical properties of Hydrogen in ice molecule • 2 protons of Hydrogen close to each Oxygen atom • Orientation of water molecule protons point toward 2 adjacent Oxygen atoms • 1 proton between 2 adjacent Oxygen atoms

5. Snow Crystal Formation • Condensation of water vapor never reaches liquid state • Sublimation – transition of water from gas phase to solid phase with no intermediate liquid phase • Dependent on specific atmosphericpressure and temperature

6. Snow Crystal Formation • Water droplets accumulate on ice crystals and freeze, ice crystals change into snow crystals • Formation of symmetrical 6-sided structure, hexagon, occurs uniformly • Hexagonal prism includes 2 hexagonal basal facets and 6 rectangular prism facets • Prism called plate-like or columnar, depending on which surface grows quickest Hexagon

8. Snowflakes • Variety of forms • Each one unique • Only common characteristic, 6-sided

9. Formation of Snow Crystals

10. Faceting • Frozen water molecules stack together • Produce geometric, crystalline lattice • Controls overall shape of snow crystal • Molecularscale-dangling chemical bonds available for growth • Growing facet surfaces produce hexagonal prism • Simplest form of snow crystal

11. Branching • Branches sprout from the 6 corners of hexagon • Refers to presence of small protrusion on flat ice crystal surface • Water molecules attach and grow • Other protrusions appear • Bumps on branches becoming side-branches

12. Formation of Snow Crystal • Branching applies itself over and over again • Result is an ice dendrite • Dendrite-refers to a tree-like shape • Formation depends on balance between faceting and branching • Relationship depends on temperature and humidity

13. Snowfall • Colder areas, snow falls as separate pieces • Warmer areas experiencing snow, pieces stick together, consisting of thousands of pieces • Snow flake design depends on several factors… 1.Velocity from the clouds 2.Temperature 3.Humidity 4.Electrical conditions

14. The Changing Snow Pack • Ground snow is tumbled by the wind, fractured and compacted, vaporized and re-condensed, melted and frozen again • Metamorphism-undergoing change, alters ice crystal structure and internal structure of snow pack • 3 processes of metamorphism influenced by time, internal snow pack characteristics and external weather conditions

15. Destructive Metamorphism • Initial deterioration of snow flake and formation of ice grain, destructive or equi-temperaturemetamorphism - reordering of water molecules - redistribution of molecular energy - net reduction in surface area and structure • Ice grains combine with others, increasing size • Metamorphism increases with warm temperature

16. Destructive Metamorphism • Air spaces in snow pack reduced in size, ice grains pack together • Snow pack density and mechanical strength increases, within hours • Construction of snow caves/shelters

17. Constructive Metamorphism • Changes within vertical structure of snow pack, constructive or temperature gradient metamorphism -results from migration of water vapor upward in snow pack and influenced by 2 snow pack properties… 1. existence of temperature gradient between top and bottom of snow pack 2. interconnecting system of air pockets within snow pack

18. Constructive Metamorphism • Thermal gradient caused by low thermalconductivity of snow • Temperature difference affects water vapor distribution in snow pack • Air pockets remain saturated with 100% relative humiditybased on processes of sublimationand condensation

19. Constructive Metamorphism • 2 results from upward migration of water vapor… 1.Condensation in upper regions of snow pack super saturates the air, resulting in ice grain growth 2.Sublimation in lower regions of snow pack replaces water vapor, maintaining saturated atmosphere

20. Constructive Metamorphism • Depth hoar-loosely arranged ice crystals formed at base of snow pack because of temperature gradient • Formation allows movement of mammals under snow • Constructive metamorphism creates loss of mechanical strength of snow pack, increasing potential for avalanche

21. Melt Metamorphism • Snow pack alteration influenced by warming temperatures • Surface snow warms, becoming same temperature as bottom, with cold layer sandwiched between • Changes occur slowly

22. Melt Metamorphism • Surface melt water percolates through snow pack releasing heat energy • Latent heat of fusion-energy gained or lost during the phase changes in water • Melt water acts as heat pump • Also occurs when rain falls or fog forms on the snow pack • Amount of heat transferred relative to temperature and quantity entering snow pack

23. Thank You References available upon request