Meteorologists are very careful in choosing a cloud that is suitable for seeding. Before a cloud is seeded, it must pass three tests. The cloud top must be colder than about -5 ºC, as warmer clouds won’t develop much ice, even though they may be slightly supercooled. The cloud must have a steady updraft, to provide a continuing supply of the moist air that allows the ice particles to keep growing. Finally, the cloud must not have much natural ice. If the cloud has already developed ice, it need not be seeded, as nature is already being efficient. Thus, the cloud must be cold, ice-free, and have an updraft.