CEN/TR 17909:2023

SLOVENSKI STANDARD
01-maj-2023
Hidrometrija - Merjenje globine snega in višine snežnih padavin na kraju samem
Hydrometry - On-site measurement of snow depth and depth of snowfall
Vor-Ort-Messung der Schneehöhe und der Schneefalltiefe
Mesurage sur site de la profondeur de neige et de la profondeur de la chute de neige
Ta slovenski standard je istoveten z: CEN/TR 17909:2023
ICS:
07.060 Geologija. Meteorologija. Geology. Meteorology.
Hidrologija Hydrology
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

CEN/TR 17909
TECHNICAL REPORT
RAPPORT TECHNIQUE
March 2023
TECHNISCHER REPORT
ICS 07.060
English Version
Hydrometry - On-site measurement of snow depth and
depth of snowfall
Mesurage sur site de la profondeur de neige et de la Vor-Ort-Messung der Schneehöhe und der
profondeur de la chute de neige Schneefalltiefe

This Technical Report was approved by CEN on 6 February 2023. It has been drawn up by the Technical Committee CEN/TC 318.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2023 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 17909:2023 E
worldwide for CEN national Members.

Contents Page
European foreword . 4
Introduction . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Symbols . 10
5 Principles . 10
5.1 General . 10
5.2 Snow depth . 11
5.3 Depth of snowfall. 12
6 Measurement principles . 12
7 Siting and exposure . 12
7.1 General . 12
7.2 Terrain . 13
7.3 Wind . 13
7.4 Thermal radiation . 13
7.5 Obstacles . 13
7.6 Disturbance by humans and animals . 13
7.7 Maintenance of measurement site . 13
8 Measurement of snow depth . 14
8.1 General . 14
8.2 Calibration . 14
8.3 Manual measurements . 15
8.3.1 Measurement techniques . 15
8.3.2 Procedure. 15
8.4 Sources of error . 16
8.4.1 General . 16
8.4.2 Reading errors . 16
8.4.3 Misjudgement of the base level . 17
8.5 Automated measurements . 17
8.5.1 General . 17
8.5.2 Ultrasonic measurements . 18
8.5.3 Optical measurements . 20
8.5.4 Data quality control . 23
8.6 Sources of error . 23
8.6.1 General . 23
8.6.2 Optic measurements . 24
8.6.3 Ultrasonic measurements . 24
8.7 Choice of measuring method . 24
9 Measurement of depth of snowfall . 24
9.1 General . 24
9.2 Measurement . 25
9.2.1 Measurement techniques . 25
9.2.2 Procedure . 25
9.3 Sources of error . 26
10 Quality assurance . 26
Annex A (informative) Snow stake . 27
Annex B (informative) Manual snow depth measuring devices . 28
Annex C (informative) Electronic probe . 29
Annex D (informative) Ultrasonic snow depth measuring station . 30
Annex E (informative) Laser snow depth measuring station . 31
Annex F (informative) Snow board . 32
Bibliography . 33

European foreword
This document (CEN/TR 17909:2023) has been prepared by CEN/TC 318 “Hydrometry”, the secretariat
of which is held by BSI.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
Introduction
Snow depth, representative within a given area, is one of the most difficult weather parameters to be
measured in an accurate and consistent manner. Together with snow density it is the most important
factor in the estimation of snow water equivalent and thus of crucial importance for the assessment of
threatening hazards such as flooding, snow avalanches and building collapses. Preventive measures due
to the knowledge of snow amounts can save lives, properties and infrastructure. The data has a wide
variety of users, including national weather and hydrological services, waterpower industry, snow
avalanche forecasters, climate researchers, water resource managers, construction engineers, winter
resort managers, farmers, and many others.
In addition to weather forecasts, measurements of depth of snowfall (also called new snow height) are
essential in the preparedness of winter road plowing and airport snow removal. Resources can be
adapted to the current weather situation and serious traffic break downs can be reduced.
Much of the information in this document is based on the World Meteorological Organization (WMO)
Guide to Meteorological Instruments and Methods of Observation, Volume II – Measurement of
Cryospheric Variables, published in 2018.
1 Scope
This document defines the requirements for on-site measurements of snow depth and depth of
snowfall. This document provides guidance on manual and automatic measuring techniques, and
information about sources of errors and measurement uncertainty.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— IEC Electropedia: available at https://www.electropedia.org/
— ISO Online browsing platform: available at https://www.iso.org/obp
3.1
ablation
combined processes (such as sublimation, fusion or melting, evaporation and movement due to wind
and avalanches) that remove snow or ice from the surface of a glacier or from a snow field
[SOURCE: EN ISO 772:2022, 10.1]
3.2
blowing snow
snow being transported by wind high (approximately 2 m) above a snowpack (3.27) surface, where
visibility is noticeably reduced
Note 1 to entry: See also drifting snow (3.3).
[SOURCE: EN ISO 772:2022, 10.2]
3.3
drifting snow
snow being lifted from the snow surface and transported by wind just above the snow surface (3.23),
where visibility is not noticeably reduced
Note 1 to entry: See also blowing snow (3.2).
[SOURCE: EN ISO 772:2022, 10.6]
3.4
new snow
snow layer that is not transformed, densified or settled, from current or recent precipitation having
characteristic grain size range of 1 mm to 3 mm
Note 1 to entry: New snow height can be measured by use of a snow board (3.8).
[SOURCE: EN ISO 772:2022, 10.11]
3.5
old snow
snow layers deposited from earlier precipitation, prior to fresh fallen snow, composed of
metamorphosed snow crystals
[SOURCE: EN ISO 772:2022, 10.12]
3.6
snow accumulation
all processes that add mass to the snowpack (3.27)
EXAMPLE Solid and liquid precipitation, ice deposition from atmospheric water vapor, snow deposited by
wind, avalanches, etc.
Note 1 to entry: Snow accumulation is the opposite of ablation (3.1).
[SOURCE: EN ISO 772:2022, 10.19]
3.7
snow avalanche
rapidly moving snow masses in volumes exceeding 100 m and with a minimum length of 50 m
Note 1 to entry: Large snow avalanches may contain rocks, soil, vegetation, and/or ice.
[SOURCE: EN ISO 772:2022, 10.20]
3.8
snow board
specially constructed board used to measure new snow (3.4) height manually
[SOURCE: EN ISO 772:2022, 10.21]
3.9
snow course
established line, or transect, of measurements of snow water equivalent (3.25) across a snow-covered
area in a representative terrain, where snow accumulation (3.6) is not homogeneously distributed in the
terrain
[SOURCE: EN ISO 772:2022, 10.23]
3.10
snow cover
accumulation of snow on the ground in its natural consistency
Note 1 to entry: See also snowpack (3.27).
[SOURCE: EN ISO 772:2022, 10.24]
3.11
snow cover extent
areal extent of snow-covered ground in relation to the total catchment
Note 1 to entry: It is usually expressed as per cent of total area in a given region.
[SOURCE: EN ISO 772:2022, 10.25]
3.12
snow creep
internal deformation of the snowpack (3.27) due to gravity- and metamorphism-driven densification
[SOURCE: EN ISO 772:2022, 10.26]
3.13
snow density
mass per unit volume of snow
Note 1 to entry: Sometimes total and dry snow densities are measured separately. Total snow density
encompasses all constituents of snow (ice, liquid water, and air) while dry snow density refers to the ice matrix
and air only.
[SOURCE: EN ISO 772:2022, 10.27]
3.14
snow depth
total height of snowpack (3.27) measured vertically from the base to the snow surface (3.23)
Note 1 to entry: The slope-perpendicular equivalent of snow depth is the snowpack thickness (3.28).
[SOURCE: EN ISO 772:2022, 10.28]
3.15
snow distribution
spatial and temporal variability of snow cover (3.10) affected by snowfall, wind speed, elevation,
topography, vegetation and ablation (3.1)
[SOURCE: EN ISO 772:2022, 10.29]
3.16
snow glide
downhill movement of the snowpack (3.27) relative to the ground
[SOURCE: EN ISO 772:2022, 10.51]
3.17
snow height
vertical distance from a base to a specific level in the snow, or to the snow surface (3.23)
Note 1 to entry: Ground surface is usually taken as the base, but on firn fields and glaciers it refers to the level of
either the firn surface or glacier ice. The snow height is used to denote the location of layer boundaries but also
measurements such as snow temperatures relative to the base. Where only the upper part of the snowpack (3.27)
is of interest, the snow surface may be taken as the reference. This should be indicated by using negative
coordinate values. Snow depth (3.14) is the total height of the snowpack.
[SOURCE: EN ISO 772:2022, 10.32]
3.18
snow layering
stratification of the snowpack (3.27), where each layer is characterized by grain shape, grain size, layer
hardness, temperature, water content and density
[SOURCE: EN ISO 772:2022, 10.33]
3.19
snow probe
instrument to manually measure large snow depths (3.14)
[SOURCE: EN ISO 772:2022, 10.40]
3.20
snow redistribution
distribution of previously deposited snow that was eroded and transported by the wind
Note 1 to entry: Redistribution features such as snowdrifts are usually formed from densely packed and friable
snow.
[SOURCE: EN ISO 772:2022, 10.17]
3.21
snow season
time period when the ground usually is covered by snow
[SOURCE: EN ISO 772:2022, 10.54]
3.22
snow stake
instrument for manual measurements of the snow depth (3.14)
[SOURCE: EN ISO 772:2022, 10.44]
3.23
snow surface
uppermost part of the snow cover (3.10), forming the interface to the atmosphere
[SOURCE: EN ISO 772:2022, 10.55]
3.24
snow survey
process of determining snow parameters, most often depth and density, at representative points,
usually along a snow course (3.9)
[SOURCE: EN ISO 772:2022, 10.45]
3.25
snow water equivalent (SWE)
height of the water layer, which would develop after the melting of the snowpack (3.27), if the melting
water remained without infiltration or evaporation on a given horizontal surface
Note 1 to entry: It can represent the snow cover (3.10) over a given region or a confined snow sample over the
corresponding area. The snow water equivalent is the product of the snow height (3.17) and the snow density
(3.13) divided by the density of water. It is typically expressed in millimetres of water equivalent, which is
equivalent to kilograms per square metre or litres of water per square metre.
[SOURCE: EN ISO 772:2022, 10.47]
3.26
snowmelt
change of the physical state of snowpack (3.27) from solid to liquid phase, mainly affected by various
meteorological factors (e.g. temperature, air humidity, radiation, wind, rain)
[SOURCE: EN ISO 772:2022, 10.36]
3.27
snowpack
accumulation of snow on the ground at a given site and time
Note 1 to entry: It often consists of various layers with different physical and mechanical properties.
Note 2 to entry: See also snow cover (3.10).
[SOURCE: EN ISO 772:2022, 10.49]
3.28
snowpack thickness
total height of the snowpack (3.27) measured perpendicularly from the base to snow surface
Note 1 to entry: See also snow depth (3.14).
[SOURCE: EN ISO 772:2022, 10.53]
4 Symbols
Symbol Description Unit
φ Slope angle °
DS Snowpack thickness (slope-perpendicular measurement) cm
DN Thickness of new snow (slope-perpendicular measurement) cm
D Snow thickness (slope-perpendicular measurement) cm
L Snow layer thickness (slope-perpendicular measurement) cm
P
HS Snow depth, height of snowpack (vertical measurement) cm
HN Depth of snowfall, new snow height, (vertical measurement) cm
SOURCE: The International Classification for Seasonal Snow on the Ground, IACS-UNESCO 2009.
5 Principles
5.1 General
Snow depth and depth of snowfall are measured vertically from a base level up to the snow surface.
Snow depth measurements normally have the ground surface as the base, while depth of snowfall is
measured on a snow board placed on the old underlying snow surface. Depending on the type of
instrument, measurements can be obliged to be performed perpendicularly to the ground. The vertical
component, though, is calculated through the ground slope angle.
5.2 Snow depth
Figure 1 — Relationship between snow depth (HS) and snowpack thickness (DS)
Snow depth, or height of snowpack, (HS) is the vertical distance from the base to the snow surface
(Figure 1). Unless otherwise specified, a snow depth measurement refers to a measurement at a single
location at a given time. The perpendicular distance from the base to the snow surface is defined as
snowpack thickness (DS), which is related to HS through the slope angle (φ) as follows:
DS HS⋅ cosϕ (1)
( )
The slope angle is the acute angle measured from the horizontal plane of the slope. Conversely, HS can
be derived from DS as follows:
−1
HS DS⋅ cosϕ (2)
( )
The vertical distance to a certain coordinate inside a snowpack (Figure 2) is defined as snow height (H)
and the slope-perpendicular distance as snow thickness (D), both related to each other as described for
HS and DS in Formula 1. The measurement is used for location of snow layering boundaries or
measuring points inside the snow for, for example, snow temperature or liquid water content. Where
only the upper part of the snowpack is of interest, the snow surface may be taken as the reference. This
should be indicated by using negative coordinate values.
=
=
Figure 2 — Relationship between snow height (H) and snow thickness (D)
5.3 Depth of snowfall
Depth of snowfall, or new snow height, (HN) is the vertical depth of freshly fallen snow that has
accumulated on the base or on a snow board during a specific period, usually of 24 h. When reporting
depth of snowfall for observation periods other than for 24 h, the period is added in parentheses to the
symbol, for example, the symbol for an 8-h measurement becomes HN(8h). Thickness of new snow (DN)
is the perpendicular equivalent of HN. DN is related to HN via the slope angle (φ) as described for HS
and DS in Formula 1.
6 Measurement principles
Measurement of snow depth can be made manually by readings on fixed scales or by graduated devices
pushed through the snowpack. A fixed scale can also be read remotely by a camera mounted at the site
or send photos automatically by broadband or GSM for manual post-processing. The measurement can
also be recorded continually by use of automatic sensors mounted above the snow surface. A manual
measurement, though, is the reference for all types of automated snow depth measurement since
calibration and verification of the sensor’s reading shall be done manually at site.
The fact that the snowpack is compacting during the accumulation of new snow makes it difficult to find
automated methods useful for measurements of the depth of snowfall. Operative measurements are
made manually on snow boards, which are swept free from snow after measurement and placed on top
of the old snow surface.
7 Siting and exposure
7.1 General
The accumulation of snow is often extensively affected by redistribution during snowfall and by drifting
and melting between the snowfall events. To find a representative site for measurement of snow depth
there are a number of factors to take into account, of which some important are listed in 7.2 to 7.5.
7.2 Terrain
The measurement site should be as horizontal and flat as possible within a range of at least 100 m
radius, sheltered by vegetation from the influence of strong winds and safe from snow glide and snow
creep. Steep slopes, troughs, terrain edges or large rocks in the immediate vicinity of the measuring site
should be avoided as well as hollows and gullies, ridges and humps, where snow accumulation or snow
redistribution due to wind effects often occur. Inconsistencies of the ground within the measurement
site should be minimized by preparation with sand or gravel. At automatic measuring sites, constructed
plates sometimes could be preferred. It is important though that the constructions have thermal
properties which do not differ too much from the ground.
7.3 Wind
To ensure representative values it is essential to perform the measurements in locations where the
effects of blowing and drifting snow are minimized. In open areas where windblown snow cannot be
avoided, the mean value at the site should be estimated from several measurements, enough to give a
representative value.
7.4 Thermal radiation
Measuring sites should not be situated in urban areas where thermal effects could influence the snow
cover, nor at locations with unrepresentative sun exposure or shadow. To avoid ablation due to
absorption of solar radiation, stationary snow measurement equipment should be painted with, or
constructed by material in, light colour.
7.5 Obstacles
Structures and obstructions that could affect the wind patterns close to the measuring site should be
avoided. Recommended locations of measurement sites are at places sufficiently distant from larger
trees or rock outcrops and buildings, which could disturb natural snow accumulation and melting. The
minimum recommended distance between the sampling point and the nearest obstacle is roughly equal
to the height of the obstacle. In heavily forested locations, it is important to use clearings to avoid the
interception of snow by the tree canopy. Dense grass can cause a layer of air between the ground and
the bottom layer of the snow, particularly early in the winter season. This space between the ground
and the snow should not be included in the snow depth. Though, it may be very difficult to estimate if
this layer is present or not.
7.6 Disturbance by humans and animals
To prevent disturbance of the snow surface, measurement sites are preferably situated where people or
animals are not passing by frequently. It is recommended to use marker bands and reflectors to make
the equipment more visible even during night-time. If necessary, the installation should be surrounded
by a fence. It is important, though, that the fence does not disturb the general snow accumulation at the
site.
7.7 Maintenance of measurement site
After the growing season ground vegetation should be cut at the target area and any objects that could
disturb the measurements must be removed. If necessary, holes in the ground should be filled up,
stones and grass tufts removed, and the measuring site prepared with filling material in order to level
the ground. Adequate preparation of the target area increases the measurement accuracy.
Trees and bushes might be removed or trimmed before they have grown too high or too dense. Fences,
marks, reflexes, and signs should be looked over and the stability of the installations should be checked
to withstand soil frost or storms.
8 Measurement of snow depth
8.1 General
Snow depth can vary significantly within short distances. For a correct estimation it might be necessary
to perform measurements at several points. Before establishment of a new measuring site the
accumulation of snow should be studied.
If a snow depth measurement is carried out together with measurement of snow density the snow
water equivalent (SWE) at the site can be calculated. For assessment of SWE in whole catchments it is
important to take into account the variability of snow depth within the landscape, which normally is
significantly bigger than snow density. This can be made by snow surveys along carefully selected
transects (snow courses) in the study area. The number of samples and the length of snow courses
should be determined by the snow variability. Snow courses should be selected to represent the terrain
characteristics of the catchment, considering parameters such as elevation, slope, inclination,
coordinate, aspect, curvature, and the proportion of forest and open field. Snow courses can be short
with manual readings on 5 to 10 snow stakes installed 5 to 10 m apart or up to several kilometres
where measurements are made by use of ground penetrating radar (GPR) and located by Real-time
kinematic positioning (RTK).
Prior to the installation of a fixed snow measuring structure, it is important to ensure that its
measurement range is within the maximum expected snow depth. The installation needs to be solidly
constructed and anchored to prevent movement, for example due to wind and soil frost, but yet keep
interference with the accumulation and ablation of snow at the target area minimized. Steel mounting
beams and vertical poles firmly secured in the ground, for example with concrete foundations, are
recommended. For sites where the measurement structure necessarily has to be very high, due to deep
snow, the installation must be more substantial to stand firmly against strong winds and extra care
must be taken to protect sensors from wind vibrations.
Personnel performing snow measurements and site maintenance must be trained to consider relevant
factors that can affect the snow accumulation at the site, or the measurement itself. Untrained
personnel may hamper the continuity of measurements and increase the uncertainties. To ensure that
the snow surface is as unperturbed as possible it is important to keep single paths to the sampling
points, throughout the whole winter. Measurement sites can be situated in remote mountainous areas
where weather conditions might change rapidly. Recurrent training in field work safety and in
management of equipment in harsh weather conditions is recommended.
8.2 Calibration
Snow depth stations are calibrated to measure zero snow depth, often called the zero level or zero
point, at the base level. The equipment can be lifted by frost heave and the zero level accordingly ending
up above the ground, or the target surface under automatic sensors can settle. To ensure a correct
reading it is important to calibrate the zero level before each snow season. A zero level check should
also be made directly after the snow has melted, but before melting of the ground frost, which could
further move the installation. Zero level drift could potentially produce small but incremental errors in
snow depth data that are difficult to assess or to adjust. Preferably, manual control measurements
should be made at least twice during the winter, which makes it possible to determine a possible zero
level drift before the end of the measurement season.
Automatic sensors should have the calibration procedures described in the manufacturer’s instructions.
8.3 Manual measurements
8.3.1 Measurement techniques
8.3.1.1 Snow stake
Annex A shows a snow stake.
Manual snow depth measurements are often made by use of fixed snow stakes with graduated clear
scales. Horizontal marks on the stake can facilitate readings from distance. The stake must be vertical
with enough length for measuring the highest possible snow depth and well anchored in the ground to
protect it from being pushed up by ground frost or teared down by wind, animals, or from mischief. If
the installation is seasonal, it is important that the stake is placed exactly at the same point every year.
The material of the snow stake should be of a relatively low thermal conductivity and low heat storage
capacity, such as wood or fibreglass, and painted white to minimize heat adsorption.
Readings can be made remotely, both manually and automatically, by a camera installed at the site at
the lowest grazing angle practical from the snow surface at the site. If the stake is to be photographed in
low light conditions, graduations delineated by reflective material may increase the visibility. Low light
photography may require specialized camera equipment such as infrared capability and/or an external
light source to improve the quality of the photographs. To minimize interpretation of the snow line
against the stake the angle between the camera and the snow stake should be as low as practical, taken
into consideration that it changes with the snow depth. Attention should be paid to the orientation of
the camera with respect to direct sunlight and shadowing. As for other types of remote readings the
observations should be checked manually.
8.3.1.2 Manual snow depth sampling
Annex B and C show examples of manual snow depth sampling devices.
Instead of using a snow stake, or in addition to, samples can be taken by a ruler, measuring rod or a
snow probe. For dense and deep snow or snowpack with ice layers it is advisable to use a heavy-duty,
sharp-headed probing rod. It might be easier to hack through thick ice layers if the rod has a flat chisel
shaped head. The head should be slightly larger in diameter than the measuring rod, preventing the rod
from getting stuck.
Another option, particularly for surveys in shallow snow depth, is the use of electronic probes. The
instrument consists of a rod along which a basket is sliding. When the rod is inserted in the snow the
basket floats on the surface and an electronic device register the penetrated depth. The electronic probe
is especially efficient when a big number of samples are taken with a relatively small distance between
the measurements.
8.3.2 Procedure
Snow stakes might affect the wind or cause ablation in the immediate vicinity of the stake. Therefore, it
is important to perform the survey as parallel to the surrounding snow surface as possible. Some
interpolation by the observer may be required when there is uneven snow distribution around the
snow stake (Figure 3), such that the mean snow depth around the snow stake is reported. If there is any
doubt about the reading on the snow stake additional measurements around the stake are necessary. It
is recommended that the observation is made from the same observation point each time.
A manual snow depth sample is taken vertically. On inclined surfaces a clinometer may be used to
ensure that the measurement is made at the correct angle to the slope. A measurement or estimate of
the slope angle should then be included in the metadata of the observation point.
If the snow cover extent at the site is less than 50 % the snow depth is reported as 0, even though there
is snow at the stake and some snow-covered areas have a significant depth.
Figure 3 — Measurement of snow depth with snow stake
Make sure that the snow measuring equipment is in good working order before each snow season.
Measuring sticks and scales should be checked to measure the correct snow depth. The zero level on
snow stakes should be flush to the ground. If necessary, white paint on stationary snow measuring
equipment should be maintained.
8.4 Sources of error
8.4.1 General
If the purpose of the measurement is to be representative of the surroundings, the wind conditions at a
site is of crucial importance. A poorly chosen site could result in measurements being located within a
snow drift or scoured area. The magnitude of error depends on the site mean and the variance. This
magnitude could potentially be as high or higher than the mean at the site, with relative errors
increasing with decreasing snow depth.
Snow stakes, as with all other structures anchored in the ground in cold climates, are exposed to frost
heave. This might result in the lifting and changing of the instrument’s zero level. Such errors are
usually within a few centimetres in magnitude.
The observer’s assessment of the measurement quality is very important. Any factor affecting a
representative snow depth should be noted and the measurement complemented by additional
samples.
Errors in manual measurements may result from erroneous readings on scales or entries into the
logbook. In very cold and harsh weather the notes may be entered with the gloves on and the writing
more difficult to interpret, making extra efforts to write carefully and clearly important.
Errors can be found by data quality control processes and data outliers can be highlighted for further
checks.
8.4.2 Reading errors
Reading errors in interpretation of the snow depth can occur if the viewing angle is high, and also if the
snow around the stake in uneven (mounding, welling, or sloping). These errors can be as large as a few
centimetres but can be minimized with experience and by following best practice guidelines for snow
stake observations.
Electronic probes have a tendency of overestimating the snow depth in steep terrain when the basket is
not flush with the surface, producing errors up to a few centimetres.
8.4.3 Misjudgement of the base level
A measurement error occurs if the observer misjudges the location of the base. An ice layer may be
mistakenly interpreted as being the base, which results in underestimation of the snow depth, and vice
versa, a ground surface consisting of a soft organic material might be difficult to detect, thus resulting in
an overestimation of the snow depth. These errors can be avoided with the observer’s experience and
their knowledge of the ground conditions at the site. The magnitude of these errors, depending on the
nature of the snowpack and the ground surface, could range from a few to tenths of centimetres.
8.5 Automated measurements
8.5.1 General
Automatic measurements allow continuously recording of the changes of snow depth, thus giving a lot
more information than what can be obtained by manual readings, for example the increase during
snowfall events, the settlement after snowfall due to compaction, melting and redistribution by wind.
Measurements employing ultrasonic or optical technology are most used and are the only techniques
described in this report. Both are measuring the distance from the instrument to a target (here the
snow surface) rather than measuring the actual snow depth. The snow depth is the vertical component
between the instrument’s reading on the base, calibrated before each snow season, and it’s reading of
the snow surface.
An adequately prepared target area will increase the instrument’s capability to detect the first
accumulations of snow occurring on the bare surface. Inconsistencies under the instrument can be
minimized by an artificial surface target. Material with similar texture and thermal properties as the
natural ground should be used, for example artificial turf or a perforated and textured surface
construction.
The sensors must be installed at a sufficient height above the maximum anticipated snowpack. The
minimum and maximum height above the target should be verified with the instrument manufacturer.
The peak snow depth can vary substantially from station to station in a network, even within a short
distance, and before installation of the sensor the maximum snow depth should be estimated.
Prior to the snow season the zero level should be calibrated, the stability of the installation should be
checked, and it should be verified that electronic and telemetry systems work properly.
The relative distance between the sensor and the target may change due to settling (positive change) or
frost heaving (negative change), which would impact the snow depth measurement. Known or
suspected changes in the zero level during the measuring season should be noted in the station’s
metadata.
Automatic snow depth readings should be validated with manual control measurements. Manual
observations also allow estimation of the measurement’s representation of the surrounding snow
depths and to detect any sources of error which could cause erroneous data, for example frost heaving,
settling, or objects disturbing snow accumulation or blocking the sensor. As a minimum, it should be
done twice during the snow season, one in the beginning and one in the end.
Snow on mounting structures which could fall off into the target area should be removed carefully.
Some instruments have heating capability to minimize this potential and should be used where
required. Heating of the mounting beams with heat tape and the use of angled mounting beams may
also assist in prevention of snow, frost and ice build-up around the sensor.
Data are preferably stored in dataloggers at the measurement site and transferred by GSM or
broadband networks. The sampling and data transmission intervals depend on the purpose and need of
data as well as the access of power supply available at the measuring site. Common sampling
frequencies are between 10 min and 1 h, and data transmission carried on at least once per 24 h.
8.5.2 Ultrasonic measurements
Annex D shows a complete ultrasonic snow depth station.
Ultrasonic distance sensors consist of a transducer which transmits a high frequency sound pulse and
receives the returning echo reflected against the target. The distance is calculated from the time lag
between transmission and reflection by use of the speed of sound in air. The minimum installation
height of the sensor depends on the expected maximum snow depth in addition to a blind zone within
measurements are not possible. This blind zone is caused by oscillating resonant energy at the base of
the transducer. Higher frequency transducers have smaller ringing-decay times, thus reducing the blind
zone, but as a consequence the sensing range is reduced. This blind zone, normally between 0,4 and
1,0 m, should be stated in the equipment specification sheet and in the instruction manual.

A transducer transmits a pulse (1) which is reflected against the target and detects the returning echo (2). The
object must be out of the blind zone (3) to be detected. The snow depth (HS) is calculated by subtracting the
sensor’s distance measurement to the snow surface (h1) from its previous measurement to bare ground (h0).
Figure 4 — Ultrasonic distance sensor’s working principle
Figure 4 shows the principle of measurement with an ultrasonic snow depth sensor. Correcting for the
speed of sound with concurrently measured air temperature, the measured distance to the target (h1) is
calculated as follows:
T
air
hh1= (3)
0°C
273,15
where T is the actual temperature in kelvin, assuming that the raw instrument reading is correct for
air
the speed of sound at 0 °C (h ). HS is derived by subtracting h1 from the previously derived distance to
0°C
the snow-free base (h0):
HS h0− h1 (4)
The beam is projected against the ground as a cone, normally with an angle between 10 and 30 degrees.
The target area, the conical footprint, is dep
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