Snow macrophysical properties (Snow covered area (or snow extent), snow presence, snow depth, snow water equivalent, snow liquid water content (or wetness), snow density, snow temperature, snow layer thickness (stratigraphy), snow hardness, penetrability of snow surface, snow strength (shear resistance), snow roughness length (aerodynamic roughness), snow surface roughness, runoff)

Snow covered area (Snow extent)                           Snow covered area is defined as the areal extent of snow-covered ground, usually expressed as a fraction (%) of the total area investigated. The latter must be defined, e.g., observation site, catchment, district, country, continent. Unless otherwise specified, only seasonal snow cover is considered. Hence, on glaciers and névés, ground refers either to glacier ice or to an old firn surface (Fierz et al., 2009).

Instruments:

Optical remote sensing

Camera or web-cam

 

Snow presence Visual determination of presence or not presence of snow on the ground.  Snow is present only if more than 50% of the 360deg field of view is covered by snow.

Instruments:

Visual

Camera or web-cam

Snow depth sensor

Thermometer

Infrared sensor

 

Snow depth (HS)             Snow depth denotes the total height of the snowpack, i.e., the vertical distance in centimetres from base to snow surface. Unless otherwise specified snow depth is related to a single location at a given time. Thus, manual snow depth measurements are often made with one or more fixed snow stakes. On the other hand, portable snow depth probes allow for measurements along snow courses and transects. The slope-perpendicular equivalent of snow depth is the total thickness of the snowpack denoted by DS (Fierz et al., 2009).

Instruments:

Stakes

Rulers

Snow probe

MagnaProbe

Ultrasonic and laser depth sensors

Time-lapse photography

Terrestrial and air-borne Laser Scanner

Unmanned Aerial Vehicle + Photogrammetry

Ground Penetrating Radar

 

Liquid water content (LWC)        Liquid water content is defined as the amount of water within the snow that is in the liquid phase. This parameter is synonymous with the free-water content of a snow sample. Liquid water in snow originates from either melt, rain, or a combination of the two. Measurements of liquid water content or wetness are expressed as either a volume (LWC_V) or mass (LWC_m) fraction. Both can be reported as a percent (%), which usually requires a separate measurement of density (Fierz et al., 2009).

Instruments:

Melting/freezing calorimetry

Alcohol calorimetry

Dilution method

Time Domain Reflectometry (TDR)

Snow Fork

Denoth meter

 

Snow Water Equivalent (SWE)  Snow water equivalent is the depth of water that would result if the mass of snow melted completely. It can represent the snow cover over a given region or a confined snow sample over the corresponding area. The snow water equivalent is the product of the snow height in metres and the vertically-integrated density in kilograms per cubic metre. It is typically expressed in millimetres of water equivalent, which is equivalent to kilograms per square metre or litres per square metre, thus referring to the unit surface area of the considered snow sample (Fierz et al., 2009).

Instruments:

Snow sampling tube

Snow cutters + scale

Snow pillow

Scale (i.e. Sommer scale)

Snow gauge

Passive snow water equivalent sensor (i.e.  Campbell CS 725)

Snowpack analyzer

Gamma and cosmic rays sensors

Ground penetrating radar

Acoustic sensor

 

Snow Density                    Density, i.e., mass per unit volume (kg m^–3), is normally determined by weighing snow of a known volume. 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 (Fierz et al., 2009).

Instruments:

Snow sampling tube

Snow cutter + scale

Scale (i.e. Sommer scale)

Neutron probe

Snow MicroPen

 

Snow temperature (Ts)                 The temperature of the snow (not of the air above the snow). It can be taken at the snow surface and/or at various depths of the snowpack.

Instruments:

Manual thermistor probe

Fixed thermistor string

Pyrgeometer (only surface)

Infrared sensor (or camera) above the surface

Probe with infrared sensors

i-buttons

 

Snow layer thickness (stratigraphy)        The snow layer thickness (measured in centimetres or fractions thereof) is an essential parameter when characterizing the current state of a snowpack. Layer thickness is usually measured vertically. If the measurement is taken perpendicular, i.e., slope normal, layer thickness should be denoted by L_p (Fierz et al., 2009).

Instruments:

Visual observations + ruler

NIR photography

Translucent profile

 

Snow hardness                 Hardness is the resistance to penetration of an object into snow. Hardness measurements produce a relative index value that depends on both the operator and the instrument. Therefore, the devise has to be specified (Fierz et al., 2009).

Instruments:

Hand test

Push-pull gauges

SnowMicroPen

 

Penetrability of snow surface    Penetrability is the depth that an object penetrates into the snow from the surface. It can be used as a rough measure of the amount of snow available for transport by aeolian processes or the ability of a snowpack to support a certain load. The depth of penetration of some suitable object, such as a ramsonde element, a foot, or a ski, is measured in centimetres (Fierz et al., 2009).

Instruments:

Ram penetrometer

Foot penetration

Ski penetration

 

Snow strength/shear resistance      Snow strength can be regarded as the maximum or failure stress on a stress–strain curve. It is the maximum stress snow can withstand without failing or fracturing. Snow strength depends on the stress state (σ: compressive or tensile; τ: shear) in pascals and the strain, ε, which is dimensionless, as well as on their rates in pascals per second and per second, respectively. Snow strength depends also on microstructure and on the homogeneity of the sample. To make measurements meaningful, all of these parameters must be considered. Moreover, failure types such as ductile or brittle fracture or maximum stress at low strain rates must be given (Fierz et al., 2009).

Instruments:

Shovel Shear test

Loaded column test

Trapezoidal Tensile test

Flat jack Shear test

Shear frame test

Rutschblock test

SnowMicroPen

 

Snow roughness length (aerodynamic roughness):  is a parameter of some vertical wind profile equations that models the horizontal mean wind speed near the ground; in the log wind profile, it is equivalent to the height at which the wind speed theoretically becomes zero.

Instruments:

Wind profile

Snow surface roughness: refers to the roughness of a snow surface caused by the precipitation, or the wind, as well as by uneven evaporation, sublimation or melt. It does not refer to the roughness due to the snow microstructure.

Instruments:

Laser scanner

Photography

 

Runoff  Melted water that flows over land as surface water

Instruments:

Lysimeter

Electromagnetic properties (snow bidirectional reflectance distribution function, polarization of snow optical reflectance, snow directional reflectance, snow spectral albedo, snow broadband albedo, snow e-folding depth, snow optical transmittance, snow brightness temperature, snow backscattering)

Snow Bidirectional Reflectance Distribution Function (BRDF)                    It is defined as the ratio between the reflected radiance at a specific reflection angle and the incidence irradiance at a specific incident solar angle for each solar shortwave wavelength. In other words, it is the directional distribution of the diffuse radiation reflected by the snow surface in the solar shortwave wavelength range (350-2800nm).

Instruments:

Gonio-spectro-radiometer

Camera + white Lambertian-reflecting target

 

Polarization of snow optical reflectance               Direction of oscillation of the optical electromagnetic waves that have been reflected from the snow surface.

Instruments:

Gonio-spectro-polarimeter

Snow directional reflectance    It is defined as the integral of BRDF over a particular reflection cone.

Instruments:

Spectro-radiometer

Snow Spectral Albedo (α_λ)                          It is defined as the spectral directional-hemispherical reflectance, i.e. the integral of BRDF over all reflection angles. In other words, the spectral albedo is the upward shortwave irradiance divided by the downward shortwave irradiance at each shortwave wavelength.

Spectro-radiometer connected to cosine receptor or integrating sphere

Snow broadband albedo (α)       It is defined as the integration of the spectral albedo over all shortwave wavelengths. In other words, the broadband albedo is the upward broadband shortwave irradiance divided by the downward broadband shortwave irradiance.

Instruments:

Spectro-radiometer connected to cosine receptor or integrating sphere

Pyranometers

 

Snow e-folding depth The depth of snow over which light intensity reduces to 1/e.

Instruments:

Fiber-optic probes

 

Snow optical transmittance                        It is the property of snow to transmit light through it. It determines the portion of incoming light that is transmitted through the snow instead of being absorbed or scattered.

Instruments:

Array of spectro-radiometers (two or more)

Snow brightness temperature   A descriptive measure of radiation in terms of the temperature of a hypothetical black-body emitting an identical amount of radiation at the same wavelength

Instruments:

Microwave radiometer

Snow backscattering     The reflection of waves, particles, or signals back to the direction from which they came.

Instruments:

Radar

Snow electrical conductivity      Property of snow that controls the conduction of electrical current

Instruments:

Snow fork

 

Snow microphysical properties (snow grain shape, snow grain size, snow optical equivalent grain size, snow SSA, correlation function, correlation length, snow porosity, snow permeability, snow tortuosity, snow stickiness, snow thermal conductivity)

Grain shape       The main morphological classes of grain shapes are: Precipitation Particles (PP), Machine Made snow  (MM), Decomposing and Fragmented precipitation particles (DF), Rounded Grains (RG), Faceted Crystals (FC), Depth Hoar (DH), Surface Hoar (SH), Melt Forms (MF), Ice Formations (IF). This basic classification is augmented by subclasses, where a process-oriented characterization of all sub-classes supplements the morphological classification. This side-by-side representation of morphological classification and physical processes should help various user groups arrive at a more reliable classification and an easier physical interpretation of their observations (Fierz et al., 2009).

Instruments:

Visual

Visual + magnifying glasses

Macro-photography + image processing

Casting + Micro-computed tomography + image processing

 

Grain size            The classical grain size (E) of a snow layer is the average size of its grains. The size of a grain or particle is its greatest extension measured in millimetres (Fierz et al., 2009). Alternatively, E can be expressed by using the terms: very fine ( < 0.2 mm), fine (0.2–0.5 mm), medium (0.5–1.0 mm), coarse (1.0–2.0 mm), very coarse (2.0–5.0 mm), extreme (> 5.0 mm) . Some users will want to also specify the average maximum size E_max or even a distribution of sizes. Note that grain size must be regarded as a property of the snow layer and not of the grain shape or shapes.

Instruments:

Visual + scale

Visual + scale + magnifying glasses

Macro-photography + image processing

NIR-photography

Casting + Micro-computed tomography + image processing

 

Optical-equivalent grain size     The optical-equivalent grain size (OGS) can be defined as the radius of a collection of mono-disperse modelled crystals (often spheres) with the same volume-to-surface ratio as the non-spherical snow particle (Grenfell & Warren, 1999). Optical-equivalent grain size is related to the specific surface area and therefore to the microstructure of snow. It is calculated using snow-radiative transfer models with snow reflectance measurements as input.

Instruments:

Contact probe of Spectro-radiometer + snow-radiative transfer model

Spectro-radiometer + snow-radiative transfer model

 

Microwave equivalent grain size            The definition of the microwave-equivalent grain size (MGS) is analogous to the definition of the optical-equivalent grain size, except that MGS is calculated inverting snow microwave emission models with snow brightness temperature as input parameter. MGS is related to the optical-equivalent grain size via a scale factor that depends on the stickiness of the snow particles.

Instruments:

Microwave radiometers + snow microwave emission model

Specific Surface Area (SSA)         is defined as the total surface area of the air/ice interface either per unit mass of a snow sample, SSA_m, or per ice volume, SSA_V, given in m² kg^–1 or m²m^–3, respectively. The density of ice, ρ_i, simply relates SSA_m to SSA_V = ρ_i SSA_m. Snow metamorphism generally reduces the specific surface area even when other parameters, such as density, remain constant.

Instruments:

IceCube

ASSSAP

NIR-photography

SnowMicroPen

Casting + Micro-computed tomography + image processing

CH₄ absorption

 

Snow correlation function           It describes how snow microstructures at different positions are related. More specifically, correlation function quantifies how microstructure variables co-vary with one another on average across space and time.

Instruments:

Casting + Micro-computed tomography + image processing

Snow correlation length               Derivative of the three-dimensional, spatial autocorrelation function A(x), with A(0)=1, and with x being the scalar displacement (Debye et al., 1957)

Instruments:

Casting + Micro-computed tomography + image processing

SnowMicroPen

 

Snow tortuosity               It is defined as the ratio of two distances: the path between two points through the ice or pore space and the straight line between them. The tortuosity of the ice matrix may be the primary factor determining the thermal conductivity of snow. It also affects the elastic modulus and heat and mass fluxes through the pore space. Currently, tortuosity measurements require three-dimensional reconstructions of the ice/air matrix combined with numerical simulations.

Instruments:

Casting + Micro-computed tomography + image processing

Snow porosity   It is defined as the volume of the pore space divided by the total volume. Porosity can be calculated from the snow density. Direct measurements are possible with specialized equipment, but are difficult in the field.

Instruments:

Casting + Micro-computed tomography + image processing

Air permeability of snow             The air permeability of snow is the property of snow that controls the ease with which a fluid, typically air or water, can move through the snow (Domine et al., 2008)

Instruments:

Permeameter, or permeability apparatus (controlled air flow through a snow sample)

Casting +micro-computed tomography

 

Snow stickiness                                It is a measure of the degree in which snow particles are attached to each other’s (by sintering and interlocking)

Instruments:

Casting + Micro-computed tomography+ image processing

 

Snow thermal conductivity         Property of snow that controls the heat conduction

Instruments:

Heat flux plate

Transient needle probe

Thermistor string

Micro-computed tomography + image processing

 

Solid Precipitation (height of new snow, precipitation intensity, drifting snow occurrence, number flux and particle size distribution of drifting snow)

 

Height of new snow, depth of snowfall                Height of new snow is the depth in centimetres of freshly fallen snow that accumulated on a snow board during a standard observing period of 24 hours. Additional observation intervals can be used, but should be specified. For example, the notation HN(8h) or HN(2d) denotes an observation interval of 8 hours or 2 days, respectively. The corresponding slope-perpendicular measurement is denoted by DN (Fierz et al., 2009).

Instruments:

Stakes

Rulers

Snow probe

MagnaProbe

Ultrasonic and laser depth sensors

Time-lapse photography

Terrestrial and air-borne Laser Scanner

Photogrammetry

 

Precipitation Intensity                                 The intensity of precipitation is defined as the mass flux that would fall over a given interval of time if the intensity were constant over that time period. It is typically expressed in terms of length (depth) per unit time, for example, millimeters per hour, or inches per hour. In the case of frozen precipitation liquid water equivalent measure is used.

Description of the instrument and site is needed to define the possible error sources. For instrument: size of the orifice/measurement area, position with respect to wind, installation height (typically for snowfall measurements c.a. 1.5 m), what kind of wind shield is used (e.g. none, Tretyakov, Alter, WMO DFIR (Double Fence Intercomparison Reference)). In case of instruments with gauges: is heating used at the orifice, is anti-freeze used and what are the amount with respect to max capacity, how snow capping or blowing snow is prevented. For surrounding: specification of wind velocity at the height of the orifice or measurement probe, description of surrounding obstacles, including distance/direction from, height, and type (Generally a flat and open area within 10 m of instrument is selected). This area surrounded by generally open space with a slope of less than 1:3 (19°).

 

Instruments:

Weighing-recording gauge

Present weather detector based on extinction

 

Hydrometeor fall velocity and size distribution                Measurement of size distribution (particle size distribution, PSD) and vertical fall velocity of the hydrometeors as a function of the diameter. The measured diameter varies among different instruments. The definition of diameter, detectable size range and observation directions, e.g. 1D, 2D, view angle should be given.  PSD is typically integrated e.g. one-minute-time interval.

Description of the instrument and site is needed to define the possible error sources. For instrument: installation height (typically for snowfall measurements c.a. 1.5 m), what kind of wind shield is used (e.g. none, Tretyakov, Alter, WMO DFIR (Double Fence Intercomparison Reference)), position with respect to wind, measurement volume, how snow capping or blowing snow is prevented. For surrounding: description of surrounding obstacles (including distance/direction from, height, and type), wind velocity at the height of the measurement probe. Generally a flat and open area within 10 m of instrument is chosen. This area is generally surrounded by open space with a slope of less than 1:3 (19°).

 

Instruments:

Optical disdrometer

Drifting snow occurrence             Presence or absence of drifting snow

Instruments:

Visual observation

Number flux of drifting snow    It is defined as the number of snow particles flowing through a unit area per second.

Instruments:

Snow particle counter

Mechanical trap + wind speed measurements

Camera system with light source

FlowCupt acoustic sensor

Disdrometer

 

Particle size of drifting snow      It is defined in the same way as snow grain size (see Snow microphysical properties)

 

Instruments:

Sticking slides + photography + image processing

Camera system with light source + image processing

Disdrometer

 

Snow composition (snow impurity, snow isotopes)

Snow impurity            The type of impurity should be fully described and its amount given as mass fraction (%, ppm). Common impurities are dust, sand, soot, acids, organic and soluble materials. Low amounts of impurities do not strongly influence the physical properties of snow but are of hydrological and environmental interest.

Instruments:

Ion chromatography 

Organic and Elemental Carbon analyzer (OC/EC)

Single Particle Soot Photometer (SP2)

Filter + spectrophotometer

 

Snow isotopes            The most relevant stable isotopes for water are ¹⁸O for oxygen (corresponding to the most abundant isotope ¹⁶O), and ²H (or Deuterium, D) for hydrogen (corresponding to the most abundant isotope ¹H). The respective heavy water molecules are then  and HDO. Generally, stable isotopes are quantified as relative ratios of rare toward abundant isotope abundance with respect to a mean standard (the Vienna Standard Mean Ocean Water V-SMOW), denoted with the symbol δ and expressed in per mille. Thus, snow isotopic composition is reported as δD, δ¹⁸O, and deuterium excess d.

Instruments:

Mass spectrometer

Laser absorption methods (Cavity ring down-spectroscopy-analyzer, Integrated cavity output spectroscopy-analyzer)

 

Most of the definitions are from:

Domine, F. Albert, M., Huthwelker, T. Jakobi, H.-W., Kokhanovsky, A. A., Lehning, M, Picard, G., and Simpson, W. R.: Snow physics as relevant to snow photochemistry, Atmos. Chem. Phys., 8, 171–208, 2008.

Fierz, C., Armstrong, R.L., Durand, Y., Etchevers, P., Greene, E., McClung, D.M., Nishimura, K., Satyawali, P.K. and Sokratov, S.A: The International Classification for Seasonal Snow on the Ground. IHP-VII Technical Documents in Hydrology N°83, IACS Contribution N°1, UNESCO-IHP, Paris, 2009.

Grenfell T. C., and Warren S. G.: Representation of a nonspherical ice particle by a collection of independent spheres for scattering and absorption of radiation. J. Geophys. Res., 104, 31697-31708, 1999.

Harrison, W. L., and Russo, R. S.: Quantitative methods of snow strength measurements, University of Utah Geography, 5260, 1994.

Sigrist, C.: Measurement of fracture mechanical properties of snow and application to dry snow slab avalanche release, Diss. ETH 16736, 2006.

Schweizer, J., and Jamieson, J.B., 2003. Snow stability measurements. Proceedings International Seminar on Snow and Avalanche Test Sites, Grenoble, France, 22-23 November 2001, CEMAGREF Editions: 317-331.

Snow, Weather, and Avalanches: Observational Guidelines for Avalanche Programs in the United States: http://www.avalanche.org/research/guidelines/

http://glossary.ametsoc.org/wiki/Precipitation

Guide to Meteorological Instruments and Methods of Observation: (CIMO guide): https://3920fa727af316d4a002d14303005d900630223b.googledrive.com/host/0BwdvoC9AeWjUZW1iQ2JYNDNDdUE/wmo_8-2012_en.pdf