When an ice particle grows at about -5 ¿C by sweeping up supercooled water drops in a cloud having a wide range of drop size, secondary ice particles are ejected as the drops freeze. In this way the concentration of ice particles in the cloud may increase rapidly. Present experiments show that this ''splinter'' formation can be considerably reduced by injecting small quantities of ammonia into the cloud. This supports the theory that splintering is a consequence of the fromation of a strong ice shell round the periphery of a freezing drop. As the drop freezes the pressure within the shell increases until it is relieved by cracking of the shell. In extreme cases fragments of ice may be ejected. The addition of ammonia is thought to weaken the ice structure, thus allowing pressure to be relived by deformation or cracking without ejection of splinters. This interpretation is supported by experiments on the freezing of larger drops ''doped'' with NH3 or HCl.