One way to define the Randers metric is using the data $(h,W)$ associated to the Zermelo problem. Here $h$ is the Riemannian metric and $W$ is the wind. In order to define the Randers metric we must have $h(W,W)<1$.

Now the question is that what happens if $h(W,W)\geq 1$? I mean we are faced with a new metric? Why in Randers $h(W,W)\geq 1$ is a necessary condition?