Snow Made in a Wet-Bulb Window

Snowmaking is gated on wet-bulb temperature, not on the thermometer. That single fact is why this page exists, and it is the thing that separates people who make snow from people who watch the forecast and wonder why the guns are running.

Here is the mechanism. A snow gun does not freeze water by putting it in cold air and waiting. It atomises the water into droplets a few tens of micrometres across and throws them into the air with as much hang time as it can buy. Each droplet immediately starts evaporating from its own surface, and evaporation takes latent heat out of the droplet — about 2,500 kJ for every kilogram that leaves. The droplet cools itself. It keeps cooling until it reaches the wet-bulb temperature of the air around it, which is by definition the temperature at which evaporative cooling and conductive warming balance. So the temperature that matters to the droplet is the wet bulb, and the dry-bulb reading on the lodge wall is very nearly irrelevant.

The consequence runs in both directions and both surprise people. At +2 °C with 15% relative humidity the wet bulb is about −3.3 °C, and the guns will run and make good dry snow while the air is above freezing. At −1 °C with 95% humidity the wet bulb is barely below −1.2 °C, the droplets have nowhere to dump their heat, and nothing freezes — a crew that trusted the thermometer runs pumps and compressors all night and puts water on the trail. The second case is the expensive one, and it is why every snowmaking operation watches a wet-bulb readout rather than a temperature.

The working thresholds most operations use: around −2.5 °C wet bulb to start marginally, with wet heavy snow and high losses; −5 °C for reliable production; −10 °C and below for full output and dry snow that grooms well. A "window" is the run of hours the wet bulb stays under the threshold, it is usually overnight, and it closes without regard for how nearly finished you were.

The loss fraction has a physical floor and it is not zero. To freeze a droplet you must remove about 334 kJ/kg of latent heat of fusion, and the only mechanism available in flight is evaporating some of it, which costs about 2,500 kJ/kg. The ratio is roughly one to seven and a half: about an eighth of every droplet must evaporate to freeze the rest. That is 12% before wind, before drift, before anything lands in the trees. Real operations run 10 to 30% overall, and the losses are worst in exactly the dry, windy conditions that give the most attractive wet bulb — which is the standing irony of the trade.

Two closing honesties about the arithmetic. It assumes a constant flow across the whole window, and real operators throttle the water as the wet bulb moves, taking more water per gun as it gets colder. And it says nothing about snow quality: the marginal hours at each end of a window make wet, dense snow that a groomer will complain about, and it is the same cubic metres in this equation as the good stuff made at −12 °C.