Rock salt works by lowering the freezing point of water -- a process called "freezing point depression". In a nutshell, the salt breaks into its component ions in a small amount of liquid water. The added particles make it more difficult for the water to freeze into ice, lowering the freezing point of the water. So, for rock salt to work, there needs to be a tiny bit of liquid water. This is part of the reason why rock salt is not effective in extremely cold weather when water would freeze too easily. Rock salt is used to control snow and ice. As the temperature drops, salt's effectiveness slows to the point that when you get down near -10 degrees (C) and below, salt is barely working.