Snow Volume from Water Volume

This is a mass balance and nothing else. The water does not change amount when it becomes snow; it changes how much space it occupies. So the only interesting quantity in the equation is the density of the snow being made, and the reason the page exists is that almost everybody guesses that density from natural snowfall and is wrong by a factor of four.

Machine-made snow is dense on purpose. Natural snow is dendritic — six-armed crystals that grew from vapour and interlock into an open structure full of air, which is why fresh natural snow can be 30 to 50 kg/m³. Machine-made snow is not crystals at all. It is a spray of atomised water droplets that froze in flight, so it lands as small solid ice spheres that pack tightly. 350 to 500 kg/m³ is normal, and that is a feature rather than a defect: a base layer has to survive grooming machines, skier traffic, thaw cycles and a whole season, and open dendritic snow would not last a week. Someone planning water on a ten-to-one assumption will order between a quarter and a fifth of what the hill needs.

At 400 kg/m³ the ratio is two and a half to one. One cubic metre of water becomes two and a half cubic metres of snow. Put that on a trail: covering a 1,000 m run 30 m wide to a depth of 30 cm needs 9,000 m³ of snow, which is 3,600 m³ of water — nearly a million US gallons, or about three acre-feet, for one run, one base layer, before the first thaw asks for a touch-up.

The equation assumes every drop lands on the hill, and it does not. Between 10 and 30% of the water pumped never becomes snow on the trail. Part of that is unavoidable physics: freezing water releases about 334 kJ per kilogram and evaporating it absorbs about 2,500, so roughly one part in eight of every droplet must evaporate to carry away the latent heat that freezes the other seven. Part is wind, drift and plume carrying finished snow into the trees. The irony of the trade is that the losses are worst in exactly the dry, breezy weather that gives the best wet bulb.

There is a second thing the balance cannot see: snow settles. What lands at 400 kg/m³ will be denser within days, so the same water covers less depth a week later than this arithmetic suggests. The mass is still there. The volume is not, and depth is what a skier stands on.

Finally, the water accounting itself deserves a fair statement. Snowmaking water is mostly borrowed rather than consumed: the great majority runs back into the same watershed at melt-out, minus the evaporative fraction. But it is borrowed at the wrong time of year — drawn during winter low flow and returned in spring, when the river had plenty anyway — and it is drawn at rates that matter to a small stream. That timing, not the total, is why withdrawals are licensed.