Abstract: Water is the most abundant compound at the surface of the earth and, as such, plays a fundamental role in many physical, chemical, biological, geological and atmospheric processes. This dissertation comprises studies aimed at enhancing our understanding of the physical and chemical processes that give water its unique properties. In Chapters 2-4, experimental studies of evaporation and fractionation of water and its isotopomers are presented. When evaporation occurs into an under-saturated gaseous atmosphere, diffusion through a boundary layer causes additional isotopic fractionation due to differences in the transport properties of the isotopomers. In Chapter 2, the magnitude of this diffusion-controlled fractionation for natural water samples is quantified, and the ramifications of its proper treatment in regional and global models of the hydrologic cycle are examined. In Chapters 3 and 4, evaporation and fractionation of deuterium-rich samples of water are observed as they evaporate into a vacuum. The isotopic fractionation observed depends upon the liquid isotopic composition, demonstrating that free molecular evaporation of liquid water is an activated process and that evaporation of water proceeds at a slower rate than the theoretical maximum. Transition state theory is used to develop a microphysical model of evaporation consistent with the observations. Implications of these results for understanding cloud formation processes are briefly discussed. In Chapter 5, a computational study of the contribution of spectrally narrow cavity resonances to absorption of shortwave solar radiation by clouds is presented. Although absorption due to cavity resonances is significant, models wherein the absorption of solar radiation is calculated at low resolution perform sufficiently well due to substantial cancellation of errors. In Chapters 6--9, soft X-ray absorption spectroscopy (XAS) experiments on liquid water and aqueous solutions are discussed. XAS is an element specific technique sensitive to changes in the local electronic environment around atomic centers; for water, this is evidenced by variations in the oxygen K-edge XAS with phase, temperature or solution composition. The experimental and associated computational studies presented in this dissertation detail the perturbations of dissolved inorganic salts and acids on the liquid water XAS, specifically focusing on the effects of halide anions (Chapter 6), the hydrated proton (Chapter 7), and both monovalent and divalent cations (Chapter 8). In Chapter 9, a comparison between the total ion yield (TIY) XAS of liquid water acquired using a newly constructed X-ray endstation and previously obtained TIY spectra is given.