Interstellar clouds are concentrations of cold (T ≲ 100 K) neutral gas (cf. Spitzer 1978) which are immersed within an intercloud medium. It is worthwhile to distinguish between diffuse clouds (roughly those with E[B-V] ≳ 0.5) and dark clouds (those with E[B-V] ≳ 0.5). This distinction is useful in the sense that diffuse clouds are relatively warm (T ∼ 100 K), they are composed mostly of atomic species except for hydrogen which can be appreciably molecular, and they are dynamically controlled by their interaction with the intercloud medium. Dark clouds are relatively cold (T ∼ 10 K), they contain a rich variety of molecules, and self-gravity is important in their evolution. Because the interstellar extinction is a rapid function of wavelength, most ultraviolet observations have been of diffuse clouds. The IUE satellite is sufficiently powerful that observations of some dark clouds are possible, and an important area of future research will be to delineate more quantitatively the similarities and differences between diffuse clouds and dark clouds.With ultraviolet observations, considerable progress has been made in understanding the physical characteristics of clouds including determinations of their densities, temperatures, chemical compositions and dynamics (cf. Spitzer and Jenkins 1976). Because particular progress has been made on understanding the abundances within diffuse clouds and because of the limitations of space, we restrict this review to a discussion of abundances within diffuse clouds. These abundance measurements provide a set of fundamental astrophysical data.
Ultraviolet Observations and a Theory of STEVE