The skier is on the snow.
The skier is standing in snow
Being on snow when skiing is a sufficient condition for standing. Standing is required when skiing.
exists p1 x1 x2.(n_snow(x2) & n_skier(x1) & on(x1,x2))
exists e1 x1 x2.(n_skier(x1) & in(e1,x2) & n_snow(x2) & Actor(e1,x1) & v_stand(e1))