How deep does the snow have to be to open an average trail. I know every trail is different, and probably each trail has different requirements caused by unevenness, stumps, rocks, undergrowth that may not be cut down fully, etc. I was watching a groomer moving snow around on the upper portion of the terrain park at Crotched when I was there on the 31st and it seemed they were not going to have enough to make it skiable because of the drainage ditches running across the trail in a few places.
Camelback has been blowing snow on some trails and not grooming it out during this warm snap. They claim that the snow in piles will last longer due to refrigeration and will groom it out with the resumption of colder weather.
Having hiked a few of the slopes at Crotched for lift evac practices, etc, the slopes are in really nice shape for only having been cleared less than four years ago.
The snowmakers at crotched dumped far more snow Galaxy, Meteor, and Plutos than was really needed to open them. Having drilled holes for signage, the based depth really was 18+ inches. This allowed them to maintain very good cover during some pretty snow-hostile weather, including building a big kicker for a freestyle competition.
I guess the point I'm making is just hitting the minimum and moving onto another trail isn't always the best strategy.
As for the minimum, it depends on the area. In the west, some places need several feet of snow to cover the rocks. In the midwest, some areas plant rye grass and let it grow to about a foot. Before the season starts the grass is rolled to provide a "p-tex friendly" natural ground barrier. At those areas, a few inches of natural snow often will suffice.
i would imagine different ski areas have different standards. are you specifically wondering about snow making and grooming or any trail including natural snow ungroomed?
generally, ungroomed natural snow has lower base depth requirements than man made. some areas will open trails on as little as 6-12" of fresh depending upon consistency whereas other areas would want twice as much. additionally it would depend on what that snow is covering up and how thin the cover is. two to three feet is a good rule of thumb for the woods.
I would say that it depends on the surface of the trail. For example, Okemo opened Upper Mountain Road the other day, with no snowmaking. However, under that is a paved road. That is the only non snowmaking trail Okemo has open right now. I guess it also depends on standards for example Stratton vs. Mad River Glen.
like stated before, it all depends on whats underneath. at the local ski hill i work at, we opend trails with only 6 inches of snow. this is on a slope which has mowed grass and no rocks. other slopes it can take up too 2-3 feet.