A chilling wind whips through the hollow. Winter's bite grips your spine. The very air you breath stings. Snow swirls all around you, yet, the trees... the trees are motionless in the gale, unaffected by the cold, biting winds.
If a creature is transformed into another creature (say, a human into a dog), then that creature will become more and more accustomed to the new form until a point is reached where reversing the spell is impossible. A second transformation is possible, but the creature will have to go through the whole process again.