Ice surfaces: macroscopic effects of microscopic structure

The balance between ice and water controls the habitability of an important fraction of the globe and influences the majority of the world'population. The freezing of water to form ice is one of the most common phase transformations in the natural environment. However, a complete understanding of its microscopics and their influence on macroscopic phenomena still eludes us. As this millennium comes to a close, we are beginning to understand how the microscopic interfacial structure of ice controls pattern formation during ice-crystal growth, the evolution of the polycrystalline fabrics of the great ice sheets, the dynamics of ground freezing, ozone destruction, and the mechanism of charge transfer that drives thunderstorm electrification. This paper describes our evolving understanding, its implications for the basic principles of melting and freezing, and their environmental consequences.