“In order to simulate actual outdoor skiing conditions, provisions are made to vary the steepness of the slope from place to place. In addition, facilities are provided to produce random simulated moguls or an entire mogul field. Thus, during one run of the slope, most, if not all, of the conditions encountered on natural outdoor slopes may be simulated and incorporated into the run”

By Nobuyuki Matsui, via Google Patents, via Futility Closet.