# How is machine-made snow produced on the ski area?

Producing machine-made snow depends on several weather and technical conditions coming together at once. Les 3 Vallées ski area draws on a network of sensors, artificial reservoirs and installations that stretch across dozens of kilometres. The water needed for this production is also strictly shared with other, higher-priority uses.

## Four conditions for producing machine-made snow

Four factors must align for a snow gun to run. The first is humidity: the air's moisture level, which determines the wet-bulb temperature from the dry-bulb temperature measured by a standard thermometer. A dry-bulb temperature of 0°C with 30% humidity gives a wet-bulb temperature of around -4.3°C, whereas 100% humidity makes the two temperatures identical. Fan-type snow guns, which run at low pressure, start up from -2°C wet-bulb; lance-type snow guns, which run at high pressure, start from -4°C wet-bulb. A total of 150 sensors positioned across the ski area measure these temperatures to within a tenth of a degree, each one triggering a group of three to five snow guns.

The second factor is wind: strong wind scatters the snow produced beyond the piste, while wind blowing directly into a snow gun's head causes icing up and produces water instead of snow. The third factor is the temperature of the water used, ideally between 1°C and 3°C. This water comes from reservoirs — artificial bodies of water such as those at la Masse, le Montaulever and Boismint in Les Menuires — whose contents are churned by an air-bubbling system so they cool on contact with the outside air. The fourth factor is the availability and capacity of the installation itself, which can be limited by pump flow rate, the diameter of the pipe network, or technical faults affecting pumps, networks or power supply.

## Water drawn from the Doron and strictly rationed

The water used to make machine-made snow comes from reservoirs, which are fed from two sources. Part is drawn from the Doron, the local river fed by melting snow; this supply allows snow production in the Saint-Martin-de-Belleville sector. The rest comes from a reservoir belonging to Suez. These withdrawals are overseen by authorities including the water authority and the prefecture. A strict order of priority governs use of this resource: fire services' reserves are supplied first, then the needs of local residents and tourists, and only then, if any allowance remains, the filling of reservoirs earmarked for snow production. During the 2025/26 season, drought limited reservoir filling to 75% of capacity.

## A role seen as essential in extending the season

Machine-made snow is presented as a necessary complement to natural snowfall. Being denser, it melts more slowly and forms a base layer that protects natural snow falling on top of it and extends its life. On this account, without machine-made snow, the ski area's operating seasons would be shorter.

## Optimised water use in the face of climate change

A study carried out with a specialist consultancy, drawing on Météo France's climate models and cross-referencing the number of available cold hours with natural snow cover, indicates that, in the most unfavourable scenario considered, producing snow before 31 December would remain possible until sometime between 2080 and 2100.

Water consumption stayed relatively stable over the decade before the 2025/26 season, despite year-on-year variation in natural snow cover. This control relies partly on Snowsat, a system fitted to grooming machines that measures snowpack depth in real time. It lets operators produce only the volume of snow needed, take snow from where the pack is thickest, optimise grooming time and cut energy use. Modernising production facilities has also supported more careful consumption, while how busy the ski area is affects how fast the snow melts: heavy footfall scatters more snow towards the sides of the pistes.

## Key points

- Producing machine-made snow requires four conditions: favourable humidity, no unfavourable wind, water between 1°C and 3°C, and a working installation.
- Fan-type (low-pressure) snow guns start up from -2°C wet-bulb, compared with -4°C wet-bulb for lance-type (high-pressure) snow guns.
- The water comes partly from the Doron, the local river, and partly from a reservoir belonging to the water utility Suez; its use follows an order of priority that places machine-made snow after fire services and local populations' needs.
- During the 2025/26 season, drought limited the fill rate of reservoirs (artificial lakes used to store water for snowmaking) to 75%.
- A study based on Météo France's models estimates that, in the most unfavourable scenario, producing snow before 31 December would remain possible until sometime between 2080 and 2100.
