Minecraft wiki entry for Ice, associated text: Water source blocks in a snowy biome eventually freeze into ice if exposed to the sky from directly above, the light level immediately adjacent to the water block on all sides is less than 13, and there is at least one horizontally adjacent non-water block. This can happen at any time of day, and in any weather condition. If the highest adjacent light level is 12, an ice block alternately melts and re-freezes when it receives a block tick.