Ski wax: types, temperature and application

Ski bases are made of UHMWPE (Ultra High Molecular Weight Polyethylene) — a hard, porous plastic. Without wax, the surface oxidizes and becomes rough. The ski gets slow.

Wax fills the micropores of the base and forms a thin film that optimizes the water layer between ski and snow. Too much water (wet snow) causes drag through capillary suction, too little water (cold, dry snow) increases dry friction.

Regular waxing protects the base from oxidation, UV radiation, and mechanical abrasion from aggressive snow crystals. A well-waxed ski doesn't just glide better — it also lasts longer.

Base Wax: Soft, penetrating wax that soaks deep into the base. Forms the foundation and protects against oxidation. Applied first.

Race Wax: Harder, temperature-specific wax. Applied over the base wax and determines the glide properties for current conditions.

Finish (Topcoat): Liquid or powder overlay as the final layer. Particularly effective in humid conditions (hydrophobic effect). For racing, recommended from wet snow onward (on moist snow only with high air humidity, on machine-made snow always as a protective layer); often unnecessary in dry/cold conditions. In racing, an ironed-in race powder (P stage) can additionally sit between race wax and finish; the finish then forms the topmost layer.

More on this in the article: Base wax vs. race wax

Many manufacturers use a similar color scheme: Yellow stands for warm conditions (0 to -6°C), Red for mid-range (-4 to -12°C), and Blue for cold conditions (-8 to -20°C). Green marks extreme cold. Watch out for the exceptions: Rex assigns Blue to the mid range (Green = cold), and Toko and HWK partly name their lines Warm/Mid/Cold.

The ranges deliberately overlap — in the transition zone you can test both colors. When in doubt, the colder (harder) option is usually the safer choice: wax that's too soft gets penetrated by the snow crystals, brakes, and wears out quickly — wax that's too hard glides only marginally worse.

More on this in the article: Which wax for which temperature

Since the 2023/24 season, all fluorinated waxes have been banned in FIS competition. Reason: environmental and health concerns (PFAS). FIS tests with handheld devices on the ski base — a positive test means disqualification.

Manufacturers have responded: Holmenkol Syntec FF, Swix HS/Pure, HWK HX, Toko HP/Jet, and Rex NF/NFX are the current fluorine-free race lines. Performance is comparable thanks to new additives, but application has partly changed (different iron temperatures, longer cooling times).

More on this in the article: The fluorine ban in ski wax

The sliding friction between ski and snow is complex. Five mechanisms act simultaneously:

1. Plowing: The ski pushes snow grains aside (dominant in soft snow).
2. Adhesion: Molecular attraction between base and ice (dominant in cold conditions).
3. Hydrodynamic lubrication: Water film reduces contact (optimal at ~-3°C).
4. Capillary suction: Water "sucks" the ski down (dominant in wet snow).
5. Contamination: Dirt particles increase friction.

At approximately -3°C snow temperature, frictional heat and pressure melting create an optimal water film just a few micrometers thick. The friction coefficient reaches its minimum here — in the lab (polyethylene on ice) down to µ ≈ 0.005 (Bäurle/ETH Zürich); real values on snow are around 0.02–0.05.

Colder than -3°C: Too little water → dry friction increases. Warmer than -3°C: Too much water → capillary suction creates drag. Wax and structure shift this optimum: hydrophobic waxes for wet snow, soft waxes for cold conditions.

Measurements by Theile at SLF (WSL Institute for Snow and Avalanche Research) show: the actual contact area between ski base and snow is only 0.4% of the geometric surface. Contact occurs at tiny points of ~110µm diameter.

This explains why fine base structures are so effective: they control how many and which contact points form. A coarse structure reduces capillary suction in wet snow, while a fine one minimizes the contact area in dry conditions.

Machine-made snow is produced by atomizing water under compressed air. This creates small, spherical ice pellets (0.2–0.5 mm) instead of the hexagonal snow stars of natural snowfall. These round crystals pack much more densely: Fresh machine-made snow weighs 300–500 kg/m³ — natural fresh snow only 50–150 kg/m³.

1. Harder wax needed: The densely packed, hard ice pellets cause more abrasion on the ski base. A harder (colder) wax resists this penetration better. The rule of thumb from all major manufacturers: Choose one color step colder.

2. Molybdenum and graphite help: Additives like MoS₂ or graphite act as solid lubricants, reducing dry friction on hard crystals while extending wax durability.

3. Finer structure: The uniformly round grains need less water drainage structure. For cold machine-made snow, a very fine linear structure (0.01–0.02 mm) suffices.

More on this in the article: Ski wax for man-made snow

Snow temperature — not air temperature — is the decisive parameter for wax selection. It determines how much melt water forms between ski and snow, and therefore how hard the wax needs to be.

Snow temperature ≠ air temperature: On a sunny day, the snow surface on a south-facing slope can be 5–10°C warmer than the air. In shade or wind, it can be colder. The app calculates snow temperature using a physics-based energy balance model (V3) that accounts for solar radiation, slope angle, slope aspect, wind speed, cloud cover, snow properties, and the diurnal cycle since nighttime. In race mode, users can specify aspect, start time, and slope angle — otherwise default values (south-facing, 10:00, 25°) are used.

Current research (Wolfsperger et al., 2021) shows: snow moisture explains 77% of the variance in ski friction — more than any other single factor. The app calculates moisture according to ICSSG classes (International Classification for Seasonal Snow on the Ground):

Dry (T_snow < -3°C): No free water. Hard wax, fine structure.
Moist (-3°C to -1°C): Minimal water film. Transition zone.
Wet (-1°C to 0°C, low humidity): Visible water film. Finish recommended.
Very wet (-1°C to 0°C with high humidity, or 0°C+): Water film management critical. Hydrophobic finish + structure.
Soaked (0°C+ with heavy precipitation or very high humidity): Maximum water transport needed. Coarse structure + finish mandatory.

The boundaries between "Wet", "Very wet" and "Soaked" depend not only on snow temperature, but also on humidity and precipitation over the last 48 hours — the app considers all three factors.

Fresh snow has fine, branched crystals with a large surface area — the ski "sinks in" and friction increases through capillary effects. Old snow has round, compact grains with fewer contact points.

Freeze-thaw cycles (freezing at night, thawing during the day) transform snow into aggressive, icy crystals that grind down the base. Artificial snow is particularly hard and aggressive — it requires more durable wax and more frequent rewaxing.

The sharp fracture edges of fresh machine-made crystals smooth out through thermal metamorphosis within about 1–2 weeks. However, the high density (and thus hardness) persists — machine-made snow slopes typically last 2–4 weeks longer than comparable natural snow slopes.

For wax selection: even with older machine-made snow, a slightly harder wax is advisable. The app therefore distinguishes between "Machine-made (fresh)" and "Machine-made (compact)" with different correction strengths.

High humidity (>80%) means more moisture in the snow and a thicker water film — hydrophobic waxes/finishes are particularly effective then. In dry air (<50%), the snow sublimates faster and becomes more aggressive.

The app displays humidity in three levels: low (<50%), medium (50–80%), high (>80%). Wind amplifies sublimation, which further transforms the snow.

The orientation of the slope relative to the sun has an enormous impact on snow temperature. A south-facing slope receives up to 500–600 W/m² of direct radiation on the slope face under clear skies — 2–3 times that of a horizontal surface. This can warm the snow surface by 5–10°C compared to a north-facing slope.

In race mode, the app asks for slope aspect (N/E/S/W) and calculates the slope-specific solar irradiance using the NOAA/Meeus algorithm. The snow temperature is then computed via an energy balance model that accounts for the actual incidence angle of radiation on the tilted slope.

Before new wax is applied, the base must be thoroughly cleaned. Old, oxidized wax, dirt particles, and abrasion from slopes and artificial snow block the pores of the UHMWPE base and prevent fresh wax from penetrating deeply.

A properly prepared base absorbs measurably more wax, retains it longer, and demonstrably glides better. Professional service teams invest more time in preparation than in the actual waxing.

In hot-scraping, a soft cleaning wax (e.g., Toko Base Performance Cleaning or HWK Servicewax Soft) is ironed on and immediately scraped off while still warm. The warm wax dissolves dirt particles and old, oxidized wax from the pores.

Procedure: Iron on the cleaning wax at the manufacturer-specific temperature (Toko BP Cleaning: 110–125°C; HWK Servicewax Soft: only 55–65°C) → immediately (don't let it cool!) scrape off with a sharp plexiglass scraper → the wax shavings are darkly discolored, indicating the dissolved dirt. For heavily soiled bases, repeat the process 2–3 times until the shavings come out clean.

This method is gentler and more thorough than chemical cleaners (wax removers), which can dry out the base. Sources: Toko Alpine Tech Manual, Swix Wax Manual.

After hot-scraping, the cooled base is thoroughly brushed out. A steel brush (or brass brush for sensitive bases) opens the structure and removes the last wax residues from the fine grooves of the stone grind.

Technique: 15–20 strokes with the steel brush from ski tip to tail — always in one direction, never back and forth. Then 10 strokes with a nylon brush to smooth the fibers and remove loose debris.

Only then is the base ready for the first wax layer (base wax). This step is often skipped but makes a noticeable difference in wax durability.

The ski base (UHMWPE) melts at ~135°C — at this temperature the base begins to irreversibly shrink. The iron temperature must be high enough for the wax to melt and penetrate the pores, but low enough to avoid damaging the base.

Soft waxes (warm/yellow) need lower temperatures (approx. 115–130°C), hard waxes (cold/blue) need higher ones (approx. 130–160°C). Fraunhofer IWM has shown: the optimal ironing technique is a single slow pass (15–20 seconds tip-to-tail), not multiple quick back-and-forth movements.

More on this in the article: How to wax your skis

A poorly brushed ski with premium wax glides worse than a well-brushed ski with average wax. Brushing removes excess wax from the structure and exposes the optimized surface. Only the wax in the pores counts — anything on the surface creates drag.

Why sequence matters: Each brush has a specific purpose. Hard brushes (steel, brass) open the structure and remove coarse material. Medium brushes (horsehair, coarse nylon) refine the surface. Soft brushes (fine nylon) polish. Reversing the order smears the structure instead of exposing it.

Protocol A (T_snow ≥ -4°C, warm/wet): Coarse nylon → Fine nylon → Polishing pad. For soft, warm waxes a nylon brush is sufficient. Light pressure — the wax is soft and comes off easily.
Protocol B (-15°C ≤ T_snow < -4°C, all-round): Steel → Horsehair → Fine nylon → Polishing pad. Standard for most conditions. Steel opens, horsehair cleans, nylon polishes.
Protocol C (T_snow < -15°C, extreme cold): Fine steel → Steel → Horsehair → Fine nylon → Polishing pad. Hard, cold waxes require more mechanical effort.

Technique: Always brush in one direction (tip → tail), never back and forth. 15–20 strokes per brush, with consistent pressure. Clamp the ski base-up and work in good lighting — you should see the structure being exposed. After the finish: Only 5–10 light nylon strokes to polish, without pressure.

The stone grind is the permanent structure in the ski base, applied by a grinding machine. It determines the base performance of the ski and is renewed every 30–50 ski days.

Fine grind: For cold, dry conditions. Creates few contact points and minimizes friction with little water film. Typical for continental conditions and hard-packed slopes.
Medium grind: The all-round grind for most conditions. A good compromise between cold and warm performance. The right choice if you only have one pair of skis.
Coarse grind: For warm, wet conditions. The deeper structure channels excess melt water away and reduces capillary suction. Ideal for spring skiing and slush.

Professional racers have multiple pairs of skis with different grinds ("Quiver") and select the appropriate ski based on conditions. Exact grind parameters vary by machine manufacturer and service shop — which is why an experienced ski service that matches the structure to typical conditions is recommended.

Hand rilling (also Structurite) is a technique where temporary grooves are pressed into the ski base with a tool. This technique originates from Nordic skiing (cross-country, biathlon) and was developed for the specific demands of flat gliding.

Why Nordic: In cross-country skiing, skis glide relatively flat across the snow with constant ground contact. The applied grooves control the water film across the entire base surface — especially with classic skis that lie flat during the glide phase. Research by Breitschädel (NTNU/Norwegian University of Science) and Moldestad confirms the measurable effect in Nordic disciplines.

Why not alpine: Alpine skis are ridden almost exclusively on edge. Through carving and high speeds, the actual base contact with the snow is minimal and distributed differently than in cross-country. Temporary grooves are destroyed within a few runs under alpine loads (edge pressure, vibrations at 80–130 km/h, aggressive snow crystals on groomed slopes). Additionally, common rilling tools are designed for cross-country ski widths and don't fit optimally on wider alpine skis.

Conclusion: For alpine racing and recreational skiing, a suitable stone grind is significantly more effective. The app therefore does not recommend hand rilling, but focuses on choosing the right grind and optimal wax combination.

Modern top finish products come in three forms:

1. Liquid (Spray/Sponge): Apply with fleece or sponge, let dry for at least 20 min, then polish with roto-fleece and brush out. No iron needed. Swix TS Liquids are applied with roto-fleece.

2. Powder: Sprinkle onto prepared race wax and iron in at higher temperature (150–180°C depending on manufacturer). Rex NFX Powders require up to 180°C (NFX 21/21G/41/00) — the highest iron temperature of any system; exception NFX 11 Yellow: 130°C.

3. Block/Rub-On: Rub directly onto the prepared base and cork in. Fastest method, ideal just before the start. HWK UHX Block and Rex OLOS/NFX Blocks work this way.

Toko Jet System: Offers Liquid and Powder in the same three temperature ranges (Warm 0 to -4°C, Mid -2 to -12°C, Cold -10 to -20°C); the Jet Bloc complements the mid range as a rub-on.

Swix is one of the largest ski wax manufacturers in the world. Current fluorine-free system: Pro by Swix with PS (Training) → HS (Race) → TS (Top Race). WC Finish: PF (Fine Snow) + PC (Coarse Snow) + PM (Molybdenum).

PS Series (Training, 6 products): PS5 Turquoise to PS10 Yellow plus PS Polar for extreme cold (-32 to -14°C). Affordable all-round waxes (~€8–12/60g).

HS Series (Race wax, 5): HS5–HS10 with optimized biodegradable additives. Iron temperatures 150–160°C depending on hardness.

TS Black Series (Top Race, 5): TS5–TS10 with MoS₂ additive for dirt repulsion. Particularly effective on contaminated and aggressive snow.

Pure WC Finish (7): PF25/PF35/PF100 (Fine Snow) and PC25/PC35/PC100 (Coarse Snow) — differentiated by snow type AND moisture class. PM Pure Moly for contaminated snow. Premium priced (~€97/20g).

Liquid Finish (5): LF35 Lotus, LFC100 AmphiGlide and TS6/TS7/TS8 Liquid — roto-fleece application.

Toko is the oldest of the five brands (founded 1916, BRAV Switzerland AG). Three-tier system: Base Performance → Performance → WC High Performance.

Important: Toko mixes color names (Yellow/Red/Blue for Base Performance, Performance, Powder/Liquid) and climate designations (Warm/Universal or Mid/Cold for High Performance and Jet). For the hot-wax lines, the iron temperature is the same per color: Yellow 130°C, Red 140°C, Blue 150°C; powders require higher temperatures (~160°C).

Base Performance (Training, 4): BP Yellow/Red/Blue plus BP Cleaning Wax for hot-scraping. Plus RS Premix 1916 and All-in-One Hot Wax (2 universal additions). Performance (Race, 3): Mid-tier level. Natural Speed (Race, 3): TripleX technology hot waxes.

WC High Performance (Race, 3): HP Warm (-4 to 0°C), HP Universal (0 to -16°C), HP Cold (-12 to -24°C). Highest performance tier.

Jet Top Finish System (7): Jet Liquid Warm/Mid/Cold + Jet Powder Warm/Mid/Cold + Jet Bloc Mid. Liquid and Powder cover the same three temperature ranges; the Jet Bloc complements the mid range as a rub-on. Plus Jet Black (antistatic, dirt-repelling).

Powder & Liquid (10): HP Powder Y/R/B, HP Liquid Paraffin Y/R/B, Performance Liquid Paraffin Y/R/B, X-Cold Powder (-30 to -15°C).

Holmenkol (founded 1922) is regarded as the inventor of modern ski wax and is the first SWAN ECO-certified ski wax company. Headquarters: Ludwigsburg. Current race system: Syntec FF.

Alpha/Beta/Ultra/Delta Mix (Base, 4): Alpha Yellow (-4 to 0°C), Beta Red (-14 to -4°C, World Cup bestseller), Ultra Blue (-20 to -8°C), Delta Universal (-12 to 0°C).

Syntec FF 21 Race Basis (1): All-temperature race base as a critical interlayer. FF Bar Race Wax (4): Yellow/Red/Blue/Green.

Syntec FF1 (World Cup, 6): Liquid Yellow/Red/Blue + Powder Yellow/Red/Blue. Highest performance tier, iron temperature for powder: 150–160°C.

Syntec FF2 (Racing, 3): Liquid Yellow/Red/Blue — second performance tier at a lower price (~€64/100ml vs. ~€140–150/50ml for FF1). FF1 > FF2 in the hierarchy.

HWK Performance (HWK-Kronbichler GmbH) from Ebbs, Tyrol produces all waxes by hand. Official supplier for ÖSV and DSV since the 2007/2008 season. System: LX (Training) → HX/ARP (Racing) → UHX (World Cup Finish).

LX Basewax (3): Warm (-2 to +8°C), Middle (-8 to -2°C), Cold (-15 to -5°C).

HX Racewax (5): Warm/Middle/Cold plus HX Newsnow Warm/Cold for fresh snow conditions.

Alpin Racing PRO (2): ARP Warm (-2 to +15°C) and ARP Cold (-20 to +2°C). Co-developed with Edi Unterberger (Marcel Hirscher's service technician). No ARP Middle — the two products together cover +15 to -20°C seamlessly.

UHX Finish (12): Liquo Spray Warm/Middle/Cold, UHX Block Warm/Middle/Middle-Black/Cold, UHX Powder Warm/Middle/Middle-Black/Cold-Polar, plus Prototype WM25. The most extensive finish system of any manufacturer.

Servicewax Soft (1): Ultra-low iron temperature (55–65°C) for hot-scraping.

Rex (founded 1952, first product: ignition cartridge for the Helsinki Olympic torch) brings decades of experience from Nordic skiing. Technology: N-KINETIC™ (3rd generation), UHW (Ultra Hard Wax), Power Polymers.

NF Sisu Base (2): Sisu White (-20 to +5°C, all-round) and Sisu Black Hard (-25 to 0°C, abrasive). Wide temperature ranges.

NF Race Wax (5): NF11 Yellow, NF21 Blue (according to Rex, the fastest fluorine-free wax in its range), NF21G Graphite (fresh snow), NF31 Green, NF41 Pink/Green (UHW for artificial snow). Rex does not publish iron temperatures for NF Blocks.

NFX N-KINETIC Powder (5): NFX 11/21/21G/41 plus NFX 00 SISU Black. Up to 180°C — the highest iron temperature of any system; exception NFX 11 Yellow: 130°C.

NFX Blocks Rub-On (4): Gold/Blue/NEW/OLD — quick finish application without an iron, rub on directly and cork.

Liquid (8): NFX Blue Liquid, NFX Extra Tuned Blue/Black/Pink, NF Liquid Gliders (NF11/NF21/NF41), Gold Liquid Top Coating. Plus OLOS Finishing Powder (rub-on). NF Liquids have broader temperature ranges than the identically named Solid Blocks.

The app calculates snow temperature using a full energy balance model. Instead of a simple formula (air temperature minus a flat offset), the V3 model simulates the real physical process: How much energy does the snow surface gain or lose per hour?

The 6 energy flows:
1. Shortwave absorbed (Q_sw): Solar irradiance with dynamic CLASS albedo (α_max=0.70 → α_min=0.40, exponential decay with snow age).
2. Longwave in (Q_lw_in): Atmospheric counter-radiation using Brutsaert (1975) clear-sky emissivity + Crawford-Duchon cloud correction.
3. Longwave out (Q_lw_out): Emission from the snow surface (ε_snow × σ × T⁴).
4. Sensible heat flux (H): Bulk aerodynamic formulation, proportional to wind speed and the temperature difference between air and snow.
5. Latent heat flux (LE): Sublimation and condensation at the snow surface.
6. Ground heat flux (G): 3-layer coupling from deeper snow layers, with groomed piste properties (ρ=450 kg/m³, k≈0.43 W/(m·K) — much harder than natural snow).

3-stage refinement: Physics model → rule-based residual correction → Kalman bias correction: a validation run compares the model directly at the measuring stations every 2 hours (Switzerland: SLF IMIS, Tyrol: LWD Tirol); whenever a station lies within 15 km, the forecast is corrected with the bias learned at that station. The dynamic lapse rate is interpolated from pressure level data (8 levels, 1000–700 hPa), with a fallback of -0.55°C/100m.

The model starts at night (radiative equilibrium without sun) and then calculates snow temperature hourly until start time. The temperature is capped at 0°C — snow cannot get warmer without melting.

More on this in the article: How the model is tested

Slope aspect (orientation) has an enormous influence on snow temperature — greater than most skiers realize. Direct solar radiation hits south-facing slopes almost perpendicularly, while north-facing slopes receive it only at a grazing angle or not at all. A concrete example, calculated with the V3 model, illustrates the difference.

Scenario: Laterns, January 15, South Slope, 1'600 m

Laterns in Vorarlberg, south-facing slope. Race start 11:00 AM. Assumed conditions: Air temperature -5°C, cloud cover below 20%, groomed piste (albedo 0.60, density 450 kg/m³). Slope angle 25°, aspect south.

The Racer's Rule of Thumb

Many racers estimate snow temperature with a rule of thumb: "Snow temperature ≈ air temperature minus 2–3°C". At -5°C air, that gives approximately -7 to -8°C. The wax choice falls on Toko Performance Red (-12 to -4°C).

What the V3 Model Calculates

The energy balance model computes snow temperature hourly — using the NOAA/Meeus solar position, Brutsaert (1975) atmospheric physics, and the 3-layer model. All values are verified with the actual algorithm.

The hourly progression shows how quickly a south-facing slope warms up in January:

From -12°C to -3°C in just 3 hours — driven by the 2x radiation concentration on the south-facing slope at low sun angles.

Rule of Thumb vs. Model (11:00 AM)

Why the Rule of Thumb Fails on South-Facing Slopes

• Radiation concentration: A 25° south-facing slope captures 2–3 times the radiation of a flat surface at low sun angles (13–19° in January). The sun hits the slope almost perpendicularly.
• Low heat capacity: Only the top ~7 cm of the snow surface respond to radiation. This thin layer heats up by 8°C+ in one hour — much faster than soil or water.
• Sensible heat: At night, the snow (-12°C) is significantly colder than the air (-5°C). This temperature gradient drives additional heat from the air into the snow and accelerates the morning warming.

Under overcast skies, aspect barely matters: without direct radiation there is no concentration effect. The rule of thumb then comes surprisingly close to the calculated value. That is why the app specifically asks for slope aspect in race mode.

To calculate solar irradiance on a tilted slope, the app needs to know the exact sun position: azimuth (compass direction) and elevation (altitude angle). For this, the V3 model uses the NOAA/Meeus algorithm — the same one used by professional solar energy software.

From date, time, latitude, and longitude, the app calculates:
• Solar elevation: How high is the sun above the horizon?
• Solar azimuth: In which compass direction is it?
• Incidence angle on the slope: How steeply does the radiation hit the tilted surface? (Depends on slope angle, slope aspect, and sun position)

A south-facing slope at 25° receives significantly more radiation per square meter than a horizontal surface in winter at low sun angles — the incidence angle is more favorable. Conversely, a north-facing slope in winter beyond a certain slope angle can receive no direct solar radiation at all.

The app sources hourly weather data from Open-Meteo (free API, no personal data collected). Queried parameters: air temperature, relative humidity, cloud cover, wind speed, precipitation, snowfall, direct and diffuse solar radiation.

Altitude correction: The API provides data for the model elevation (typically a grid point in the valley). The app corrects to the selected reference altitude: dynamically interpolated from 8 pressure levels (1000–700 hPa), fallback -0.55°C/100m.

48h history: For snow classification and the V3 model, 2 days of historical data are retrieved. From this, the app identifies: fresh snow, freeze-thaw cycles, wind transport, rain on snow.

The algorithm follows the same three-step process as professional ski service teams:

Step 1 — Snow grain type: Determines the product family (fresh snow waxes vs. old snow waxes).
Step 2 — Snow moisture: Determines hydrophobic needs and whether a finish is worthwhile.
Step 3 — Snow temperature: Determines the exact wax hardness within the product family.

Each wax is scored on a 100-point scale: temperature match (up to 50 points), grain type match (up to 25 points), moisture match (up to 15 points), and feedback bonus (up to 10 points). For products with a verified manufacturer humidity band, the app shifts the weights to 45/25/20/10 — the moisture axis then counts more.

After every recommendation, you can rate whether the wax performed well. In race mode, you can additionally provide the measured snow temperature. This data feeds into the feedback bonus of the scoring algorithm.

Waxes that receive positive ratings under certain conditions rise in the ranking. Waxes with negative feedback drop. The more users provide feedback, the more precise the recommendations become — especially in borderline situations where two waxes score similarly.

The V3 model automatically classifies snow based on the weather history of the past 48 hours. Instead of a single question ("What does the snow look like?"), the app evaluates multiple indicators:

Fresh snow detection: Significant snowfall in the last 6–12 hours → fresh snow. At air temperature < -5°C and low humidity: dry fresh snow (albedo 0.85, density ~80 kg/m³). At borderline temperatures (wet bulb -2 to +0.5°C): moist fresh snow.
Freeze-thaw cycles: At least 3 cycles with amplitude > 5°C in the 48h temperature history → firn/crust. The surface becomes icy and aggressive.
Wind transport: Fresh snow (< 24h old) + wind > 30 km/h → wind-packed snow. Denser, harder, different crystal structure than normal fresh snow.
Rain on snow: Precipitation at wet bulb temperature > 1.5°C → wet snow/slush.

The classification additionally provides a confidence level (0–100%) and reasoning that explains why this snow type was selected. Users can override the snow type at any time — for example when local conditions (snowmaking, groomers) differ from what the model assumes.

raceday.ski automatically shifts wax selection toward harder waxes for machine-made snow — based on the physical effect that artificial snow creates a friction regime that only occurs at colder temperatures with natural snow. Fresh machine-made snow (< 3 days) receives a stronger correction than aged artificial snow.

Specifically: At a measured snow temperature of -5°C, the app selects wax for fresh machine-made snow as if the snow were -8°C. Additionally, waxes with molybdenum or graphite additives receive a bonus score.

When you expand the Calculation Details in a recommendation, you see all the intermediate values the model uses. Here is what each one means:

1. Air Temperature
The temperature at the chosen altitude, measured 2 m above the ground. It is corrected using a dynamically calculated lapse rate (see below), not the raw value from the nearest weather station.

2. Snow Low Point (Last Night)
The coldest temperature the snow surface reached overnight. Snow radiates heat into the sky and can cool 5 to 15 °C below the ambient air temperature, especially under clear skies with low humidity.

3. Slope Concentration & Absorbed Radiation
Shows how much more or less solar radiation the slope receives compared to a flat surface. A steep south-facing slope in winter can receive several times more energy than a shaded north face. The absorbed radiation value accounts for slope angle, aspect, and albedo.

4. Lapse Rate
The rate at which temperature decreases per 100 m of altitude gain. Instead of a fixed standard value, the app calculates the lapse rate dynamically from 8 pressure levels (1000 to 700 hPa) in the weather model.

5. Emissivity (Brutsaert)
Describes how much longwave (thermal) radiation the atmosphere returns to the ground, on a scale from 0 to 1. High values (near 1) indicate cloudy or warm, humid conditions; low values indicate clear, cold, dry skies that allow the snow to radiate heat away more freely.

6. Albedo (CLASS)
The reflectivity of the snow surface, ranging from 0 to 1. Fresh natural snow reflects up to 85 % of incoming solar radiation (albedo up to 0.85); for the groomed piste, the app follows the CLASS scheme with 0.70 (freshly groomed) down to 0.40 (old, dirty snow), aged via the snow age. Lower albedo means the snow absorbs more sunlight and warms faster.

7. Sky View Factor
The fraction of the sky that is visible from the slope, between 0 and 1. A wide-open plateau has a sky view factor near 1; a narrow valley or tree-lined run has a lower value. This affects the radiation balance both during the day (less diffuse radiation) and at night (less radiative cooling).

8. Sublimation
The energy lost when snow transitions directly from solid to vapor without melting first. This process extracts heat from the snow surface and has a cooling effect. Typical values range from -5 to -20 W/m², depending on wind speed and humidity.

9. Layer Temperatures
The model tracks three snow layers: surface (top 3 cm, where your ski base glides), subsurface (around 17 cm depth), anddeep (around 80 cm depth). The surface layer reacts quickly to sun and wind; deeper layers change slowly and anchor the temperature profile.

10. Rule-based Correction
A rule-based post-correction that adjusts the modeled snow temperature for known systematic errors. For example, the model may consistently underestimate cooling under clear skies or overestimate warming on shaded slopes.

11. Measuring Station (IMIS / LWD Tirol)
The nearest automatic station that measures a real snow surface temperature — in Switzerland the IMIS stations of the SLF (Swiss Institute for Snow and Avalanche Research), in Tyrol the stations of the avalanche warning service (Land Tirol). The Kalman bias — the systematic deviation between model and measurement — is learned every 2 hours by a validation run directly at the stations; if a station lies within 15 km, the forecast is corrected with it, anchoring the forecast to real measurements in Switzerland and in Tyrol.

12. Solar Position
The sun's elevation above the horizon and its azimuth (compass direction). These values drive shadow detection: if the sun is below the horizon or blocked by terrain, direct radiation is zero and only diffuse light remains.

13. Weather Flags
Special conditions the model detects automatically:Foehn (warm, dry downslope wind that can raise temperatures rapidly),Inversion (cold air trapped in the valley while higher elevations are warmer), and Rain-on-Snow (liquid precipitation on the snowpack, creating wet, aggressive surfaces).

14. Artificial Snow Correction
When machine-made snow is selected, the app shows two temperatures: the display temperature (physical snow temperature) and the scoring temperature (shifted colder for wax selection). The offset reflects how much harder artificial snow feels compared to natural snow. An additive bonus also favors waxes with graphite or molybdenum additives.

15. Uncertainty Band (±) & Confidence
Every snow temperature is shown with an uncertainty band (e.g. −6.5 ±1.5°C). It is derived from the confidence score (cloud cover, wind, temperature stability, station proximity, terrain, among others) and ranges from ±0.5°C (very reliable conditions) to ±4°C. The underlying confidence value (n/100) is shown in the detail panel directly below the band.

Input: Laterns-Gapfohl (1'785 m), April 3, start time 10:00, southeast slope (135°; the app asks for the four main aspects — computed exactly here for illustration), slope angle 25°, clear sky.

Step 1 — Weather data (Open-Meteo):
Air temperature 10:00: -3°C (altitude-corrected). Cloud cover: 15%. Wind: 8 km/h. Direct radiation: 420 W/m². Diffuse radiation: 85 W/m².

Step 2 — Solar position (NOAA/Meeus):
Solar elevation: 32°. Solar azimuth: 148° (SSE). Incidence angle on SE slope: cos(i) = 0.91 → Nearly perpendicular irradiance! Absorbed radiation: (420 × 0.91/0.53 + 85 × 0.93) × (1 - 0.65) = ~280 W/m² after albedo (compact old snow, albedo 0.65).

Step 3 — Night equilibrium:
Atmospheric clear-sky emissivity (Brutsaert 1975): 0.65. Night temperature: -8°C. Radiative equilibrium without sun → T_s,night ≈ -13.8°C.

Step 4 — 3-layer model (night → 10:00):
Hourly integration: From sunrise (~06:30), T_s rises through absorbed radiation. Simultaneously, wind cools (sensible heat flux) and sublimation removes energy. After 3.5h of solar irradiance on the SE slope: T_s ≈ -11.0°C.

Result: Snow temperature = -11°C at air temperature -3°C. The snow surface is 8°C colder than the air — because nocturnal radiative cooling chilled the surface heavily and the morning sun has not yet delivered enough energy. Confidence: 78/100, shown as an uncertainty band of ±1.5°C (clear conditions, good model reliability).

It sounds counterintuitive, but snow temperature can actually be higher than air temperature — though never above 0°C (melting point). This typically occurs:

Afternoon on a south-facing slope: Under clear skies and low air temperature (-1 to -5°C), strong direct radiation can warm the snow surface to 0°C while the air is still cold. The snow then begins to melt at the surface.

When is snow colder than air? Almost always — especially in the morning. At night, the snow surface radiates heat away (longwave radiation) and cools significantly. Even if the air temperature is at -5°C, the snow surface can drop to -15°C or colder. This effect is strongest under clear skies and low wind.

For wax selection, this means: In the morning, harder wax may be needed than air temperature suggests. In the afternoon on sun-exposed slopes, it can be significantly warmer than expected. The V3 model accounts for this diurnal cycle.

The scientific foundation of the app:

Glide friction & wax:
Wolfsperger, F. et al. (2021): "Snow Conditions and Ski Wax Performance." Cold Regions Science and Technology.
Bäurle, L. et al. (2006): "Sliding Friction of Polyethylene on Snow and Ice." ETH Zürich.
Theile, T. et al. (2009): "Mechanics of the Ski–Snow Contact." Tribology Letters, SLF/WSL.
Colbeck, S.C. (1992): "A Review of the Processes That Control Snow Friction." CRREL Report.
Breitschädel, F. (2012): "A New Wax for Cross-Country Ski Base Preparation." Norwegian University of Science.
Moldestad, D.A. (1999): "Some Aspects of Ski Base Sliding Friction and Ski Base Structure." NTNU.

Snow temperature model (V3):
Meeus, J. (1991): "Astronomical Algorithms." (Solar position)
Brutsaert, W. (1975): "On a Derivable Formula for Long-Wave Radiation from Clear Skies." Water Resources Research. (Atmospheric clear-sky emissivity)
Crawford, T.M. & Duchon, C.E. (1999): "An Improved Parameterization for Estimating Effective Atmospheric Emissivity for Use in Calculating Daytime Downwelling Longwave Radiation." J. Applied Meteorology. (Cloud correction)
Verseghy, D.L. (1991): "CLASS — A Canadian Land Surface Scheme." Int. J. Climatol. (Dynamic albedo)
Boone, A. & Etchevers, P. (2001): "An Intercomparison of Three Snow Schemes (ISBA-ES)." J. Hydrometeorol. (3-layer snow model)
Sturm, M. et al. (1997): "The Thermal Conductivity of Seasonal Snow." J. Glaciology. (Snow thermal conductivity)
Rolland, C. (2003): "Spatial and Seasonal Variations of Air Temperature Lapse Rates." J. Climate. (Dynamic lapse rate)
Alduchov, O.A. & Eskridge, R.E. (1996): "Improved Magnus Form Approximation." J. Applied Meteorology. (Saturation vapor pressure)
SLF/WSL: IMIS — Interkantonales Mess- und Informationssystem. (Snow station calibration data)
LWD Tirol: open station data with snow surface temperature for Austria/Tyrol. Datenquelle: Land Tirol - data.tirol.gv.at (LWD Tirol). CC BY 4.0.

Every recommendation is based on a combination of public weather data, measurement networks, terrain models, and our curated wax database. Here is the complete list:

1. Weather data — Open-Meteo (api.open-meteo.com/v1/forecast)
Free, commercially usable API without an API key. Hourly forecasts for air temperature, humidity, cloud cover, wind, precipitation, and snowfall. Plus 8 pressure levels (1000–700 hPa) for the dynamic lapse-rate correction. Model ensemble includes ICON-D2, AROME, ECMWF IFS. Responses are cached server-side for 30 minutes.

2. Snow temperature reference — SLF IMIS (CH) & LWD Tirol (AT) (measurement-api.slf.ch, wiski.tirol.gv.at)
From the IMIS network of the WSL Institute for Snow and Avalanche Research SLF we use around 130 snow stations in the Swiss Alps; in Austria, around 150 stations of the Tyrolean avalanche warning service provide the surface temperature (the open Tyrolean dataset additionally lists stations of the hydrographic service without a surface-temperature sensor). The snow surface temperature (TSS or OFT) is paired with the model every 2 hours by a validation run directly at the stations; from these pairs the Kalman bias is learned, which corrects forecasts near a station (≤15 km). Licences: CC BY 4.0 (SLF/WSL); Datenquelle: Land Tirol - data.tirol.gv.at (LWD Tirol), CC BY 4.0.

3. Terrain model — pre-computed SVF and horizon profiles
For every one of the >1,100 resorts we have pre-computed the Sky-View Factor (sky visibility, 0–1) and a full horizon profile (36 azimuths × 11 distances from 150 m to 18 km) from a hybrid elevation model (SRTM ≈90 m; EU-DEM 25 m in northern Scandinavia). The shading math follows Dozier & Frew (1990): SVF = 1 − (1/n)·Σ sin²(h_i). Profiles ship as a static JSON file with the build (svf-profiles.json) — no runtime dependency. Deeply incised valleys show low values (Ernen/Goms 0.59, Trient 0.66, Val d’Anniviers 0.73), while glacier sites (Rettenbach 0.86) are considerably more open.

4. Wax database — curated
127 products from five manufacturers (Swix, Toko, Holmenkol, HWK, Rex). Temperature ranges, iron temperatures, snow type suitability, and application form are sourced directly from manufacturer documents (wax manuals, product data sheets, FIS approvals). Reviewed each season and updated when products change.

5. Feedback loop — anonymised
Logged-in users can rate every recommendation as “Perfect / Too warm / Too cold / Wrong wax”. A wax's score is slightly boosted on consistently positive feedback (max +10 points). No personal data is stored — only wax ID, temperature, and rating. Opt-out possible at any time.

6. Resort master data — internal
Over 1,100 ski resorts across the Alps and Scandinavia with base/peak altitude and coordinates. Based on official tourism websites and OpenStreetMap.

What we do NOT use:

• No direct ECMWF integration (licensing costs) — we rely on Open-Meteo as aggregator.
• No GPS or position tracking.
• No third-party data sharing (Google Analytics, Facebook, etc.).
• No sponsored wax recommendations — all brands are treated equally.

Data sources are continuously updated. The sources in use are documented on the methodology page.