  In this paper it is demonstrated that based on two-dimensional distributions
in semi-inclusive deep inelastic scattering (SIDIS) data, obtained recently by
the HERMES experiment at DESY on different nuclei, which contains data for
charged pions produced in $z$ slices as a function of $\nu$, and in a $\nu$
slices as a function of $z$, it it is possible to parametrise a ratio of
multiplicities on nucleus and deuterium (per nucleon) $R_M^{h}$ in a form of a
function of a single variable $\tau$, which has the physical meaning of the
formation time of hadron. We call this effect $\tau$ scaling. $\tau$ is a
function of two variables $\nu$ and $z$. It is also shown that $R_M^{h}$ can be
presented in a form of a linear polynomial of $\tau$, $a_{11}$ + $\tau a_{12}$,
where parameters $a_{11}$ and $a_{12}$ do not depend on $\nu$ and $z$.
