transformation of snow to ice

8/11/2019 Transformation of Snow to Ice

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28.5.20

Training on glacier studies, climate change and remote sensing

27 May – 06 June 2014

Indian Institute of Science, Bangalore, India

Divecha Centre for Climate Change

Transformation of snow to ice

Basic concept of mass balance and flow

Helgi Björnsson,

Institute of Earth Sciences, University of Iceland

Cryosphere:

those portions of Earth‘s surface where water is in solid form

Snow

Sea ice

Lake ice and river ice

Frozen ground (permafrost)

Glaciers and ice sheets (ice movin under its own wei ht)

Firnline

Ablation area

Accumulation area

Glaciers form by accumulation of snow andconversion of snow to glacier ice

snow becomes denser by the

weight of the increasing snow pack

Snow

Firn

ice

ice

Mass moves due to its own weight

t1 autumn

t2 spring

t3 autumn

Basic concept of mass balance and flow Annual changes in glacier surface elevation

Equilibrium line

Ablation area Accumulation area

312

ACCUMULATION

• Snow fall• Wind drifted snow

• Avalanches deposition

• Freezing of water (rain,

meltwater

ABLATION

Melt (surface, basal)• Wind erosion

• Calving (e.g. Antarctica,

Greenland)

• Sublimation (tropical glaciers)

Balance velocity

u is the average velocity in a vertical cross section beneath the equilibrium line (S j).

Glacier in balance:

All mass Bnc collected on the surface Sc goes through a verical

cross section beneath the equilibrium line, with mean velocity u

Bnc Sc = u S j

Along a flowline: b x = u h

Firnline

Ablation area

Accumulation area

Glaciers form by accumulation of snow andconversion of snow to glacier ice

snow becomes denser by the

weight of the increasing snow pack

snow

ice

Mass moves due to its own weight



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From snow to firn to ice:

• Once snow crystals have fallen to the ground, theyundergo significant changes (both the individualcrystals as well as the snowpack as a whole):

Metamorphism

•snow is compressed by the weight of the snow on top

fragile crystals break

•Snowflakes change to grains, which become rounded and

granular like coarse sugar.

•Grains recrystallize and larger crystals of ice begin to form

at the expense of smaller ones

density increases





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At 830 kg/m3 the air spaces close off to leave behind

pockets of air.

These bubbles are important in environmental change

studies as they contain the atmospheric composition at the

time of formation.

At great depths it may be that no bubbles are visible in the

ice. However, air is emitted upon melting. This occurs

because the air is imprisoned within cages in the molecular

structure - a so-called clathrate hydrate.

• dry metamorphism: no melt

• wet metamorphism: melt, changes by moving

water, refreezing, internal accumulation

Metamorphism

• warm, wet snow

turns into firn with

spherical grains

that are well

interconnected(strong bonding)

Snow d ensity variation with depth

’warm’

Wet

metamorphismChange from snow to

ice may take a few

years

’Cold’

Dry metamorphism

Change from snow to

ice may take hundreds

of years

Depth and age of f irn-ice transition

(a)

(b)



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DEPTH HOAR

- forms especially in autumn on glaciers

- when a steep temperature gradient is in a snow layer

- evaporation at depth, condensation further up• in extremely cold and dry conditions• Water vapor travels from warm ground to

colder surface, steep temperature gradient

• formation of large cup-shaped depth hoarcrystals near the bottom of the snowpack

• formation of denser crusts near snowsurface

• Depth hoar : weak layer (grains are notconnected to one another), avalanchedanger, may form in autumn of glacier

Warm

( –5 ˚C)

Cold ( –20 ˚C)

Humidity gradient metamophism- depth hoar hoar

Avalanches

•Depth hoar: weak layer (grainsare not connected to oneanother),

•avalanche danger

Zonation of the accumulation zone

o

elting

Snow and ice densitiesThese materials are mixtures of a ir, ice and water

•New snow 50 - 70 kg m3

•Damp, new snow 100 - 200

•Settled snow 200 - 300

•Depth hoar 100 - 300

•Wind packed snow 350 - 400

•Firn 400 - 830

•Very wet snow & firn 700 - 800

•Glacier ice 830 - 910

•Water 1000

How to measure

snow density ?

•Take a container of

known volume and fill it

with snow

•weigh the snow



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Snow density

measurement Density = Mass / volume

Firnline

Glacier ice

Snow/ firn

Ice crystals grow during flow downglacier

Ablation & accumulation area(Blue Glacier, WA)

At end of summer

accumulation

ablation

Ice flow

What keeps a glacier alive ?

ACCUMULATION = Any addition of snow/ice

• Snow fall• Wind drifted snow

• Avalanches deposition

• Freezing of water (rain,

meltwater

ABLATION = Any removal of ice/snow

• Melt (surface, basal)• Wind erosion

• Calving (e.g. Antarctica, Greenland)

• Sublimation (tropical glaciers)

The balance between snow accumulation and ablation

over the entire glacier

End of winter End of summer



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transformation of snow to ice

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