Before snow making, ski resorts were at the mercy of Mother Nature. A bad year with no snow meant no business, for ski areas and for hotels, restaurants and shops as well. Since even the smallest local resorts all have some form of snow making now, business can continue during most months as long as it stays cold. It's more than just point a fire hose at the hill though, the science is very precise.
One kind of snow gun uses water and compressed air. The air has to be at high pressure to split the water stream into tiny droplets and force it up hill, and higher into the air, so it can freeze and fall like real snow. The physics have to be just right to compensate for the topography of the land and the wind currents. Other systems use an electric fan rather than high pressure air to blow water droplets into flakes. The weather is still a factor however. Dry air makes the best snow, regardless of what system because evaporation cooling is great for flake makers. The snow then has to be groomed with large equipment because no matter what force or temperatures are at play, no amount of physics can move large amounts of man made snow around to perfect moguls and trails.