In the science of snowmaking, temperature alone doesn’t tell the full story. What truly determines whether snow guns can fire isn’t just how cold it is — it’s how dry the air is, too. This is where wet bulb temperature comes in: the key metric that combines air temperature and humidity into a single, snowmaking-ready number.
The wet bulb temperature is the lowest temperature that can be reached by evaporating water into the air. It’s measured by wrapping a moist cloth around the bulb of a thermometer and ventilating it — the evaporative cooling lowers the reading below the standard (dry bulb) temperature. The drier the air, the more evaporation occurs, and the cooler the wet bulb reading.
This temperature reflects not only the ambient temperature but also the relative humidity of the air. It's especially relevant to snowmaking because the process of turning water into snow relies on evaporative heat loss.
If the air is humid, evaporation slows, heat is retained, and snowmaking is inefficient or even impossible. But when the air is dry, evaporation accelerates and cools the droplets more efficiently. That’s why a 29°F night with low humidity may be better for snowmaking than a 27°F night with high humidity.
The magic threshold for snowmaking generally sits around a wet bulb temperature of 27°F or lower. This is the operational green light for snow production in most climates.
The concept of wet bulb temperature dates back to the early 19th century. Meteorologists and engineers needed a more accurate measure of atmospheric moisture content, particularly for military, agricultural, and industrial applications.
The first practical instrument to measure it was the psychrometer, developed in the 1820s. This device uses two thermometers — one with a dry bulb and one with a wet bulb — spun through the air on a sling or fanned mechanically. The difference in temperature between the two readings is then used to calculate relative humidity and, by extension, the wet bulb temperature.
In snowmaking, this became vital as ski resorts began installing the first artificial snow systems. Technicians quickly learned that merely watching the mercury wasn’t enough — snow production failed when humidity was high, despite subfreezing temps.
By the 1980s, automated wet bulb sensors began replacing manual psychrometers in mountain operations. These sensors now feed real-time data into modern snowmaking systems, optimizing when and how much snow is made — while conserving water and energy.