The Cryosphere, 9, 1819–1830, 2015
www.the-cryosphere.net/9/1819/2015/
doi:10.5194/tc-9-1819-2015
© Author(s) 2015. CC Attribution 3.0 License.
Thermal energy in dry snow avalanches
W. Steinkogler1,2, B. Sovilla1, and M. Lehning1,2
1WSL Institute for Snow and Avalanche Research SLF, Davos Dorf, Switzerland
2CRYOS, School of Architecture, Civil and Environmental Engineering, EPFL, Lausanne, Switzerland
Correspondence to: W. Steinkogler (steinkogler@slf.ch)
Received: 24 September 2014 – Published in The Cryosphere Discuss.: 20 November 2014
Revised: 19 August 2015 – Accepted: 21 August 2015 – Published: 17 September 2015
Abstract. Avalanches can exhibit many different ﬂow
regimes from powder clouds to slush ﬂows. Flow regimes
are largely controlled by the properties of the snow released
and entrained along the path. Recent investigations showed
the temperature of the moving snow to be one of the most
important factors controlling the mobility of the ﬂow. The
temperature of an avalanche is determined by the temperature
of the released and entrained snow but also increases by
frictional processes with time. For three artiﬁcially released
avalanches, we conducted snow proﬁles along the avalanche
track and in the deposition area, which allowed quantifying
the temperature of the eroded snow layers. This data set allowed
to calculate the thermal balance, from release to deposition,
and to discuss the magnitudes of different sources
of thermal energy of the avalanches. For the investigated dry
avalanches, the thermal energy increase due to friction was
mainly depending on the effective elevation drop of the mass
of the avalanche with a warming of approximately 0.3 ◦C
per 100 vertical metres. Contrarily, the temperature change
due to entrainment varied for the individual avalanches, from
−0.08 to 0.3 ◦C, and depended on the temperature of the
snow along the path and the erosion depth. Infrared radiation
thermography (IRT) was used to assess the surface temperature
before, during and just after the avalanche with high spatial
resolution. This data set allowed to identify the warmest
temperatures to be located in the deposits of the dense core.
Future research directions, especially for the application of
IRT, in the ﬁeld of thermal investigations in avalanche dynamics
are discussed.
1 Introduction
Avalanches can exhibit many different ﬂow regimes (Gauer
et al., 2008) depending on (1) the released and entrained
amount of snow, (2) the properties of the snow and (3) the
topography (slope, curvature) (Naaim et al., 2013). Studies
showed that avalanches can increase their mass due to
entrainment by multiple factors (Sovilla et al., 2007; Bates
et al., 2014) which in turn inﬂuences the run-out distance.
Even though important, the amount of snow entrained is not
the main controlling factor that determines the ﬂow form of
the avalanche (Bartelt et al., 2012). The ﬂow regimes and in
turn mobility are strongly inﬂuenced by the properties of the
entrained snow (Steinkogler et al., 2014). Data on front velocities,
run out, ﬂow regimes and powder clouds revealed
that different avalanches can form with similar release conditions
and on the same avalanche path depending on the
inherent snow cover properties. Advancements in avalanche
dynamics models allow to account for the properties of the
ﬂowing snow with more and more detail (Vera Valero et al.,
2015).
Recently, it has been shown that snow temperature inside
an avalanche can signiﬁcantly change its ﬂow dynamics
(Naaim et al., 2013; Steinkogler et al., 2014), mainly by
changing the granular structure of the ﬂow (Steinkogler et al.,
2015). Laboratory studies on the granulation of snow showed
a distinct dependency on snow temperatures with a fundamental
change in snow structure at a threshold of −1 ◦C.
Therefore, signiﬁcant changes in ﬂow dynamics can be expected
with relatively small changes in temperature around
this threshold.
Measuring temperature inside a ﬂowing avalanche or in
its deposit with traditional methods has proven to be difﬁcult
due to technical constraints or because measurements cannot
Published by Copernicus Publications on behalf of the European Geosciences Union.
1820 W. Steinkogler et al.: Thermal energy in dry snow avalanches
Figure 1. Avalanches at the Flüelapass ﬁeld site released by artiﬁcial triggering of the cornices on the ridge. Avalanche #1a and #1b (a)
were released on 23 January 2013, #2 (b) on 5 February 2013 and #3 (c) on 31 January 2014. Note the signiﬁcant secondary release and
entrainment of deeper layers below the rock face for (b) avalanche #2 and (c) avalanche #3.
be conducted due to safety reasons. In addition to manual
snow proﬁles we therefore investigate the application potential
of infrared radiation thermography (IRT) technologies.
IRT is a non-contact, non-intrusive technique, which enables
us to see surface temperature in a visible image. Meola and
Carlomagno (2004) give an overview on existing work and
describe the most relevant industrial and research applications
of IRT.
The emissivity of a surface is a function of many factors,
including water content, chemical composition, structure
and roughness (Snyder et al., 1998) as well as the viewing
angle between observer and measurement object. Even
though many technical challenges and shortcomings of IRT
are known, possible applications on the ﬁeld of snow science
have recently been discussed (Shea and Jamieson, 2011).
Shea et al. (2012) and Schirmer and Jamieson (2014) applied
IRT to measure spatial snow surface temperatures on snow
pit walls. It was found that fast and large temperature changes
resulting from surface energy balance processes must be expected
(Schirmer and Jamieson, 2014). These energy balance
processes between air and snow are particularly important
during windy conditions, clear skies and large temperature
differences between air and snow. These ﬁndings indicate
that measuring the snow surface temperature of avalanche
deposits or erosion layers along the track must be carried out
as fast as possible. If IRT can therefore be seen as a useful
qualitative or quantitative tool for snow applications still
needs further veriﬁcation.
The aim of this study is to identify the spatial temperature
distribution in an avalanche and to quantify potential sources
of thermal energy in an order of magnitude estimation. This
is achieved by ﬁeld measurements and the application of an
IRT camera. A secondary aim is to evaluate the application
of the IRT technique to get deeper insights into the thermal
state of an avalanche.
2 Methods and data
2.1 The Flüelapass ﬁeld site
Multiple dry avalanches were artiﬁcially released during
winters 2012–13 and 2013–14 at the Flüelapass ﬁeld site
above Davos (Switzerland). Here we will discuss three
avalanches, #1 (23 January 2013), #2 (5 February 2013) and
#3 (31 January 2014), out of this data base (Fig. 1).
The avalanche path is a north-east facing slope covering
600 vertical metres. Deposits of larger avalanches typically
reach a lake located at 2374 m a.s.l. at the bottom of the
slope (Fig. 2). Observations and remote measurements can
safely be conducted from the road at the pass which is approximately
800 m away from the avalanche. The slope angle
ranges from 50◦ in the rock face in the upper part to 20◦
at the beginning of the run-out zone with an average of 30◦
of the open slope around 2600 m a.s.l.
2.2 Snow proﬁles
To assess the properties of the released and entrained snow,
manual snow proﬁles according to Fierz et al. (2009) were
conducted in the release zone (Prelease), i.e. just below the
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W. Steinkogler et al.: Thermal energy in dry snow avalanches 1821
Figure 2. Flüelapass ﬁeld site close to Davos (Switzerland). Outlines
of avalanche #1a and #1b (green), #2 (red) and #3 (blue). The
colour bar shows differences between terrestrial laser scans before
and after the individual avalanches. Prelease and Ptrack indicate locations
of snow proﬁle in the release and along the path, respectively.
Red and blue lines indicate positions of lateral investigations and
deposition snow proﬁle Pdepo.
rock face, along the track (Ptrack), in the deposition zone
(Pdepo) and in the undisturbed snow cover in the run out zone
(Prunout) (Fig. 2). The proﬁle location of the initially released
cornice is refereed to as Pcornice. In combination with release
and erosion depths, the acquired snow proﬁles allowed to
identify which layers were entrained into the avalanche.
All proﬁles were conducted as fast as possible after the
avalanche stopped. Yet, especially for the proﬁles in the release
area and the track, it took around 30 min to reach the
proﬁle locations. The temperature measurements close to the
surface must therefore be interpreted carefully due to a rapid
adaptation to the ambient conditions.
In addition to the acquired video and pictures of the powder
cloud the deposits of the avalanche were investigated for
indications of different ﬂow regimes according to the observation
criteria of Issler et al. (2008).
2.3 Lateral temperature proﬁles
In addition to the regular snow proﬁles, trenches were dug
in the deposition zone and modiﬁed avalanche probes were
used to measure lateral temperature gradients. The modiﬁed
temperature probes (BTS probes) are regular avalanche
probes for which the tip was replaced by a thermistor.
BTS probes are usually used for permafrost applications
(Lewkowicz and Ednie, 2004; Brenning et al., 2005) to measure
the temperature at the interface between soil and snow.
Their application allowed to measure the temperature of
snow layers without exposing them to the ambient air temperature.
As for the thermometers used for regular snow proﬁles
(Sect. 2.2) they measure the snow temperature with an
accuracy of ±0.1 ◦C. As for the regular snow proﬁles the upper
most layers need to be interpreted carefully in this investigation
due to an expected change in temperature over time.
The lateral temperature measurements were conducted from
the left and right side towards the snow proﬁle Pdepo, which
was situated in the centre of the deposition zone (Fig. 2), simultaneously.
The BTS probe measurements were conducted
with a vertical resolution of typically 30 cm and each individual
measurement took around 3–5 min, giving the thermistor
sufﬁcient time to adapt to the snow temperature. In
total these measurements took 1–2 h. Additionally, the snow
depth of the deposits was determined by regularly spaced pits
along the transect after the temperature measurements for
avalanche #3. For avalanche #2 a full trench was dug were
the measurements were performed.
2.4 Infrared radiation thermography (IRT) camera
The snow temperature measurements acquired from proﬁles
where supplemented with an infrared radiation thermography
(IRT) camera which allowed to record snow surface
temperatures before, during and after the avalanche (Figs. 3
and 5). Time-lapse measurements after the avalanche stopped
allowed to follow the temporal evolution of surface temperatures
(Fig. 4) and videos of the moving avalanche provided
a qualitative yet illustrative point of view (provided as Supplement).
The ﬁrst pictures were recorded as fast as possible
(usually less than 1 min) after the powder cloud disappeared
and the video recording was stopped.
We used an InfraTec VarioCAM hr 384 sl and a VarioCAM
HD 980 s that both operate in the long wave infrared spectral
range (LWIR) covering 7.5 to 14 µm. According to the manufacturer
the cameras measure with an absolute accuracy of
±1.5 ◦C and a resolution of 0.05 ◦C. The measurements were
either conducted with a 15 or 30 mm lens. With the used IRT
cameras and lenses the pixel size of the footprint is approximately
1 m with the old camera and 0.5 m with the newer
model. Since cold and dry atmospheric (determined by an
automatic weather station close by) and snow conditions prevailed
during avalanche experiments #1 and #2 an emissivity
value of 1 has been chosen for all post-processing operations.
Even though in our study we use the IRT measurements
mainly in a qualitative way, a basic veriﬁcation was con-
ducted.
The surface temperatures measured with the IRT camera
(lines in Fig. 4) were compared to the corresponding manually
measured temperatures (diamonds in Fig. 4) at the snow
proﬁle locations (Fig. 2). The release sliding surface temperature
measured with the IRT camera (IRTrelease_sliding, blue
continuous line in Fig. 4) has been compared with the corresponding
layer in the manual proﬁle performed close to the
release zone (T _Prelease_sliding, blue full diamond at 0 min).
The avalanche sliding surface temperature measured with the
IRT camera in the upper part of the track (IRTtrack_sliding, orange
continuous line in Fig. 4) has been compared with the
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1822 W. Steinkogler et al.: Thermal energy in dry snow avalanches
Figure 3. Screenshot of IRT camera videos for avalanche #1a and #1b. The ﬁrst picture was taken 12 s after the avalanche released.
Time (min)
Snowtemperature(°C)
0 10 20 30 40 50 60
−9
−8
−7
−6
−5
−4
IRTrelease_sliding
IRTtrack_sliding
IRTrelease_undisturbed
IRTtrack_undisturbed
T_Prelease_sliding
T_Ptrack_sliding
T_Prelease_undisturbed
T_Ptrack_undisturbed
Figure 4. Temporal evolution and comparison of snow temperatures
of avalanche #2 using an IRT camera and manually measured data.
Solid lines represent IRT measurements in the release (blue line)
and track (orange line) sliding surfaces. Dashed lines represent IRT
measurements performed in the undisturbed snow cover close to the
release (blue) and close to the avalanche path (orange). Small dots
on the orange dashed line correspond to the time resolution of all
IRT measurements. Data from IRT are compared with manual measurements
performed at the corresponding layer (ﬁlled diamonds)
and at the snow surface (empty diamonds) at the closest snow pro-
ﬁle.
corresponding layer in the manual proﬁle performed close to
the ﬂowing zone (T _Ptrack_sliding, orange full diamond). Further,
the surface temperature of the undisturbed snow close
to the snow proﬁle location in the release and in the track
(IRTrelease_undisturbed and IRTtrack_undisturbed, blue and orange
dashed lines) have been compared to snow surface temperature
measurements at the proﬁles (T _Prelease_undisturbed and
T _Ptrack_undisturbed, empty diamonds). Measurements are in
fairly good agreement with an absolute difference of about
±1.5 ◦C (except for T _Ptrack_sliding). This accuracy suggests
that the data cannot be used to quantify precise absolute temperature
values, but to get an order of magnitude between
relative differences.
2.5 Terrestrial laser scan (TLS) and mass balance
A terrestrial laser scanner (Riegl LPM-321) was operated
from the Flüelapass road (Fig. 2) to acquire digital surface
models before and after the avalanche releases. These
measurements facilitated the calculation of the release, entrainment
and deposition area. Furthermore, the difference
between the two laser scans allowed to calculate the spatially
averaged release and erosion depth along the path.
The combination of area, release/erosion depth and depthaveraged
snow density from the manually created snow proﬁles
(Sect. 2.2) allowed to calculate the corresponding mass.
Adding release and entrained mass results in the deposited
mass. This procedure is common practice and was applied in
multiple other studies (Sovilla et al., 2007, 2010; Steinkogler
et al., 2014).
A complete set of terrestrial laser scans is available for
avalanche #3 only. For avalanche #2 the scan before the
avalanche is only available for the release zone (Fig. 2). No
information from terrestrial laser scanning was available for
avalanche #1. Avalanche boundaries and ﬁeld measurement
locations were recorded by GPS allowing spatial referencing
with the TLS data. A summer digital elevation model (DEM)
was available from an airborne digital photogrammetry campaign
(Bühler et al., 2015) with 1 m spatial resolution. It was
mainly used to georeference our data and allow consequent
processing in a GIS system. Furthermore, the summer DEM
allowed to determine the topography below the lateral transects
in the avalanche deposits (Sect. 2.3).
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W. Steinkogler et al.: Thermal energy in dry snow avalanches 1823
3 Investigated avalanches
This section summarizes the key characteristics and available
data (Table 1) of the avalanches. All avalanches were
released after a snow storm by triggering the cornices at the
ridge at 2900 m a.s.l. with explosives. Therefore, most of the
released snow was new snow. Yet, two of the avalanches,
avalanche #2 and #3 (Fig. 1b and c), entrained signiﬁcant
amounts of snow from deeper layers due to a secondary release
in a deeper weak layer, below the rock face. Since the
main mass contribution can be assumed to be deﬁned by the
secondary releases and the entrainment along the path, we focused
our investigations on these snow masses. Mass contributions
by the cornices are usually relatively small compared
to entrained snow on the open slope below. Furthermore, entrainment
of snow in the gullies of the rock face is not assumed
to contribute a signiﬁcant amount since regularly occurring
(small) avalanches and slides continuously erode the
snow cover. In this study we use the word release to refer to
proﬁle locations at the secondary release below the rock face
(Fig. 2).
3.1 Avalanche #1a and #1b (23 January 2013)
In the days previous to the avalanche experiment 10 cm
of new snow were recorded and snow drift accumulations
formed due to strong southerly winds. The national
avalanche bulletin reported a moderate avalanche danger
(level 2) and identiﬁed the fresh snow drift accumulations
as the main danger. During the experiment clear sky conditions
prevailed and the automatic weather station (AWS)
at the Flüelapass (FLU2) measured an air temperature of
−10 ◦C. Multiple charges were exploded on the ridge to the
left (south) side, facing uphill, of the summit, resulting in
two independent small powder avalanches which followed
the gullies (Figs. 1a and 3). Due to the relatively small release
mass and no signiﬁcant entrainment, both avalanches, #1a
and #1b, stopped halfway down the open slope. Even though
the avalanches were small and a full data set of ﬁeld measurements
is not available, they are retained in this study since
they provide good quality IRT data (Fig. 3). We excluded the
snow proﬁle measurements from the analysis since the erosion
and deposition depths were very small, around 0.1 m,
and the manual measurements were conducted more than 1 h
after the release. The deposition zone was not accessible before
due to safety reasons. The TLS could not be completed
due to technical problems.
3.2 Avalanche #2 (5 February 2013)
20 cm of fresh snow that covered older snow drift accumulations
resulted in a considerable (level 3) avalanche danger.
Furthermore, the bulletin noted that avalanches in isolated
cases could be released deeper within the snowpack. The
Table 1. Summary of measurements for the investigated avalanches.
Avalanche #1a #2 #3
Date 23 Jan 2013 5 Feb 2013 31 Jan 2014
IRT camera model h 384 sl h 384 sl HD 980 s
Terrestrial laser scan no partly yes
Snow proﬁles lateral –∗ yes yes
Snow proﬁles track –∗ yes yes
IRT video yes yes no
IRT pictures yes yes yes
Released mass mr(t) – 502 818
Entrained mass me(t) – 1857 1302
Deposited mass md(t) – 2359 2120
Growth index Ig – 3.7 1.6
∗ indicate that erosion and deposition depths were too small. Growth index Ig is deﬁned as
Ig = me/mr.
AWS at Flüelapass measured −12 ◦C and a partly cloudy sky
prevailed during the experiment.
Explosions along the ridge and to the observer’s left
(South) side of the summit only produced small avalanches
that stopped shortly below the rock face. A single explosion
that triggered the cornice to the right side of the summit
caused another small powder avalanche that followed
the gully and triggered a secondary release at the start of
the open slope (Fig. 1b). Even though the avalanche almost
stopped after entering the open slope, the additional mass
which was entrained resulted in an re-acceleration resulting
in a long running medium-sized avalanche (deposition
mass 2357 t) which only stopped in the ﬂat part close to
the lake. Average snow density of the release was 170 and
210 kg m−3 for the entrained snow. No full TLS was available
before the avalanche release. Nevertheless, in-ﬁeld observations
showed the surface before the release and the entrainment
depth to be rather homogenous along the slope
which allowed to extrapolate the upper entrainment area,
in combination with the envelope of the avalanche acquired
with GPS, and thus to calculate the entire entrained mass.
3.3 Avalanche #3 (31 January 2014)
Multiple consecutive smaller snowfalls and strong southerly
winds created snow accumulations close to ridges. The national
avalanche bulletin issued a considerable (level 3) danger
level and that the weak, old snowpack could cause
avalanches to be released in near-ground layers. Moderate
winds with gusts up to 60 km h−1 from the South and
cloudy to overcast conditions prevailed during the experiment.
The automatic weather station FLU2 recorded −6 ◦C
with steadily increasing temperatures during the experiment.
Two small spontaneous avalanches already released before
the experiment. Initial bombing of the main gully and
to the observer’s left (south) of the summit did not produce
any signiﬁcant avalanches. Yet, the bombing of the cornice
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1824 W. Steinkogler et al.: Thermal energy in dry snow avalanches
Figure 5. Screenshot of IRT camera videos for avalanche #2. The ﬁrst picture was taken 3 s after the avalanche released. Note that the
temperature scale was changed by 0.5 ◦C for the last shown image (82 s).
to the observer’s right (north) of the summit resulted in a
small powder avalanche which triggered a second slide at the
lower end of the rock face (similar to avalanche #2). Consequently
a signiﬁcant amount of snow was eroded and resulted
in a medium-sized avalanche (deposition mass 2120
t) that stopped in the ﬂat run out zone (Fig. 1c). The secondary
release nearly entrained all layers to the bottom of the
snowpack (1.6 m). Average snow density of the release was
270 and 310 kg m−3 for the entrained snow. For avalanche #3
snow temperature measurements were also available for the
cornice at the ridge.
4 Results
Based on these measurements we present observed temperature
distributions during the avalanche motion as well as at
the surface and inside the deposition zone (Sect. 4.1). In a
second step potential sources of thermal energy are identiﬁed
and quantiﬁed (Sect. 4.2).
4.1 Temperature distribution
4.1.1 Avalanches in motion
The use of the IRT camera gave very interesting qualitative
insights into the temperature behaviour of a moving
avalanche (Figs. 3 and 5). Especially plume formation, entrainment
of warmer snow and the stopping of the avalanche
as the powder cloud starved and drifted aside could be very
well observed. (See Supplement for the videos).
Even though avalanche #1a was small, a signiﬁcant powder
cloud developed shortly after the release (Fig. 3). After
the avalanche entered the open slope (37 s), plume formation
stopped, accompanied by a visible decrease in velocity,
and the powder cloud drifted to the uphill-looking right side
(47 s) due to the prevailing wind, revealing the until then obscured
dense core (57 s). After that a rapid cooling of the surface
of the dense core could be observed (from pink colours
at 57 s to orange at 77 s).
The IRT video of avalanche #2 (Fig. 5) is of special interest
since a distinct acceleration of the avalanche can be observed
as it approaches the open slope below the rock face
(33 s). This can be explained by the entrainment of mass
of the secondary release (42 s). The powder cloud shows
higher temperatures than during the ﬁrst phase (50 s) and the
eroded surface becomes visible after the powder cloud drifts
aside (56–82 s). Even though continuous IRT images were
acquired during and after avalanche #3 this data could not be
used in this study as clouds were preventing an undisturbed
view on the slope most of the time.
4.1.2 Surface temperature distribution
The IRT camera images acquired shortly after the avalanches
stopped (Fig. 6) allowed to identify exposed deep, and thus
warmer, layers in the release and along the path as well as
in the deposited snow. In Fig. 6b the secondary release, below
the steep rock part, showed a much deeper erosion in
the, looking uphill, left corner. In the lower part of the track
erosion was spatially rather homogenous for all avalanches.
Lateral IRT surface temperature transects in the deposition
area of avalanche #1 and #2 (black lines in Fig. 6) revealed
that the warmest part of the avalanche is located in the centre
and therefore in its dense core (Fig. 7a and c). In both
cases a distinct difference in surface temperature between
undisturbed snow cover and warmer core of the avalanche
is evident. Figure 7b shows lateral proﬁles (L1 and L2 in
Fig. 6b) along the path of the avalanche. These coincide with
in-ﬁeld measurements of the undisturbed snow cover, a thinThe
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W. Steinkogler et al.: Thermal energy in dry snow avalanches 1825
Figure 6. IRT camera images for avalanches (a) #1a and #1b and (b) #2. Note the different temperature scales amongst the avalanches. Black
lines indicate positions of lateral snow temperature transects.
Figure 7. Snow surface temperatures acquired with IRT along lateral
transects for avalanche #1 (a) and in the path (b) and the deposition
area (c) of avalanche #2. Extent of undisturbed snow cover,
thin-deposit and dense core are indicated. The lateral distance was
calculated by referencing and scaling with the measured width of
the avalanche.
deposit layer that formed at the observer’s left (south) side of
avalanche #2 and the dense core.
4.1.3 Internal temperature distribution of deposits
The observed maximum temperatures in the dense core area
did not only exist on the surface in lateral extension (Fig. 7)
but also vertically in the deposits. This could be measured
for both avalanches for which lateral investigations were conducted.
Figure 8 shows the lateral temperature measurements
conducted in the deposition zone of avalanche #2 and #3.
Measurement Pdepo, corresponding to 0 m, was located in
the middle of the deposition and marked the position of
the full snow proﬁle in the deposits (Fig. 2). Temperature
measurement locations R and L were leading laterally from
the centre to the, looking uphill, right and left side of the
avalanche deposits. Furthermore, the top of the avalanche deposits
(solid line), the bottom of the deposits (dashed line)
and the terrain (pointed line) are indicated. For better distinction
the area of the undisturbed snow cover is additionally
indicated by softened colours. Even though the transect
shown in Fig. 8a only represents one half of the avalanche deposits
from avalanche #2, the extent of the dense core (area
between solid and dashed line) could clearly be observed in
the measured snow temperature. Similar measurements were
recorded for avalanche #3 where again the highest temperatures
were recorded in the centre of the deposits (Fig. 8b)
with decreasing values towards the side of the deposits.
4.2 Thermal energy sources
To explain the observed increase in snow temperatures in the
deposits of the investigated avalanches and to assign an order
of magnitude estimation of the sources of thermal energy, we
look at two important sources of energy, namely (i) friction
and (ii) warming due to entrainment of snow. Other potential
sources of thermal energy, e.g. entrainment of air or adiabatic
warming, were not further considered since an order of magnitude
estimation revealed that their inﬂuence on the temperature
of the dense core is negligibly small (not shown).
Figure 9 shows snow temperature proﬁles in the centre of
the deposition zone T _Pdepo (violet line) and compares them
to measurements conducted in the release zone T _Prelease
(blue line), along the path in the undisturbed snow T _Ptrack
(orange) and the undisturbed snow cover in the run out
zone T _Prunout (gray). Depth-averaging the deposition proﬁle
(solid violet line in Fig. 9) yielded a snow temperature
of −6.8 ◦C for avalanche #2 and −4.1 ◦C for avalanche #3
(T _P depo in Table 2). For T _Pdepo the temperature values
from the upper and lowermost layers (0.2 m thick) were excluded
from the calculations because of adaption of the temperature
with the surrounding air and undisturbed snow cover
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1826 W. Steinkogler et al.: Thermal energy in dry snow avalanches
Figure 8. Lateral snow temperature proﬁles in the avalanche deposits of (a) avalanche #2 and (b) avalanche #3. Pdepo and 0 m indicate the
centre of deposits and the index “L” and “R” represent left and right, looking uphill, measurement locations towards the lateral sides of the
avalanches. Lines indicate the top of the avalanche deposit or snow cover (solid), bottom of avalanche deposit (dashed) and bottom of snow
cover (pointed). Colours of undisturbed snow cover were softened for better distinction with avalanche deposits.
a) b)
Figure 9. Snow temperature measurements conducted in the undisturbed
snow cover close to the release zone (T _Prelease), along
the path (T _Ptrack) and in the run out zone (T _Prunout) as well
as in the deposition zone (T _Pdepo) of (a) avalanche #2 and
(b) avalanche #3. Release, entrainment and deposition depths are
indicated by solid lines, whereas the undisturbed snow cover is represented
by pointed lines. Gray areas and dashed lines indicate parts
of the temperature proﬁles in the deposits (T _Pdepo) that were neglected
in the calculations because of expected changes in temperature
over time due to the boundary conditions of the surface and
the undisturbed snow cover. Composition of deposits (granules and
ﬁne grains) are indicated for avalanche #3.
(gray areas in Fig. 9). Since T _P depo only represents the
relatively warm core of the deposits we additionally calculated
the mean of the lateral temperature proﬁles (Fig. 8) resulting
in a mean temperature of the deposits T depo_lateral of
−7.4 and −4.8 ◦C for avalanche #2 and #3, respectively.
Comparing the mean temperature of the deposits
T depo_lateral with T _P release reveals that a signiﬁcant warming
took place. This resulted in a difference in snow temperature
between released and deposited snow of T 1.2 and
1.5 ◦C for avalanche #2 and #3, respectively (Table 2).
4.2.1 Friction
The increase in temperature due to friction was calculated
by assuming that all potential energy is transformed to heat.
This means that the increase in temperature is only given by
the drop height of the average avalanche mass:
mcp T = mg h, (1)
where m is the mass undergoing the change in potential
energy, cp is the speciﬁc heat capacity of snow
(2116 J kg−1 K−1), T is the change in snow temperature,
g is the gravitational acceleration (9.81 m s−2) and h is the
difference in elevation. Thus, the temperature increase due to
friction Tfriction is given by
Tfriction =
g h
cp
. (2)
This equation has general validity for any (incremental or ﬁnite)
mass, in which potential energy is converted to heat.
This is regardless of whether this mass is added to the
avalanche by entrainment or whether it belongs to the initial
release mass as long as h is the effective height drop
of this mass. Calculating for an elevation drop of 300 m, corresponding
to the slope below the rock face until the run out
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W. Steinkogler et al.: Thermal energy in dry snow avalanches 1827
Table 2. Depth averaged temperatures of release (T _Prelease), track
(T _P track) and deposition proﬁle (T _P depo) with the corresponding
release and erosion depths (in brackets). T depo_lateral represents
the mean of the lateral temperature measurements in the deposition.
T is the difference between T depo_lateral and T _P release.
Tfriction and Tentrainment are the individual contributions to T .
Avalanche #2 #3
T _P release −8.6 ◦C (1.03 m) −6.3 ◦C (1.75 m)
T _P track −8.7 ◦C (0.37 m) −5.8 ◦C (0.3 m)
T _P depo −6.8 ◦C (0.9 m) −4.1 ◦C (1.95 m)
T depo_lateral −7.4 ◦C −4.8 ◦C
T 1.2 ◦C 1.5 ◦C
Tfriction 0.84 ◦C 0.96 ◦C
Tentrainment −0.08 ◦C 0.3◦ C
zone, we obtain an increase in temperature due to friction of
approximately 1.4 ◦C.
The calculated Tfriction can be seen as an upper limit in
our order of magnitude estimation since in nature not all mass
is released and entrained at the maximum elevation. Furthermore,
lateral temperature gradients in the deposition area are
not taken into account. In practice, avalanches entrain large
portions of mass along the avalanche path. In many cases the
entrained mass, me, is signiﬁcantly larger than the released
mass, mr. This is also the case for the investigated avalanches
which are characterized by a growth index of 3.7 and 1.6 for
avalanche #2 and #3, respectively. If one further assumes that
entrainment is happening uniformly along the path and that
the vertical extension of the release area is small compared
to the total path vertical drop, the entrained mass only experiences
on average half the height drop of the released mass:
he = 0.5 hr. The effective h for Eq. (2) can then be cal-
culated:
h =
hr (mr + 0.5me)
mr + me
. (3)
For avalanche #2 and #3 this corresponds to a warming due
to friction, Tfriction, of 0.84 and 0.96 ◦C, respectively.
4.2.2 Entrainment at a different temperature
The above development assumes that there is no difference
between initial snow temperatures of the released and entrained
snow. The snow proﬁles (see locations in Fig. 2) enabled
a quantiﬁcation of the properties of the released and
entrained snow showing some difference as discussed above.
Assuming that the snow that is entrained along the track is
completely mixed with the released snow in the deposition
zone results in the mean temperature difference between entrained
(T _P track) and released snow (T _P release). This difference
leads to the following difference in temperature at the
deposited snow mass:
Tentrainment =
T _Ptrack − T _Prelease me
mr + me
. (4)
This results in a change in temperature due to entrainment
Tentrainment of −0.08 0.3 ◦C for avalanche #2 and #3, respectively.
Since we assume that the heating due to friction
is independent of the initial snow temperature either from release
or entrainment, at least as long as the snow remains dry
during warming, the temperature change due to a different
snow temperature of the entrained snow can simply be added
to the one from friction in Eq. (2).
5 Discussion
It has been noted in other studies (Vera et al., 2012) that
potential sources of thermal energy in snow avalanches are
friction processes or entrainment of snow with differing temperatures.
The investigated avalanches in this study indicate
that the thermal energy increase was mainly deﬁned by frictional
heating, which in turn depends only on the effective
elevation drop (Sect. 4.2.1). Yet, it is well known that
avalanches can signiﬁcantly increase their mass along the
path via entrainment (Sovilla et al., 2006). Also for the investigated
avalanches the growth index were Ig of 3.7 and 1.6 for
avalanche #2 and #3, respectively. Therefore, the calculated
(maximum) value of approximately 0.46 ◦C per 100 altitudinal
metres (Eq. 2) has to be adapted to consider the actual
mass that enters the avalanche at a certain point along the
track (Sect. 4.2). For dry and cold snow avalanches far away
from the melting point, the warming due to friction alone is
not expected to have a substantial inﬂuence on ﬂow dynamics.
Yet, if the overall avalanche temperature is already close
to the critical temperature threshold of −1 ◦C (Steinkogler
et al., 2015) the warming by frictional processes can cause
drastic changes of the granular structure inside the avalanche
and consequently affect ﬂow behaviour.
Contrarily, the warming due to entrainment varied for the
individual avalanches. These variations depend on the temperature
of the snow and the erosion depth as shown in the
proﬁles along the avalanche track (Fig. 9) and the IRT pictures
(Fig. 6). Typically, the alpine snow cover shows a positive
temperature gradient towards the ground (Armstrong and
Brun, 2008). Except for areas with permanent permafrost,
the temperature at the soil–snow interface can be assumed to
be approximately 0 ◦C if there has been a signiﬁcant snow
cover for several weeks. Consequently, the erosion of deeper
snow layers leads to warmer snow temperatures (Fig. 9).
Also changes of snow temperature due to elevation gradients
have been proven to be quite variable and directly inﬂuence
ﬂow dynamics (Steinkogler et al., 2014). As in the case
of avalanche #2 even a cooling of an avalanche is possible
when the avalanche released into deep layers but entrained
only superﬁcial (cold) layers of snow, e.g. blue and orange
lines in Fig. 9. Overall, the contribution of the temperature
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1828 W. Steinkogler et al.: Thermal energy in dry snow avalanches
of the entrained snow to the temperature change was smaller
than by friction for the investigated avalanches (Table 2).
Our temperature measurements on the surface (Fig. 7) and
in depth (Fig. 8) of the deposit indicate that the highest temperatures
are located in the dense core of the avalanche. The
interface between the bottom of the avalanche deposits and
the subjacent undisturbed snow cover featured a very clear
and sharp transition (violet lines in Fig. 9). The shape of
the temperature curve indicates the warmest temperatures in
the lower parts of the deposits proﬁle (−0.4 to −0.8 m and
−1.2 to −1.9 m for avalanches #2 and #3, respectively) and
close to the sliding surface. This would support the expectation
of the most pronounced friction at the bottom of the ﬂow,
typical for this kind of avalanche. Unfortunately, a cooling of
the lowest deposition layers to the temperature of the subjacent
undisturbed snow cover has to be expected and thus prevents
a deﬁnite conclusion on this observation. Also, whether
the small temperature variations in the upper part of the deposition
proﬁle between 0 and −1 m of avalanche #3 (violet
line in Fig. 9) are a result of a mixture of broken parts of the
eroded snow cover, with varying temperatures, and formed
granules could not be fully answered. Yet, granules embedded
in ﬁne grained snow were still clearly observable in this
area of the deposition.
It is without question that reaching the deposits after an
avalanche release to measure the snow surface temperature
with traditional methods, e.g. thermometers, takes too long
and the surface as well as the upper most layers would have
changed their temperature already. It could be observed in
the video of avalanche #1 (see Supplement) that right after
the dense core stopped it started to cool. In all those cases
for which a real-time measurement is necessary, IRT technology
provides a valuable addition to traditional measurements.
Even though in our study we only applied the IRT
camera in a qualitative way, the presented basic veriﬁcation
(Fig. 4) with manually measured snow surface temperatures
showed a fairly good agreement with an accuracy of about
±1.5 ◦C. Although further investigations are necessary to deﬁne
whether absolute values of surface temperature can be
acquired without signiﬁcant uncertainties, the relative accuracy
of the IRT cameras are usually high, around 0.05 ◦C
in our case as speciﬁed by the manufacturer. This facilitates
tracking relative changes in temperatures even if the absolute
value might not be accurate.
Recently IRT was mainly tested and evaluated for
snow proﬁle applications at short distances (Schirmer and
Jamieson, 2014). Dozier and Warren (1982) investigated the
effect of viewing angle on the infrared brightness temperature
of snow and found differences of up to 3 ◦C. Similar
values have been found by Hori et al. (2013), yet they concluded
that for viewing angles less than 40◦ from the nadir,
the error in temperature is less than −0.8 ◦C. The effect of
moisture has been studied extensively (Wu et al., 2009, and
references therein), basically concluding that the presence
of water causes a strong absorbance and consequently a decrease
in reﬂectance in the near-infrared spectra of soils. In
general, low signal attenuation can be expected for (peak)
winter month atmospheres, especially for clear sky conditions,
due to relatively low humidity levels. An effect that
still illustrates challenges for the interpretation of IRT images
is due to the roughness of the investigated surface (Wu
et al., 2009). In most studies, it is assumed that the scene
elements are isothermal, smooth and homogenous (Danilina
et al., 2006). Consequently supposing that the object of interest
is Lambertian, i.e. behaves as a perfect diffuser and emits
and reﬂects radiation isotropically. Mushkin et al. (2007) observed
that the effective emissivity spectra of rough surfaces
are different from those of perfectly smooth surfaces of the
same composition due to multiple scattering among roughness
elements. Yet, they only found an up to 3 % reduction in
the spectral contrast due to sub-pixel surface roughness variations.
This might also be the case for situations similar to
the presented application as size of the granules, i.e. the sub
pixel structures, are much smaller than the pixel size (0.5 to
1 m).
Also whether the surface temperature, and possibly even
the composition, of the aerosol mixture of the powder cloud
can be measured is an open question. Visualization of air
ﬂows on the qualitative level is common practice for various
applications (Narayanan et al., 2003; Carlomagno and
Cardone, 2010) and, as presented in this study, provides impressive
footage of powder snow avalanches. Usually a tracer
is injected into the ﬂow ﬁeld. In our case the tracer is already
present by snow particles of the entrained snow which are
transported into the powder cloud.
A possible further application of IRT could be the differentiation
of ﬂow regimes in the deposition area. As shown
earlier the warmest part of an avalanche is located in the
dense core, e.g. center (red and pink) of avalanche #2 in
Fig. 6b, whereas layers with less mass or where less friction
occurred are cooler (yellow and orange areas in Fig. 6b).
Especially the observer’s left (south) side thin-deposit area in
Fig. 6b might be associated with the deposits of a ﬂuidized
layer (Issler et al., 2008). The IRT observations of this thindeposit
area are in agreement with the ﬁeld observation criteria
for ﬂuidized layers as described by Issler et al. (2008):
(1) rapid decrease of deposit thickness, (2) snowballs of various
sizes embedded in a matrix of compacted ﬁne-grained
snow, (3) large snowballs lying on top of the deposit and
(4) fewer snowballs per unit area than on the dense deposit.
The powder clouds of the investigated avalanches (see
Figs. 3 and 5 and corresponding videos) had consistently
lower temperatures than the warm dense core despite the fact
that the powder cloud (at least from one avalanche) travelled
as far downhill as the dense core. Two distinct processes may
contribute to this fact: (i) a preferential ejection of colder and
lighter surface with colder snow while the dense core may
have a higher fraction of snow from lower layers in the proﬁle
and therefore with a higher temperature. (ii) The particle
concentration in the suspension layer is low and therefore
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W. Steinkogler et al.: Thermal energy in dry snow avalanches 1829
molecular dissipation of kinetic energy and exchange of sensible
and latent heat happens largely between air and snow
and not between snow and snow particles as in the dense
core. This leads to a rapid adoption of temperatures close
to the air temperature for the suspended snow.
Furthermore, the IRT results can be qualitatively interpreted
in a similar way as a laser scan to identify areas where
deeper or shallower erosion occurred, e.g. see entrainment
by secondary release below the rock face for avalanche #2
(Fig. 6b). For this avalanche, we (for the sake of example)
calculated the release mass solely by using information from
the IRT pictures and manually measured snow proﬁles in
the release. Therefore, the IRT picture was georeferenced
in a GIS software and shallower and deeper release layers
were identiﬁed. The (IRT) surface temperature of these layers
were combined with the snow height of the corresponding
temperature in the conducted snow proﬁle in the release.
This resulted in a calculated release mass of 457 t, which is
similar to the mass measured with the terrestrial laser scan
(502 t). This depicts a rough yet quick and efﬁcient method
to estimate the release mass of an avalanche. As shown in this
study, the release and entrainment depth does not only deﬁne
the overall mass of snow but equally important its temperature.
IRT pictures and videos provide an intuitive and easy
way to identify these relevant erosion processes (Fig. 6).
6 Conclusions
We conducted full-scale avalanche experiments at the Flüelapass
ﬁeld site above Davos (Switzerland) to investigate the
distribution of snow temperatures in avalanche deposits and
identify the sources of thermal energy in dry avalanches. A
further goal was to test the usability of infrared radiation thermography
(IRT) in this context.
For the investigated similar avalanches the temperature increase
due to friction has been shown to dependent on the
effective elevation the mass inside the avalanche dropped.
The contribution to the total temperature increase by erosion
processes was shown to be quite variable, depending on the
release depth and snow temperatures of the entrained snow.
The warmest temperatures were observed in the centre of the
avalanche deposits and thus represented the dense core of the
ﬂowing avalanche.
The IRT camera allowed to observe the avalanche phenomenon
“with different eyes” and provides a lot of potential
for more detailed research in the ﬁeld of avalanche dynamics,
both quantitative and qualitative. It is still necessary to
further verify the measurements and deﬁne to which extent
absolute snow surface temperatures can be measured. Then,
the spatial distribution of surface temperatures can help in
the interpolation of proﬁle temperatures measured by hand.
Our results allow for a more comprehensive understanding
of snow temperatures in avalanche ﬂow and their consequences
on ﬂow regimes. This information can directly be
used to verify and enhance the performance of avalanche dynamics
models and is thus of great interest for practitioners.
The Supplement related to this article is available online
at doi:10.5194/tc-9-1819-2015-supplement.
Acknowledgements. Funding for this research has been provided
through the Interreg project STRADA and STRADA 2 by the
following partners: Amt für Wald Graubünden, Etat du Valais,
ARPA Lombardia, ARPA Piemonte, Valle d’Aosta, Regione Lombardia.
The authors would like to thank all the people who helped
gathering the data during the ﬁeld experiments and Perry Bartelt
for his valuable input. Thanks to Vali Meier from the SOS Service
Jakobshorn (Davos) and his team for the great support. We thank
the anonymous reviewers and the editor who helped to signiﬁcantly
improve the quality of the paper.
Edited by: F. Dominé
References
Armstrong, R. L. and Brun, E.: Snow and Climate: Physical Processes,
Surface Energy Exchange and Modeling, Cambridge
University Press, Cambridge, UK, 222 pp., 2008.
Bartelt, P., Bühler, Y., Buser, O., Christen, M., and Meier, L.: Modeling
mass-dependent ﬂow regime transitions to predict the stopping
and depositional behavior of snow avalanches, J. Geophys.
Res., 117, F01015, doi:10.1029/2010JF001957, 2012.
Bates, B., Ancey, C., and Busson, J.: Visualization of the internal
ﬂow properties and the material exchange interface in an entraining
viscous Newtonian gravity current, Environ. Fluid Mech., 14,
501–518, 2014.
Brenning, A., Gruber, S., and Hoelzle, M.: Sampling and statistical
analyses of BTS measurements, Permafrost Periglac. Process.,
16, 383–393, 2005.
Bühler, Y., Marty, M., Egli, L., Veitinger, J., Jonas, T., Thee, P.,
and Ginzler, C.: Snow depth mapping in high-alpine catchments
using digital photogrammetry, The Cryosphere, 9, 229–
243, doi:10.5194/tc-9-229-2015, 2015.
Carlomagno, G. and Cardone, G.: Infrared thermography for convective
heat transfer measurements, Exp. Fluids, 49, 1187–1218,
doi:10.1007/s00348-010-0912-2, 2010.
Danilina, I., Mushkin, A., Gillespie, A., O’Neal, M., Pietro, L.,
and Balick, L.: Roughness effects on sub-pixel radiant temperatures
in knetically isothermal surfaces, in: RAQRS II: 2nd International
Symposium on Recent Advances in Quantitative Remote
Sensing, 25–29 September 2006, Torrent (Valencia), Spain,
2006.
Dozier, J. and Warren, S. G.: Effect of viewing angle on the infrared
brightness temperature of snow, Water Resour. Res., 18, 1424–
1434, doi:10.1029/WR018i005p01424, 1982.
Fierz, C., Armstrong, R., Durand, Y., Etchevers, P., Greene, E., McClung,
D., Nishimura, K., Satyawali, P., and Sokratov, S.: The
international classiﬁcation for seasonal snow on the ground, IHPVII
Technical Documents in Hydrology No. 83, IACS Contribution
No. 1, UNESCO-IHP, Paris, 2009.
www.the-cryosphere.net/9/1819/2015/ The Cryosphere, 9, 1819–1830, 2015
1830 W. Steinkogler et al.: Thermal energy in dry snow avalanches
Gauer, P., Issler, D., Lied, K., Kristensen, K., and Sandersen, F.:
On snow avalanche ﬂow regimes: inferences from observations
and measurements, paper presented at International Snow Science
Workshop ISSW 2008, Whistler, Canada, 717–723, 2008.
Hori, M., Aoki, T., Tanikawa, T., Hachikubo, A., Sugiura, K.,
Kuchiki, K., and Niwano, M.: Modeling angular-dependent
spectral emissivity of snow and ice in the thermal infrared
atmospheric window, Appl. Optics, 52, 7243–7255,
doi:10.1364/AO.52.007243, 2013.
Issler, D., Errera, A., Priano, S., Gubler, H., Teufen, B., and
Krummenacher, B.: Inferences on ﬂow mechanisms from snow
avalanche deposits, Ann. Glaciol., 49, 187–192, 2008.
Lewkowicz, A. G. and Ednie, M.: Probability mapping of mountain
permafrost using the BTS method, Wolf Creek, Yukon Territory,
Canada, Permafrost Periglac. Process., 15, 67–80, 2004.
Meola, C. and Carlomagno, G. M.: Recent advances in the use of
infrared thermography, Meas. Sci. Technol., 15, 9, 2004.
Mushkin, A., Danilina, I., Gillespie, A. R., Balick, L. K., and McCabe,
M. F.: Roughness effects on thermal-infrared emissivities
estimated from remotely sensed images, in: Remote Sensing, International
Society for Optics and Photonics, 67492V–67492V,
2007.
Naaim, M., Durand, Y., Eckert, N., and Chambon, G.: Dense
avalanche friction coefﬁcients: inﬂuence of physical properties
of snow, J. Glaciol., 59, 771–782, doi:10.3189/2013JoG12J205,
2013.
Narayanan, V., Page, R., and Seyed-Yagoobi, J.: Visualization of
air ﬂow using infrared thermography, Exp. Fluids, 34, 275–284,
2003.
Schirmer, M. and Jamieson, B.: Limitations of using a thermal imager
for snow pit temperatures, The Cryosphere, 8, 387–394,
doi:10.5194/tc-8-387-2014, 2014.
Shea, C. and Jamieson, B.: Some fundamentals of handheld
snow surface thermography, The Cryosphere, 5, 55–66,
doi:10.5194/tc-5-55-2011, 2011.
Shea, C., Jamieson, B., and Birkeland, K. W.: Use of a thermal
imager for snow pit temperatures, The Cryosphere, 6, 287–299,
doi:10.5194/tc-6-287-2012, 2012.
Snyder, W. C., Wan, Z., Zhang, Y., and Feng, Y.-Z.: Classiﬁcationbased
emissivity for land surface temperature measurement
from space, Int. J. Remote Sens., 19, 2753–2774,
doi:10.1080/014311698214497, 1998.
Sovilla, B., Burlando, P., and Bartelt, P.: Field experiments and numerical
modeling of mass entrainment in snow avalanches, J.
Geophys. Res.-Earth, 111, F03007, doi:10.1029/2005JF000391,
2006.
Sovilla, B., Margreth, S., and Bartelt, P.: On snow entrainment in
avalanche dynamics calculations, Cold Reg. Sci. Technol., 47,
69–79, doi:10.1016/j.coldregions.2006.08.012, 2007.
Sovilla, B., McElwaine, J. N., Schaer, M., and Vallet, J.: Variation
of deposition depth with slope angle in snow avalanches: Measurements
from Vallee de la Sionne, J. Geophys. Res.-Earth, 115,
F02016, doi:10.1029/2009JF001390, 2010.
Steinkogler, W., Sovilla, B., and Lehning, M.: Inﬂuence of snow
cover properties on avalanche dynamics, Cold Reg. Sci. Technol.,
97, 121–131, doi:10.1016/j.coldregions.2013.10.002, 2014.
Steinkogler, W., Gaume, J., Löwe, H., Sovilla, B., and Lehning, M.:
Granulation of snow: from tumbler experiments to discrete element
simulations, J. Geophys. Res.-Earth, in press, 2015.
Vera, C., Feistl, T., Steinkogler, W., Buser, O., and Bartelt, P.: Thermal
Temperature in Avalanche Flow, paper presented at International
Snow Science Workshop ISSW 2012, Anchorage, Alaska,
32–37, 2012.
Vera Valero, C., Wikstroem Jones, K., Bühler, Y., and Bartelt, P.:
Release temperature, snow-cover entrainment and the thermal
ﬂow regime of snow avalanches, J. Glaciol., 61, 173–184, 2015.
Wu, C.-Y., Jacobson, A. R., Laba, M., and Baveye, P. C.: Accounting
for surface roughness effects in the near-infrared
reﬂectance sensing of soils, Geoderma, 152, 171–180,
doi:10.1016/j.geoderma.2009.06.002, 2009.
The Cryosphere, 9, 1819–1830, 2015 www.the-cryosphere.net/9/1819/2015/