The Snow-making

Snow-making has been around for decades and has become an important addition to mountain operations at many resorts. It is part of the routine at roughly 87 percent of ski resorts nationwide, most common in the Mid-Atlantic, Midwest, and Northeast. Assuming a relative humidity < 100 %, what temperature do mountain operation managers need to observe before they can start making snow (at 0 degrees Celcius, snow starts to form). Please explain with reasoning.