Given a root system $\Phi$ and a Weyl group $W$ associated with $\Phi$, it is well-known that the number of positive roots sent to negative roots by an arbitrary element $w \in W$ is equal to the length of $w$ (regardless of how we pick the positive roots).
Is there a similarly nice expression for the number of simple roots sent to negative roots by $w$?
I am interested in both the finite case as well as the affine case.