Threshold for particle clumping via the streaming instability for different particle stopping times and metallicities. The green area in the upper right part denotes the parameter space within which clumping via streaming instability is possible. The filled coloured circles show our data for different disc locations with a cooling time of t_cool = 100 yr. The lower row corresponds to a disc with solar metallicity within which the interacting solids are made of ice, and the upper row is either a high-metallicity disc or a disc with solar metallicity, where both ice and rock contribute to clumping.