As we've seen, the main job of snow-making machines is to do the work of snow-making clouds that occur naturally in the atmosphere. These machines do not make snow under just any conditions, however — you need to have the right kind of ground weather, just as you need the right kind of ground weather for natural snow to make it to the earth and then stick. To figure out when to make snow, and to make sure they get the right kind of snow, ski resorts depend on the expertise of experienced snow machine operators, commonly called snow-makers.
So how do snow-makers determine if the conditions are right? It turns out they need a lot more information than they can get from an ordinary thermometer. Standard thermometers measure the dry bulb temperature of the atmosphere; but the most important factor for snow conditions is the wet bulb temperature.
The wet bulb temperature is a function of the dry bulb temperature and the relative humidity, the amount of water vapor in the air. Liquid or solid water cools itself by evaporating some water as water vapor. This releases heat, and so lowers the energy level in the water. When there is more water vapor in the atmosphere, water or snow can't evaporate as much because the air is already saturated with water to a high degree. Consequently, water cools more slowly when the humidity is high, and more quickly when the humidity is low.
For this reason, humidity is a very important factor in determining snow conditions. If the humidity level is low enough, you can actually get snow even when the dry bulb temperature is several degrees above freezing. If the relative humidity is 100 percent, then the wet bulb temperature and the dry bulb temperature will be exactly the same. But even if both are at the freezing temperature, you might get rain instead of snow because the air saturation slows the cooling process down so much.
If the temperature is around 30 F (minus 1 C), you need a fairly low relative humidity (less than about 40 percent) for good snow-making conditions. If the temperature is less than 20 F (-6.7 C), you can make snow fairly easily even if the relative humidity is 100 percent. A temperature in the teens is ideal for snow-making.