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The Formation of Snow Flakes and the Changing Snow Pack

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The Formation of Snow Flakes and the Changing Snow Pack Winter Ecology Power Point Biology-109 John Corbett - Keystone College, La Plume, Penna. What is Snow? – PowerPoint PPT presentation

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Title: 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, transpiration
    from 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 super cooled
    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 atmospheric pressure 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
7
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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
  • Molecular scale-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-temperature
    metamorphism
  • - 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 thermal
    conductivity of snow
  • Temperature difference affects water vapor
    distribution in snow pack
  • Air pockets remain saturated with 100 relative
    humidity based on processes of sublimation and
    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
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