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6 % gas content
4
; see Palosuo 1965). Because of
the air inclusions, the thermal conductivity and strength are lower in snow-ice than in
congelation ice, and snow-ice scatters light strongly and appears opaque.
For the
890 kg m
−
3
, corresponding to 3
(860
-
-
flooding condition, the ice surface elevation from the water level, h
w
, is given
by the Archimedes
fl
'
law as
h
i
q
i
q
w
q
s
q
w
h
s
h
w
¼ 1
ð
3
:
6
Þ
where h
i
and h
s
are the thicknesses of ice and snow, respectively. The ice surface sinks
down as more snow accumulates. If the weight of the snow is enough, the ice is forced
beneath the water surface level or h
w
< 0. Cold freshwater ice is not permeable and can
resist the pressure by the snow overload. It is often reported that under overload pressure,
water
flows up like a fountain when a hole has been drilled. This was well noted in
Finnish Lake P
fl
nne, in winter 1999, when the ice surface was 12 cm beneath the water
surface level at the instant of drilling (Lepp
ä
ij
ä
ranta et al. 2003b). In saline lakes, the ice has
better permeability and overload pressure is continuously released.
The
ä
fl
flooding condition, from Eq. (
3.6
), can be expressed as h
s
>
ʳ
h
i
, where
ρ
s
= 250 kg m
−
3
, the snow thickness
must be at least about one-third of the ice thickness for the
ρ
w
− ρ
i
=90kgm
−
3
and
ʳ
=(
ρ
w
− ρ
i
)/
ρ
s
. Thus if
fl
flooding to occur. Then, if
cracks appear, water
flows through them forming slush with the snow on ice and to freeze
further into snow-ice. The theoretical upper limit of snow-ice formation by
fl
fl
flooding is
h
s
2
h
si
¼
3
h
s
ð
3
:
7a
Þ
2
1
q
q
i
1
þ
2
In this case only snow-ice forms, submerged snow is compressed, mixed with water
and frozen, and according to the Archimedes law a fraction of the snow must stay above
the water surface level. In winter 2013 snow accumulation was much more than average in
southern Finland and the timing was favourable for snow-ice formation. As a result, in
many lakes the ice thickness was about 50 cm with 90 % snow-ice, corresponding to close
to 2/3 of the snow thickness on land. When there is a congelation ice layer, snow is
rst
used to overcome its buoyancy and
is able to produce snow-ice.
In spring the day-and-night melting-freezing cycle produces snow-ice. Snow melts in
daytime and the mixture of meltwater and snow freezes in nighttime. The resulting upper
limit of snow-ice growth is
'
extra snow
'
4
In glaciology where snow transforms into ice by compression, the density is taken as 830 kg m
−
3
at the transition (e.g., Paterson 1999), corresponding to the gas content of 9.5 %.
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