Ice is a solid phase, usually crystalline, of a non-metalic substance that is liquid or gas at room temperature, such as ammonia ice or methane ice. However, the word "ice" normally means water ice, technically restricted to one of the 15 known crystalline phases of water. In non-scientific contexts, it usually describes ice Ih, which is known to be the most abundant of these phases. It can appear transparent or an opaque bluish-white colour, depending on the presence of impurities such as air. The addition of other materials such as soil may further alter the appearance.
The most common phase transition to ice Ih occurs when liquid water is cooled below 0 °C (273.15 K, 32 °F) at standard atmospheric pressure. It can also deposit from a vapour with no intervening liquid phase, such as in the formation of frost.
Ice appears in nature in forms as varied as snowflakes and hail, icicles, glaciers, pack ice, and entire polar ice caps. It is an important component of the global climate, particularly in regard to the water cycle. Furthermore, ice has numerous cultural applications, from the ice cooling one's drink to winter sports and ice sculpture.
