Process of introducing nuclei, e.g. silver-iodide crystals or solid carbon dioxide (dry ice), into clouds composed of supercooled water droplets, in an attempt to induce precipitation. Dry ice introduced (at −80 °C) from the air into cloud lowers the air temperature so that (particularly at temperatures below −40 °C) some of the supercooled water droplets are converted into ice crystals which then grow by collisions with further droplets. Silver iodide (which has a crystal structure similar to that of ice), introduced from the air or ground, is the substance most commonly used in seeding: its crystals act as ice nuclei. Other substances, e.g. common salt or fine water droplets, may also be used to encourage coalescence. Natural seeding may be significant in cases where ice crystals from a high ‘releaser’ cloud (e.g. altostratus or cirrostratus) fall into a supercooled water ‘spender’ cloud (e.g. nimbostratus) and encourage ice-crystal growth.