ABSTRACT
From our everyday life experience we all know that water vapor from the air condenses in droplets upon decrease of temperature on distinct surfaces such as windows, leaves, and the like, and that a further decrease of temperature leads to the formation of solid water (ice) below 273 K (0◦C) at about standard atmospheric pressure (1013 hPa). Up in the troposphere, the clouds have no large surfaces to condense on and for this reason they can stay there as huge wandering shape-changing beings with a milk-white appearance. Their characteristic color arises from the scattering of visible (white) light with water droplets whose sizes are comparable to the wavelength of such light (∼500 nm). Such droplets, if pure, can be cooled down to about 233 K (−40◦C) without the formation of ice (Schaefer 1946). Water is then said to be in a supercooled state. But ice in the clouds can be formed at higher temperatures (or not that low) as a result of the presence of foreign bodies such as dust particles, contaminants, nanoparticles, and so on, both from natural and anthropogenic origin, which act as nucleation agents (Langmuir 1950), in a process known in the crystal growth community as heterogeneous nucleation. This is one aspect of the watersurface interactions that is discussed at length in this book. The surface of ice plays a relevant role for atmospheric phenomena because it is able to catalyze chemical reactions such as those involved in polar ozone depletion, and those who love skiing should be grateful for the presence of a thin film of liquid water at the surface of snow below 273 K, where water would be expected to be in the solid state (again a supercooled liquid but this time on top of solid water). When the liquid water film is too thick (above 273 K) the resulting (spring) snow makes skiing a difficult task.
