  We experimentally investigate the effect of freezing on the spreading of a
water drop. Whenever a water drop impacts a cold surface, whose temperature is
lower than 0{\deg}C, a thin layer of ice grows during the spreading. This
freezing has a notable effect on the impact: at given Reynolds and Weber
numbers, we show that lowering the surface temperature reduces the drop maximal
extent. Using an analogy between this ice layer and the viscous boundary layer,
which also grows during the spreading, we are able to model the effect of
freezing as an effective viscosity. The scaling laws designed for viscous drop
impact can therefore be applied to such a solidification problem, avoiding the
recourse to a full and complex modelling of the thermal dynamics.
